Mg II Absorbers:
An On-line Review "Paper"
last update 12/99
Table of Contents
- Introduction and Statistical Properties
- High Resolution, Kinematics, and Cloud Properties
- The Connection with Galaxies
- Statistically, How Extended is Mg II Absorbing Gas Around Galaxies?
- What Types of Galaxies are Associated with Mg II Absorption? (Part I)
- What Types of Galaxies are Associated with Mg II Absorption? (Part II)
- Does Mg II Absorption Cross-Section Depend Upon Galaxy Luminosity?
- Does Mg II Absorption Depend Upon Sky-Projected Galactocentric Distance?
- Does Mg II Kinematics Depend Upon Sky-Projected Galactocentric Distance?
- Is Galaxy Morphology and Mg II Kinematics Correlated?
- A Population of Weak Systems
- High Ionization Phase and Classes of Mg II Absorbers
- Putting it All Together
Introduction and Statistical Properties
What Are Mg II Absorbers and Absorption Selected Galaxies?
Galaxies and other "metal" enriched, gaseous structures that lie between Earth and any extremely distant, background quasar give rise to absorption lines in the quasar's spectrum. For a more complete description of these "quasar absorption line systems", click here. Below is a Hubble Space Telescope, or HST, image (WFPC2) centered on the quasar 3C 336 (1622+238), which has a redshift of z=0.927. This field has been extensively studied by Steidel et al. (1997, ApJ, 480, 568). A Mg II absorption-selected galaxy is one which was identified or discovered following a priori knowledge of Mg II absorption in the spectrum of a background quasar. The view of the line of sight (LOS) in the redshift dimension is shown in the lower panel. Note that the LOS does not pass through the steller component of the galaxy (shown in green), but in front of it from this vantage point. The galaxy is at lower redshift than the quasar (red), and therefore lies between us and the quasar (thus the term intervening absorber).
Caption: A schematic of the Mg II absorbing galaxy at z=0.891, which is just to the left of quasar in the HST image. The galaxy itelf is green and the quasar is red. The extended gaseous "halo" around the galaxy is yellow and protrudes in front of the quasar. It is the material directly between us and the quasar that gives rise to the absorption.
Caption: (upper left) A deep HST image of the 3C 336 field, centered on the quasar (labeled QSO). At redshifts smaller than that of the quasar are four galaxies (labeled in white with redshifts given), whose gasesous extent protrudes in front of the quasar, as schematically shown in the panel to the right of the image. In the quasar spectrum, these gasesous components to the galaxies give rise to Mg II and C IV absorption. (bottom) Illustration of the quasar light path to the observer (a side view). HST Image courtesy of Chuck Steidel.
What Are the Statistical Properties of Mg II Absorbers?
To date, the most extensive Mg II surveys have been conducted by Lanzetta, Turnshek, & Wolfe (1987, ApJ, 322, 793), Sargent, Steidel, & Boksenberg (1988, ApJ, 334, 22), Petitjean & Bergeron (1990, A&A, 231, 309), Steidel & Sargent (1992, ApJS, 80, 1), and Churchill et al. (1999, ApJS, 120, 51). The redshift coverage and rest-frame equivalent width detection limits (i.e. sensitivity) are summarized in the figure below. The blue data are from the low resolution survey by Steidel & Sargent of 103 quasars, covering the redshift interval 0.3 to 2.2 and complete and unbaised for W_min > 0.3 Ang. The red data are from the HIRES/Keck survey by Churchill et al. of 26 quasars, covering the redshift interval 0.4 to 1.4 and complete to W_min > 0.02 Ang (unbiased only for systems with W < 0.3 Ang).
Presented here are four key statistical properties: ( panel a ) The equivalent width distribution, n(W), which gives the number of systems per unit equivalent width per unit redshift; ( panel b ) The number density of absorbers with equivalent widths, W, greater than a well-defined minimum, W_min, measured over finite redshift bins; ( panel c ) The mean number density, < dN/dz > , over the total observed redshift range, for systems with W > W_min; ( panel d ) The "evolution parameter" (the slope on the log(dN/dz) vs. log(z) plane, the log-log version of panel b) for systems with W > W_min. The blue and red data points correspond to the Steidel & Sargent and the Churchill et al. surveys, as described above and shown in the top panel.
Caption: The statistical properties of Mg II absorbers. The samples of Steidel & Sargent (1992, ApJS, 80, 1) [blue data] and Churchill et al. (1999, ApJS, 120, 51) [red data] are combined here, giving the full range of measured equivalent widths. Note, however, that the redshift coverages are not identical- if the weakest systems evolve from z=1.4 to 2.2, the results would be affected. See text for detailed descriptions of the panels.
RESULTS (for the given redshift intervals shown and W_min):
- ( panel a ) The equivalent width distribution, n(W), follows a power law with an exponential slope -1.0 +/- 0.1; there is no turnover at small W in the equivalent distribiution down to W = 0.02 Ang. This result is contrary to several previous statement in the literature that there was a turnover below W = 0.3 Ang.
- ( panel b ) The implications of the above result is that dN/dz for Mg II absorbers is dominated by the "weakest" systems; 65% of all Mg II absorbers have W < 0.3 Ang. As redshift increases, dN/dz, increases; the solid curves are fits to the data with 1-sigma uncertainties shown. This increase with redshift is such that the co-moving density (that carried along by the expansion of the universe) is constant, or non-evolving. This all has important implications for the inferred sizes and types of objects (other than normal, bright galaxies) associated with weak Mg II absorbers (see below). There are no data for the highest redshifts, z>2.2.
- ( panel c ) Consistent with these results are that < dN/dz > increases as W_min is decreased (there are more weak absorbers per unit redshift than strong ones). Note that these are not compared over the same total redshift intervals. Roughly 2.7 systems with W > 0.02 Ang are expected per unit redshift, whereas only 1.1 with W > 0.3 Ang and only 0.6 with W > 0.6 A are expected.
- ( panel d ) There is clear evolution in the number density as a function of W_min, such that the larger W_min, the more pronounced the evolution. (The no-evolution expectations are shown for qo=0.5 and 1.0, respectively, and the range is shaded yellow). That is, the strongest systems become more rare with decreasing redshift (increasing cosmic time). Since dN/dz is dominated by smaller equivalent width systems, which do not evolve with redshift, the evolution is "washed out" for W_min = 0.3 Ang. The data for W_min < 0.3 Ang are unconstraining, gamma = 0.9 +/- 1.6.
High Resolution, Kinematics, and Cloud Properties
What Does High Resolution Spectroscopy Buy?
The strong, resonant Mg II 2796,2803 doublet is easily identified in the specta of quasars. The top panel of the figure below is a R=3000 spectrum of PG 1206+459, obtained by Steidel & Sargent (1992, ApJS, 80, 1) on a 4-meter class telescope. A Mg II doublet at redshift z=0.93 is shown, which places it at the observed wavelength of ~5400 Angstroms. The 1990's and 10-meter class telescopes brought high resolution into the fold. The lower panel shows a Keck/HIRES spectrum of the highlighted wavelength region (Churchill, 1997, Ph.D. Thesis). The resolution is R = 45,000, or roughly 6 km/s in the rest frame of the absorber. Note that the 2796 transition (5383-5392 A) and the 2803 transition (5396-5406 A) have been resolved into multiple components. Also note the detection of a "weak" system at z=0.9343, about 1000 km/s to the red of the larger, complex system. A detailed study of this "triple", or complex "double" system, has been published by Churchill & Charlton (1999, AJ, 118, 59).
High Resolution Buys
Caption: Illustration of the detailed revealed when high resolution spectra are brought to bear on Mg II absorbers. (1) The detection sensitivity is increased to very small equivalent widths (~0.02 A, rest-frame) allowing very "weak" clouds to be observed. Note that most of the profiles from these weak clouds remain unresolved even with HIRES/Keck. (2) The line of sight kinematics are probed.
The HIRES spectrograph was possible because of the dedicated work of many individuals at the University of California, Santa Cruz. In particular, the PI Steven S. Vogt is due great credit for his 5+ years of effort.
Cartoon for Physical Interpretation of Mg II Kinematics
In the below schematic, a distant, "background" quasar is shown in the upper left, its light path (labeled "Line of Sight", LOS) passing through a galaxy disk and halo and being dispersed into a spectrum (lower right) of the Mg II 2796 transition. The spectrum is normalized and converted to rest-frame velocity (in km/s) relative to an arbitrary zero point. The green parts of the galaxy represent the locations of low ionization gas clouds that give rise to Mg II absorption. In this example, the LOS passes through the galactic disk and two of the halo clouds, whose velocities are represented with the blue arrows. The disk gives rise to the largest absorption, which is arbitrarily given velocity ~0 km/s. The larger of the two halo clouds is moving almost directly away from the observer, infalling into the disk at ~+100 km/s. The smaller cloud is also infalling into the disk toward the observer. Its velocity is not directly along the LOS, resulting in a velocity of ~-50 km/s, and its absorption cross section (due either to ionization condition or physical size, or both) is smaller, resulting in weaker absorption.
Caption: A cartoon illustrating the physical interpretation for the multiple component Mg II absorption profiles seen in high resolution quasar spectra. The cloud velocities are dependent upon the line of sight component of the "cloud" motions, and the absorption strengths are dependent upon the cloud absorption cross sections.
What Are the Kinematics of Mg II Absorption Like?
In view of the simplistic physical interpretation given above, one can now "appreciate" the rich variation in kinematics and absorption strengths seen in high resolution Mg II profiles (only the 2796 transition is shown). Presented here is a gallery of Keck/HIRES Mg II 2796 Profiles (in increasing redshift order). Each panel is a normalized spectrum showing a rest-frame velocity window of 480 km/s (-240 to +240 km/s), except for the z=0.8514 and z=0.9277 systems, which show 960 km/s windows. The typical line-of-sight velocity spread of the Mg II absorbing gas in galaxies is 100-200 km/s.
Caption: A gallery of HIRES/Keck Mg II 2796 profiles from Churchill (1997, Ph.D. Thesis).
What Are the Statistical Properties of Mg II Kinematics?
There are a multitude of ways to numerically quantify the kinematics of absorbing gas with each resulting in a loss of information compared to what can be seen directly in the profiles themselves. Below are shown two such indicators, the number of clouds per velocity bin for the whole sample of 50 systems and the cloud-cloud velocity two point correlation function (for the z < 1 systems).
Caption: A histogram of the number of "clouds" in each 20 km/s velocity bin. The clouds are not obtained from Voigt profile fitting, but are objectively defined absorption features isolated in velocity space within a system. Taken from Churchill (1997, Ph.D Thesis)
A velocity zero point is determined for each system by assigning v = 0 km/s at the position in the profile where the integrated optical depth is equal to either side (the optical depth median of the profile). Shown above is the resulting histogram for the full sample (red unshaded) and a subsample with limiting equivalent width detection sensitivity of 0.02 Ang (shaded green). The latter is a more accurate representation, since the former is biased against the detection of small equivalent width clouds at the higher velocities. Some systems have maximum velocity separations of 400 km/s, whereas most systems have less than 250 km/s maximum separations. There are not enough data to comment on whether the histogram actually has multiple modes (i.e. at 90, 150, and 220 km/s), but there is a slight suggestion that the kinematics are not evenly distributed in velocity for v > 20 km/s. The large signal at 0 < v < 20 km/s is not dominated by the presence of single cloud systems, it results due to the arbitrary definition of v=0 (all systems will have a cloud in this bin!). However, that the v = 0 km/s bin is so prominant and sharply drops for v > 20 km/s indicates that the bulk of the absorbing material in Mg II absorbers is confined to a narrow velocity spread (less than 20 km/s), otherwise the number of clouds would be more evenly distributed with the v > 20 km/s bins. This is a strong clue to the nature of the absorbing gas, and may suggest that the bulk of the material is in a relatively quiescent structure more akin to a galaxy disk as opposed to a full blown galactic halo (see Charlton & Churchill (1998, ApJ, 499, 181).
To the left is the cloud-cloud velocity Two Point Correlation Function (TPCF), which is obtained by taking the velocity differences between all cloud pairs in each system and binning them in velocity. The TPCF is one way to measure the velocity clustering properties of the clouds in intermediate redshift galaxies. Only systems with redshifts z < 1 are included (this is the redshift range over which the equivalent width distribution of the HIRES/Keck sample is consistent with an unbiased sample). Caption: The cloud-cloud pair velocity separations are computed for each pair in a system, then the cummulative number in each bin are obtained over all systems to make the two point correlation function. The cloud velocities are obtained from Voigt profile decomposition of the data. Taken from Churchill (1997, Ph.D Thesis)
A double Gaussian fit describes the TPCF quite well, with dispersions of 29 km/s and 140 km/s, respectively. Simulations were performed (Churchill 1997, Ph.D. Thesis; Charlton & Churchill 1998, ApJ, 499, 181) to ascertain if these two Gaussians are physical. For example, is the narrow Gaussian describing the cloud-cloud velocity separations in a "disk" while the broad Gaussian is describing that of the "halo". The simulations indicated that the narrow Gaussian is sensitive to the vertical velocity dispersion in disk gas, sigma_z, which is constrained to be ~15 km/s.
What Are the Column Density and Doppler Parameter Distributions?
Voigt profiles quantify each absorbing gas cloud with a column density, N, a Doppler parameter, b, and a velocity center. The distribution of Mg II column densities and b parameters has been measured before, in 30 km/s resolution spectra (Petitjean & Bergeron 1990, A&A, 231, 309). With the 6 km/s resolution HIRES/Keck spectra, the distributions could be measured not only for Mg II, but for Fe II and Mg I as well, and with less problems from unresolved blending. Below is shown the column density distributions for Mg II, Fe II, and Mg I. They are all well described by a power law, with the Mg II and Fe II distributions being consistent at the 1-sigma level. For log[N(MgII)] > 14 cm^-2, the Voigt profile fitting becomes quite uncertain due to saturation; this explains the increased scatter in the data for large N(MgII). It is not clear if the distribution actually is a single power-law over the full observed range, though there is no obvious physical scenario to suggest the distribution should contain a break. The same applies for Fe II. The Mg I distribution is apparently much steeper. This is consistent with the anti-correlation found between N(MgI)/N(MgII) and N(MgII), in that the larger the cloud Mg II column density, the smaller the neutral fraction of magnesium in the cloud (Churchill 1997, Ph.D Thesis). Also, take note that photoionization modeling with CLOUDY has not been able to reproduce the observed N(MgI)/N(MgII) ratios.
Caption: The power-law differential distributions of the column densities for Mg II, Fe II, and Mg I. The fits (using maximum likelihood to the unbinned data) are drawn through the binned data. The data roll over at small column densities is due to incompleteness effects. Taken from Churchill (1997, Ph.D Thesis)
The Doppler Parameter, b, measures the line widths and is obtained by Voigt profile fitting. Assumed contributions to the line widths are natural broadening (Lorentzian) and thermal plus turbulent broadening (Gaussian). Convolved, these give rise to the Voigt profile. The thermal plus turbulent broadening usually dominates so that b provides an upper limit on the gas temperature (or is directly proportional to the gas temperature if there is no turbulent component).
The panel to the left shows the distribution of b parameters for Mg II, Fe II, and Mg I measured in HIRES/Keck spectra (resolution ~6 km/s). The extended tails toward large b are due to line blends. The distribution for b(MgII) is most robust, given the Voigt profile fitting methodology. The mean b(MgII) is ~5 km/s, which corresponds to cloud temperatures of ~30,000 Kelvin. The resolution limit on b is 2.5 km/s, so b = 5 km/s suggests that some clouds are resolved- though blends of very narrow clouds cannot be ruled out, based upon simulations (Churchill 1997, Ph.D. Thesis).
In principle, the ratio of b(FeII)/b(MgII) for each cloud can be used to "deconvolve" the turbulent component to the line broadening in the cloud. This excercise was undertaken, but based upon simulations to assess the robustness of the result, all that can claimed is that the clouds are consistent with thermal broadening (i.e. a trubulent component cannot be claimed at a signficant confidence level).
Caption: The Doppler parameter distributions for Mg II, Fe II, and Mg I per 1 km/s bin. These results are obtained from Voigt profile decomposition of the HIRES/Keck profiles. Taken from Churchill (1997, Ph.D Thesis)
The Connection with Galaxies
Statistically, How Extended is Mg II Absorbing Gas Around Galaxies?
Based upon the redshift path density of absorbers, dN/dz, which gives the average number of systems per unit path of redshift, the sky-projected cross sections can be calculated for an assumed "luminosity function" (which is taken to be the space density of galaxies following the Press-Schecter formalism- in other words, the absorbers are assumed to be directly associated with galaxies). Under such assumptions, the C IV absorption-selected systems (to rest-frame detection sensitivity of 0.4 Ang) are inferred to have a 70 kpc size and the Mg II systems have a 40 kpc size (detection sensitivity of 0.3 Ang). The Lyman limit systems (those optically thick to hydrogen ionizing radiation) have sizes comparable to the Mg II selected systems. This led astronomers to infer that Mg II selected and Lyman limit selected absorbers were one and the same type of object. [We show below that this is only true for detection sensitivities of 0.3 Ang for Mg II; to a sensitivity of 0.02 Ang, Mg II systems have a size of 60 kpc (Churchill et al. 1999, ApJS, 120, 51)]. The damped Lyman-alpha systems (DLAs), which always are associated with strong Mg II absorbers, have sizes of 15 kpc. As such DLAs were thought to be the thick disks of spiral galaxies. However, at low redshifts (z<0.5) this has proven to be incorrect- DLAs are observed to be associated with a wide variety of low luminosity (~0.1 L*), morphological types (Le Brun et al. 1997 A&A, 321, 733), or sometimes are not even identifiable to these limits (Rao & Turnshek 1998, ApJ, 500, L115; Steidel et al. 1997, ApJ, 480, 568).
Caption: The sky-projected cross sections of damped Lyman alpha absorbers (DLAs), Mg II absorbers, Lyman limit systems, and C IV systems are shown (in kpc, for Ho = 50 km/s/Mpc, qo = 0.5). Figure adapted from Steidel (1993).
REFERENCES: (not on-line)
- Steidel, C. C., 1993, The Properties of Absorption-Line Selected High-Redhisft Galaxies, in "The Environment and Evolution of Galaxies", eds. J. M. Shull, & H. A. Thronson, Jr., (Kluwer Academic : Netherlands), p263
What Types of Galaxies are Associated with Mg II Absorption? (Part I)
Once Mg II absorbing systems are found, deep imaging of the quasar fields can be obtained to find candidate associated galaxies. Steidel, Dickinson, & Persson (1994, ApJ, 437, L75) performed a survey of ~30 fields, containing roughly 60 systems, and identified candidate galaxies for virtually every absorber with W > 0.3 Ang [the completeness limit of the survey by Steidel & Sargent (1992, ApJS, 80, 1)]. For 70% of the candidates, the galaxy is spectroscopically confirmed to be at the absorber redshift. The resulting luminosity functions are shown in the figure below. The "average" Mg II absorbing galaxy appears to be consistent with a normal 0.7 L*(sub B) Sb galaxy having a roughly constant star formation rate since z~1, although galaxies spanning a range of a factor of approximately 70 in luminosity are found in the absorber sample. The diffuse gas cross section imposed by studies of this kind appears to be biased against the relatively underluminous, blue galaxies which apparently dominate the number counts at faint magnitudes.
Caption: The rest-frame B-band and K-band luminosity functions of a sample of ~60 galaxies selected by the presence of Mg II absorption with W > 0.3 Ang. Roughly 70% of the galaxies are spectroscopically confirmed to actually have the same redshift as the absorbing gas. Taken from Steidel, Dickinson, & Persson (1994, ApJ, 437, L75).
What Types of Galaxies are Associated with Mg II Absorption? (Part II)
Below is a gallery of a sample of galaxies, in increasing redshift order, that give rise to Mg II absorption. The apparent sizes of the galaxies decrease with increasing redshift because of their relative distances. Note the wide variety in the apparent morphologies and orientations of the galaxies. In terms of the Mg II absorption selection process (for W > 0.3 Ang) of this sample of galaxies (of which only a small subset is shown here), it would seem that normal, bright galaxies representing a range of galaxy types, sizes, and bulge to disk ratios, are typical.
Caption: Hubble Space Telescope images (WFPC2) of several Mg II absorption selected galaxies. The location of the background quasar can be judged in the z=0.553, z=0.640?, and z=0.891 galaxies due to the diffration spikes. Images courtesy of Chuck Steidel.
Does Mg II Absorption Cross-Section Depend Upon Galaxy Luminosity?
Yes... but only very weakly. In the figure below, we show the impact parameter, D, vs. the rest-frame K-band luminosity of galaxies in quasars fields, as presented by Steidel (1995, astro-ph/9509098). The open circles are Mg II absorption selected galaxies with W > 0.3 Ang. The solid triangles are non-absorbing galaxies in the quasar fields. A "Holmberg" relation is shown (L^0.4 dependence) as a dotted line, normalized at D = 38 kpc at L/L* = 1. The dashed line is obtained by minimizing the number of non-absorbing galaxies below the line and the number of absorbing galaxies above the line. This yielded a weak Holmberg relation for the gas cross section as a function of galaxy luminosity, L^0.15. This has been interpreted as evidence that the gas is distributed in spherical halos with sharp cut-offs (at W = 0.3 Ang) and unity covering factors.
Caption: The impact parameter, D, vs. the rest-frame K-band luminosity of galaxies in quasars fields. See text for details. Taken from Steidel (1995, astro-ph/9509098).
The above interpretation, however, is not unchallenged. Charlton & Churchill (1996, ApJ, 465, 631) showed that the cross sections of the galaxy disks (once they are not assumed to be gossimer thin, but given realistic thickness), are competitive with that of spherical geometry. They performed a Monte Carlo survey of QSO fields in which the model galaxies have Mg II absorbing "clouds" in spherical halos or in randomly oriented disks. For both geometries, models that recover the observed properties of Mg II absorbers have only a 70%-80% covering factor. Therefore, regardless of absorber geometry, a survey of randomly selected QSO fields should yield a nonnegligible number of non-absorbing galaxies at small impact parameters. As shown below, few have been observed. However, based upon further modeling, Charlton & Churchill find that the selection techniques employed by Steidel, Dickinson, & Persson (1994, ApJ, 437, L75) may have played an important role in yielding a small number of small impact parameter, non-absorbers. If so, this could render the spherical halo interpretation as non-unique.
Does Mg II Absorption Depend Upon Sky-Projected Galactocentric Distance?
Yes and No. As can be seen in the figure below, there is a 3.8-sigma anti-correlation between the Mg II 2796 rest-frame equivalent width, W, and the quasar-galaxy impact parameter, D (in kpc, for Ho = 50 km/s/Mpc, q0 = 0.5). Note however that this is dominated by the relatively rare, very strongest absorbers and damped Lyman alpha absorbers (data points with concentric circles) at small impact parameters. For W <= 1.0 Ang, there is no trend. This provides evidence that the Mg II absorbing gas distribution around galaxies is patchy and not smoothly distributed (see below).
Caption: The Mg II 2796 rest-frame equivalent width vs quasar-galaxy impact parameter for the absorber-galaxy sample of Steidel, Dickinson, & Persson (1994, ApJ, 437, L75). Data points with open concentric circles are damped Lyman alpha absorbers (DLAs). Taken from Steidel (1995, astro-ph/9509098).
Does Mg II Kinematics Depend Upon Sky-Projected Galactocentric Distance?
A long-standing question has been "What is the spatial and kinematic structure of the extended gas surrounding intermediate redshift galaxies?". Early results based upon small, incomplete samples were suggestive that the gas followed an fairly smooth inverse square density law (Lanzetta & Bowen 1990, ApJ, 357, 321) and that the kinematics were systematic in that at small quasar-galaxy impact parameters, D, the gas showed disk-like rotation kinematics and at larger impact parameters showed infall-like kinematics (Lanzetta & Bowen 1992, ApJ, 391, 835). In a larger and complete, but still small sample, Churchill, Steidel, & Vogt (1996, ApJ, 471, 164) showed that the spatial distribution was not smooth and that the kinematics did not follow any simple systematic trend. They searched for correlations between the Mg II absorption properties (kinematic spreads, profile asymmetries, number of subcomponents, and total equivalent widths) and the galaxy properties (L and B luminosities, impact parameters, and B-K colors). Unlike the Lanzetta & Bowen samples, the Churchill et al. sample revealed no significant correlations between any combination of properties. The figure below shows the HIRES/Keck profiles from their sample ordered with increasing impact parameter. Note the lack of any visual trend.
Caption: The Mg II 2796 profiles in rest-frame velocity as a funtion of the quasar-galaxy impact parameter, D (in kpc, for Ho = 50 km/s/Mpc, qo = 0.5). Ticks above the spectra give the number of subcomponents ("clouds") and their velocity centers as determined using Voigt profile fitting. The labels give the galaxy ID and redshift as published in Churchill, Steidel, & Vogt (1996, ApJ, 471, 164).
Is Galaxy Morphology and Mg II Kinematics Correlated?
A statistical study of how galaxy morphology and Mg II kinematics are connected (if at all) has not yet been performed. Some data do exist for such a study. Below are shown the Mg II 2796 absorption profiles (Churchill 1997, Ph.D. Thesis) in line of sight velocity (v = 0 set by the optical depth median of the profile) and the HST images of the associated galaxies (courtesy of Chuck Steidel). These ~2" postage-stamp size images are centered on the galaxies- the background quasars lie between 2" and 10" outside the images. That is, the absorbing gas lies well outside the apparent stellar component of the galaxies. Based upon recent results by Churchill et al. (1999a,b ApJ, submitted) and Churchill & Charlton (1999, AJ, 118, 59), it is very likely that the Mg II absorption in and of itself will not reveal any fundamental results, but that high ionization absorption (i.e. C IV, N V, and O VI) and the neutral hydrogen, H I, will need to be fully incorporated into any study of relationships between absorbing gas properties and galaxy morphologies (see below).
Redshift z=0.553 (Q1148+384). A Face-on Spiral at D=15 kpc.
Redshift z=0.660 (Q1317+317). A Face-on S0 at D=21 kpc.
Redshift z=0.798 (Q1622+238). An Inclined Spiral at D=46 kpc.
Caption: The Mg II 2796 absorption profiles (resolution 6 km/s FWHM) are shown next to close-up (~2" on a side) HST images of the associated galaxies, whose morphologies and sky orientations can be clearly discerned. The histogram curves are the observed data and the smooth curves through the data are the Voigt profile model spectra. Ticks above the spectra give the number and velocity centroids of the Voigt profile components of the gas.
A Population of Weak Mg II Systems
What are "Weak" Mg II Absorbers?
Since ~1990, based upon statistical arguments and upon photoionization modeling, several researchers suggested that very small equivalent width Mg II did not exist. With the advent of very high signal to noise and high resolution spectroscopy the detection threshold dropped significantly below the the previous sensitivity levels of 0.3 Ang and the supposition could be tested directly by observations (Tripp, Lu, & Savage 1997, ApJS, 112, 1 ; Churchill et al. 1999, ApJS, 120, 51). Weak Mg II absorbers, then, are simply those having equivalent widths less than 0.3 Ang. In an unbaised survey for weak systems in 26 quasar lines of sight covering redshifts 0.4 to 1.4, Churchill et al. discovered 30 such systems and were able to measure the statistical properties of this sub-population of Mg II system. The redshift path density, dN/dz, is a factor of three times greater than that of the stronger systems! This implies very large sizes, roughly 65 kpc, if the weak absorbers are also associated with the same population of galaxies as are the strong ones (however, see below).
A gallery of a sample of weak Mg II doublets are shown in the figure below. The data are converted to rest-frame wavelengths with their redshifts labeled in red. Note that the profiles are, for the most part, comprised of unresolved subcomponents (resolution = ~6 km/s), even when multiple components are present. If you would like to view all 30 systems click here.
Caption: Representative "weak" Mg II absorbers as seen in HIRES/Keck spectra (Churchill et al. 1999, ApJS, 120, 51).
What Can be Inferred About "Weak" Mg II Absorbers?
If weak Mg II systems are in photoionization equilibrium, it can be inferred that they are optically thin to hydrogen ionizing photons and that their metallicities must be no less than 0.1 of the solar value, on average (Churchill et al. 1999, ApJS, 120, 51).
Shown in the top panel below is a grid of CLOUDY photoionization models. The important quantities characterizing the "clouds" are the total hydrogen column density, N(HI)+N(HII), shown across the horizontal axis, the observed Mg II column density, N(MgII), shown along the vertical axis, the neutral hydrogen column density, log[N(HI)], shown as solid green lines, and the ionization parameter, log(U) = log(n_ph/n_H), shown as dotted blue lines, where n_ph is the number density of hydrogen ionizing photons (lambda < 912 Ang), and n_H is the number density of hydrogen. It is assumed that the source of photons is the UV background from quasars and active galactic nuclei at z=1, as calculated by Haardt & Madau (1996, ApJ, 461, 20), and that the metallicity of the clouds is 0.1 of the solar value. The downward red arrow shows how the grid moves if the metallicity is decreases by 1 dex. The larger the ionization parameter, the higher the ionization condition in the cloud. Optically thick clouds are those having log[N(HI)] > 17.3 cm^-2. The yellow region is gives the range of observed N(MgII) for the sample of Churchill et al., based upon Voigt profile fitting.
Caption: A grid of photoionization models showing that most weak Mg II systems arise in optically thin gas (see text above for details). Taken from Churchill et al. (1999, ApJS, 120, 51).
In the photoionization grid above, note that much of the N(HI) range corresponds to the observed range of N(MgII) is in the regime of optically thin gas. This inferrence is also supported by simple arguments based upon a comparison of the redshift path densities, dN/dz, of optically thick, Lyman limit systems (LLS) and the weak Mg II systems. In the left-hand panel below, these quantities are shown for a mean redshift of
=0.9. Most all of the stronger systems are LLS absorbers, whereas the weak systems outnumber LLS absorbers by a factor of ~3. There are too many weak systems for them to be LLS absorbers, i.e. most weak systems are inferred to be optically thin. Subsequent measurements of the Lyman limit breaks in a Mg II selected sample of absorbers directly shows this inferrence to hold true (Churchill et al. 1999a, submitted). Shown in the right hand panel below is Mg II - Lyman alpha equivalent width plane. Solid red data points have measured breaks (i.e. are optically thick), open blue data points have no break (i.e. are optically thin), and open green box points are undetermined due to lack of spectral coverage at the expected location of the Lyman limit . The green dashed line at W(MgII) = 0.3 Ang segregates the weak systems. For the systems for which data are available, note that all but one of the weak systems is optically thin. Mg II aborption does arise in optically thin gas.
Caption: (left)- The redshift number density of Lyman limit systems (LLSs), strong Mg II systems (green), and all Mg II systems including weak systems (blue). Taken from Churchill et al. (1999, ApJS, 120, 51). (right)- The Mg II - Ly-alpha equivalent width plane showing the optically thick (red) and optically thin (blue) systems. Taken from Churchill et al. (1999a, submitted).
Returning to the photoionization grid above, we now see that the metallicities must be 0.1 solar or higher on average, since lower metallicity clouds require greater N(HI) for the measured N(MgII). This can be seen visually by moving the grid downward (as indicated by the red arrow) as metallicity is increased in the models.
Are Weak Mg II Absorbers Associated with Bright, Normal Galaxies?
It is hard to be conclusive about this issue at this time, however, there is mounting evidence that the weakest of the weak systems are not near normal, bright (L > 0.05L*) galaxies . Out of the 30 systems with W < 0.3 Ang reported by Churchill et al. (1999, ApJS, 120, 51), 12 have no apparent associated galaxy, 3 have confirmed galaxy hosts, 3 have candidate galaxy hosts (C. Steidel, private communication), and 12 are undetermined (i.e. there are no imaging data of the fields). The most extensive search so far was conducted on the two weak systems in the Q0454+039 field at z=0.6428 and z=0.9315 by Churchill & Le Brun (1997, ApJ, 499, 677), who studied a deep HST image of the field with follow up spectroscopy on the objects in the field. No galaxy candidates were found to m=24.
As an example of the "missing luminous counterparts" to weak systems, an HST image of the Q0002+051 is shown below with the Mg II absorbing galaxies highlighted. This field has been studied intensively, with spectroscopic confirmation of all objects within 20" of the quasar down to L < 0.05L* in the rest-frame of the objects (C. Steidel, private communication). Note that there are no candidates for the two weakest systems (the z=0.592 system is techinically a weak system with W = 0.29 Ang, but is right on the admittedly arbitrary demarcation line defining them).
Caption: An HST image of the UM 18 (0002+051) quasar field showing the galaxies identified with the Mg II absorbers at z=0.851 and z=0.592 (and a third at z=0.298; spectrum not shown). There are two weak Mg II systems that are apparently not associated with bright galaxies (L > 0.05L*) within 20" of the quasar. Image taken from the HST archive at STScI and spectra taken from Churchill (1997, Ph.D. Thesis) and Churchill et al. (1999, ApJS, 120, 51).
High Ionization Phase and Classes of Mg II Absorbers
How Does C IV Absorption Correlate with Mg II Absorption?
Since the resonant C IV 1548, 1550 doublet is also a clearly detectable feature in quasar spectra, and since it often samples an ionization phase of significantly lower density (or ionization level) than sampled by the Mg II doublet, a direct comparison can be made of these two gas tracers. Unfortunately, for the intermediate redshifts studied here ( z = 0.4 - 1.4 ), high resolution spectra are not available in the ultraviolet, where the C IV transition is observable. Below is shown a gallery of the expected locations in HST/FOS spectra (resolution ~230 km/s; lower sub-panels) corresponding to the Mg II 2796 absorption (resolution ~6 km/s; upper sub-panels). Note that both members of the C IV doublet are shown over a velocity window of 2400 km/s, whereas only the 2796 transition is shown for the Mg II profiles over a velocity window of 480 km/s. Ticks above the Mg II data give the number of Voigt profile components and their velocity centers. A "Bl", "L", or "D" in the C IV panels indicated whether the location of the doublet was blended with another feature, was not detected with no blend (a limit), or was detected, respectively. The systems are order from left to right and top to bottom in order of the increasing "kinematic spread" of the Mg II 2796 profile. The kinematic spread, omega_v, is the second velocity moment of the optical depth across the profile. See Churchill et. al. (1999, ApJ, 519, L43) for further details.
The equivalent widths of the above data are plotted to the left. Open green data are those systems with C IV detections, red data are damped Lyman-alpha absorbers (DLAs), and blue open data are upper limits for the C IV equivalent width.
The most stricking aspect of these diagrams is the apparently very tight correlation between the W(1548) and the Mg II kinematic spread (panel b). Two maximum likelihood linear fits are shown, one forced through the origin. Note that the W(2796) is also correlated, as expected, but not tightly. Also note that W(1548) and W(2796) are not correlated tightly, with the DLAs giving rise to the dramatic spread at the largest W(2796).
- (panel a) The Mg II 2796 rest-frame equivalent width, W(2796), vs. the Mg II kinematic spread, omage_v.
- (panel b) The C IV 1548 rest-frame equivalent width, W(1548), vs. the Mg II kinematic spread.
- (pabel c) W(1548) vs. W(2796).
Most intriguing are the set of data points in panel b that fall well below the correlation line in the velocity region 40-60 km/s (four are limits and one is a "low C IV" DLA). These systems can be thought of as being of "C IV-deficient" in the context of their other properties being similar to systems with stronger C IV absorption. See Churchill et. al. (1999, ApJ, 519, L43) and Churchill et al. (1999a,b ApJ, submitted) for further details.
Implications: These results have been interpreted by Churchill et al. to be suggestive of multiphase ionization conditions (also supported by detailed photoionization modeling, which showed that the C IV and Mg II absorption strengths often cannot be explained as ionization balance in single phase absorption clouds). There also seems to be two types of large W(2796) systems, DLAs and "doubles". The doubles are marked by large kinematic spreads and strong W(1548), whereas the DLAs have intermediate velocity spreads (30-60 km/s) and intermediate W(1548). This may be indicating that DLAs have C IV structures comparable to the "typical" Mg II absorber (i.e they are not unique to the DLA class) and that very strong W(1548) may signify a large kinematic spread in the Mg II gas.
What has Multivariate Analysis Revealed? Mg II Absorber Taxonomy.
Churchill et al. (1999b, ApJ, submitted) analyzed 45 Mg II absorbing systems for which they had measured the far UV transitions in 18 HST FOS spectra (Churchill et al. 1999a, ApJ, submitted). Equivalents widths or detection limits for H I (using Lyman-alpha) and C IV were obtained form almost all systems. These two species represent the two other classes of absorption selected systems (i.e. Lyman alpha forest, Lyman-limit, damped Lyman-alpha, and C IV systems; click here for more details on these classes), and are thus particularly interesting for contrasting these properties in Mg II selected systems. In HIRES/Keck spectra, the Mg II and Fe II equivalent widths and Mg II kinematic spreads are robustly measured (Churchill, 1997, Ph.D. Thesis).
Churchill et al. (1999b, ApJ, submitted) applied a multivariate clustering analysis to 45 system and found five natural groupings, or "classes" of Mg II selected absorbers. They called these classes: (1) DLA/HI-Rich, (2) Double, (3) Classic, (4) CIV-deficient, and (5) Single/Weak. The mean properties of these classes (color coded and labeled) are shown in the top panel of the figure below. The lower panel shows the data for a representative member of each class with the data aligned vertically across panels.
Consider each systems as a data point in a five dimensional space [W(MgII), W(Lya), W(CIV), W(FeII), and omega_v; where the latter is the "kinematic spread"]. A K-means analysis is used to find the minimum number of statistically significant clusters, by minimizing the "distances" between members within a cluster and maximizing the "distances" between the means of the clusters.
Caption: A K-means clustering diagram showing the mean properties of each of the five classes. For the analysis, the data were N(0,1) standardized (means set to zero and normalized by standard deviations).
To the left are the aborption profiles for a represntative member of each class. A brief description of the characrterizing properties, as shown in the K-means diagram and as seen in the profiles, is discussed in the table below.
Caption: Examples of the five taxonomic classes of Mg II selected systems when Mg II, Ly-alpha, C IV, Fe II, and Mg II kinematics are included in the analysis. Ticks above the data give the number and velocities of the Voigt profile components. For C IV, both members of the doublet are shown.
Since the Classic systems have very average, if not "defining" properties for Mg II systems (as classically measured in low resolution spectra), it is convenient to compare the properties of the other four classes in terms of the those of the Classics. Below is a table presenting a crude description and comparison of the five classes yielded by the multivariate clustering analysis. The conclusions presented below also draw upon CLOUDY photoionization modeling [see Churchill et al. (1999, ApJS, 120, 51); Churchill et al. (1999b, ApJ, submitted); Rigby, Charlton, & Churchill (2000, in prep)].
Table of Taxonomic Properties
Class Mg II H I C IV Fe II DLA/HI-Rich Black bottom saturation; velocity spreads similar to Classics- narrow range of 40-60 km/s. Very large W(Lya), some due to damping wings, others on log part of the curve of growth; not all of them have N(HI)>2e20. On average, they are not CIV deficient, but less than Classics; in separate, higher ionization phase which is small compared to Classics (perhaps due to kinematics). Black bottom saturation; W(FeII) and W(MgII) roughly equal and large. Double W(MgII) large, but less than DLAs; large kinematic spread with high level of complexity and range in cloud sizes. On average, W(Lya) larger than for Classics due to kinematic broadening, often arising in higher ionization phase. Very large compared to all other classes; correlated to MgII kinematic spread; arises in separate, higher ionization phase. On average, stronger than for Classics; arises in strongest MgII clouds. Classic Average W(MgII) and kinematics; often have small clouds at large velocities. Average W(Lya); almost always a Lyman limit system Average W(CIV); often arises in separate high ionization phase (not photoionization balance in MgII clouds). Average W(FeII); correlated to W(MgII) with large scatter. C IV - deficient Similar to Classics; not marked by small clouds at large velocity. Similar to Classics; Some are Lyman limit systems, some are not. On average, 2-sigma below mean of Classics; apparently are lacking a kinematically broadened, separate high ionization phase. Similar to Classics. Single/Weak Single, narrow, unresolved clouds. (ASIDE: apparently not associated with normal, bright galaxies.) On average, W(Lya) smaller than Classics and CIV deficient, but wide variation due to presence of separate, higher ionization phase with C IV in some; None are Lyman limit systems. Mean below classics, but show full range in W(CIV), but none as large as for Doubles; presence of CIV and of FeII in same cloud due to separate ionization phases. About half of the population has N(FeII) roughly equal to N(MgII), though small; Signifies low ionization (high density), small (10s of pc!) clouds.
We caution that the classification scheme introduced above should not be taken to suggest that Mg II absorbers group into discretized classes. Discretization is a byproduct of clustering analysis. In fact, the distribution functions of the equivalent widths are characterized by single modes and decreasing tails. The exceptions are the Mg I, Fe II, and Lyman-alpha equivalent widths, which are bimodal due to the DLA/HI-Rich class. As such, any single absorption property, viewed in this univariate fashion, is distributed continuously. However, from the perspective of a multivariate analysis, it is clear that the overall properties of Mg II absorbers group in well defined regions of a "multi-dimensional space". Interpretation of the clustering results ultimately must be tied into the issue of whether there is a systematic connection with the environments in which the various classes arise.
Mg II Kinematics and the C IV - Mg II Absorption Plane
The strong correlation between the C IV equivalent width, W(CIV), and the kinematic spread and complexity of Mg II absorption is illustrated in the figure below, which is the W(CIV) vs. W(MgII) plane, but with the Mg II 2796 profiles (HIRES/Keck) plotted at their respective W(CIV) - W(MgII) locations. Additionally, the large range of W(CIV) for the Single/Weak systems, and the smaller W(CIV) for the DLA/HI-Rich systems can be seen. This figure is to be compared with ( panel c ) of the lower figure in the section entitled "How Does C IV Absorption Correlate with Mg II Kinematics?".
Here we see that W(CIV) is not related to W(MgII) as much as it is related to the kinematic "morphology" of the Mg II absorption. A key ingredient driving the presence of large W(CIV) appears to be the presence of small clouds at large velocities. Note, for example, how the DLA/HI-Rich systems have large W(MgII) but exhibit simple kinematics (back-bottomed saturation and no higher velocity material). If the W(CIV) correlation with Mg II kinematics is relatd to the presence of small W(MgII) clouds at larger velocities, it might be inferred that these "outlier" clouds are highly ionized [this the small W(MgII) and large W(CIV)]. In other words, that the strong C IV is due to ionization balance in clouds that are spread out in velocity. However, when CLOUDY photoionization models of these systems are investigated, the large W(CIV) cannot be accounted for even under the assumption that the gas is as highly ionized as is allowed by the constraints from the low ionization data (chiefly Fe II and Mg II). The equivalent widths of these small clouds are too small to be arising in high ionization gas under the assumption of solar or subsolar metallicities in the clouds. See Churchill et al. (1999, ApJS, 120, 51), Churchill & Charlton (1999, AJ, 118, 59), and Churchill et al. (1999b, ApJ, submitted) for details.
Therefore, it is concluded that the bulk of the C IV gas arises in a higher ionization phase physically distinct from that giving rise to the Mg II absorption. This suggests a physical connection between the presence of these small clouds in the facinity of a galaxy and a high ionization structure, perhaps a corona structure not to unlike the one seen around the Milky Way.
Caption: The "W(CIV) vs. W(MgII) plane" with the Mg II 2796 profile (HIRES/Keck) of each system plotted at its location in this plane. The five taxonomic classes (see above) separate out on this plane and are identified with the color-coded outlining boxes. Adapted from Churchill et al. (1999b, ApJ, submitted). The system encircled in yellow at upper right is a higher redshift system (only z = 0.4 - 1.4 is represented here). Several very large W(MgII) systems at higher redshifts have this profile morphology. Since these systems evolve away from z=2 to z=1, it suggests that there is evolution on the W(CIV) vs. W(MgII) plane and that this is also connected to the Mg II kinematics.
Putting It All Together (under construction)
Toward an Absorption Line Perspective on Galaxy Evolution?
If we are to use quasar absorption lines as a technique for understanding galaxy evolution, we must first accept the premise that an observational database of the gaseous components of galaxies is as effective as one of the stellar components for placing constraints on models, scenarios, and theories of global galaxy evolution. Second, we then need to identify which types of systems to select (e.g. Lyman-limit, DLA, Mg II, or C IV selection, etc.). This decision should be based upon the simple criteria:
- which type of absorption selection will select a complete, uniform, and unbaised sample of galaxies (i.e. covering a wide range of galaxy masses, types, and environments)
- which type will provide direct observational constraints over as wide a redshift range as possible and over the range(s) where evolution is expected to be most pronounced (i.e. kinematics, ionization, and chemical evolution).
Arguably, the Mg II selected systems are ideally suited for a tour de force study because
- those with equivalent widths greater than 0.3 Ang are known to be directly associated with galaxies (Bergeron & Boisse' 1991, A&A, 243, 344; Steidel, Dickinson, & Persson 1994, ApJ, 437, L75; Churchill, Steidel, & Vogt 1996, ApJ, 471, 164) and/or sub-galactic metal-enriched environments (Yanny 1992, PASP, 104, 840; Yanny & York, 1992, ApJ, 391, 569) and since magnesium is an alpha-process element yielded by Type II supernovae, it is expected that the association will hold to the highest redshifts
- they arise in structures having a five decade range of H I column densities, including sub-Lyman limit systems (Churchill et al. 1999 ApJS, 120, 51), Lyman limit systems (e.g. Steidel & Sargent 1992, ApJS, 80, 1), and damped Lyman-alpha systems (e.g. Le Brun et al. 1997 A&A, 321, 733; Rao & Turnshek 1998, ApJ, 500, L115; Boisse' et al. 1998, A&A, 333, 841), which means that a large range of galactic environments will be sampled
- at z < 2.2 their statistical properties and for z < 1.4 their kinematic properties have been thoroughly documented (e.g. Lanzetta, Turnshek, & Wolfe 1987, ApJ, 322, 793; Sargent, Steidel, & Boksenberg 1988, ApJ, 334, 22; Petitjean & Bergeron 1990, A&A, 231, 309; Steidel & Sargent 1992, ApJS, 80, 1; Churchill 1997, Ph.D. Thesis; Churchill et al. 1999, ApJS, 120, 51), which means that the low redshift (z < 2.2) database is already in place and can be used for evolution studies when higher redshift data are obtained
- they are seen to give rise to a range of C IV absorption strengths in UV space-based spectra (Bergeron et al. 1994, ApJ, 436, 33; Churchill et. al. 1999, ApJ, 519, L43; Churchill et al. 1999a, submitted), which means that the more general ionization conditions can be studied in detail (e.g. Churchill & Charlton 1999, AJ, 118, 59; Churchill et al. 1999b, submitted)
Consider the following observational facts over the cosmic epoch probed by the existing Mg II absorber samples:
- from a practical point of view with regard to observational feasibility, the Mg II doublet is the only tall-tale doublet absorption that is always well redward of the Lyman-alpha forest confusion even at the highest redhifts.
- The cosmic star formation rate exhibits a dramatic ~1 dex decrease from z = 1 to z = 0, with a fairly constant, higher rate for z > 2 (e.g. Lilly et al. 1996, ApJ, 460, L1; Connolly et al. 1997, ApJ, 486, L11; Steidel et al. 1999, ApJ, 519, 1).
- For 0 < z < 1, Mg II absorbing galaxies redder than a present-epoch Sb galaxy exhibit no evolution in their number density or luminosity function, whereas the luminosity function of z > 0.5 galaxies bluer than a present-epoch Sbc galaxy evolves strongly ( Guillemin & Bergeron 1997, A&A, 328, 499). This result is consistent with the results of the Canada--France Redshift Survey (CFRS; Lilly et al. 1995, ApJ, 455, 108), which is an I-band selected sample of galaxies.
- Over the redshift range 0.3 < z < 2.2, Steidel & Sargent (1992, ApJS, 80, 1) found that the redshift number density of Mg II absorbers with equivalent widths greater than 0.6 Ang strongly evolves. There are relatively few remaining at z < 1 (the evolution is more dramatic as the equivalent threshold is increased).
These observations are somewhat suggestive of a causal connection between evolution in the cosmic star formation rate and absorbing gas cross sections, kinematics, and ionization conditions in Mg II absorbers. If so, we could hypothesize that scenarios of a cosmic mean history of galaxy evolution inferred from the global record of star formation history should be fully consistent with one based upon an "absorption line perspective". It is unfortunate that there currently are no Mg II absorption-line samples for the highest redshifts (z > 3), so an absorption line perspective cannot yet be fully appreciated (however, see Rauch, Haehnelt, & Steinmetz 1997, ApJ, 481, 601). Interestingly, the global galaxy evolution models, like those of Pei, Fall, & Hauser (1999, ApJ, 522, 604), which are constrained, to a large part, by the cosmic star formation history, may provide insights to what the absorption line data will reveal.
- The mean W(CIV)/W(MgII) ratio is observed to decrease with redshift from z ~ 2 (Bergeron et al. 1994, ApJ, 436, 33). Thus, the global ionization level of Mg II absorbers has decreased with cosmic time.
A Global Model of Galaxy Evolution and Mg II Absorbers
There are several models of global galaxy evolution and each provide a roughly similar scenario for the evolution of galaxies from the highest redshifts (now up to z~4). A nice, heuristic model has been presented by Pei, Fall, & Hauser (1999, ApJ, 522, 604), and we use this model to motivate the next section on understanding galaxy evolution from an absorption line perspective.
As seen in the figure below, Pei et. al. identify three phases to galaxy evolution: a growth period of gaseous infall at z > 3, a working period marked by higher levels of star formation from 1 < z < 3, and a retirement period when star formation diminishes as gas is no longer replenished into the galaxies at z < 1. The percentages below each of the three phases gives the approximate fraction of cosmic time over which the galaxies evolve in each phase. The redshift scale is given along the horizontal axies in the lower panel, which shows the redshift intervals and equivalent width detection limits (sensitivities) over which Mg II absorption surveys have been conducted. Note that the retirement period, from z = 1, comprises over 50% of the epoch of galaxy evolution. It is important to note that the scenario is not well constrained by the data for z > 3. It could be that the growth period terminates at a higher redshift.
Caption: NOT COMPLETE. Our current knowledge base of Mg II absorption stops at z = 2.2 and this is at a relatively low sensitivity level in low resolution spectra. The kinematics and cloud velocity splitting have been observed only from 0.4 < z < 1.4.
Having motivated Mg II absorption selected systems as the best suited for providing a absorption line database for constraining models of galaxy evolution (see above), we can now ruminate on what the absorption line signatures will be in the context of models such as that presented by Pei, Fall, & Hauser (1999, ApJ, 522, 604).
The growth period would be the epoch when galaxies accreted truly intergalactic gas and/or proto-galactic clumps (PGCs) [e.g. Rauch, Haehnelt, & Steinmetz (1997,ApJ, 481, 601)]. Note that this redshift regime and epoch of galaxy evolution remains unexplored in Mg II absorption!
The working period would be the epoch when galaxies have decoupled from the filamentary flow and PGCs were no longer abundant as fundamental building blocks, but galaxies continue to accrete local material at high mass-transfer rates [i.e. bound "satellite" objects (York et al. 1986, ApJ, 311, 610) ergo the supply of material for continued star formation]. This period would then be distinguished by having a large fraction of Mg II absorbers with profiles similar to those presented in yellow shaded region outside the upper right of the W(CIV) vs. W(MgII) plane (shown above). Note that the upper half of this redshift regime and epoch of galaxy evolution remains unexplored in Mg II absorption and is virtually unexplored at high resolution!
The retirement period, which roughly corresponds to the lower range of the intermediate redshift regime studied with this work, would be the epoch when the number of galaxy mergings and satellite accretions have diminished to the point that the cosmic star formation rate diminishes. The complex Mg II aborption profiles with multiple velocity splittings and lower total optical depths would then be a reflection of the reduced gas mass. This would imply that a majority of galaxies transform into self-regulatory systems by z~1 and begin to evolve in a more isolated fashion and in a direction dependent upon their ability to continue forming stars. Galaxies that were rich in gas and were capable of forming large numbers of molecular clouds in their interstellar media (i.e. late-type galaxies) would continue to form stars and exhibit evolution, whereas those less capable would exhibit no discernable evolution. Such a scenario is consistent with the differential luminosity evolution reported by Lilly et al. (1995, ApJ, 455, 108).
Where Do We Go from Here?
Why, we go out and observe Mg II absorption over the full redhsift range of evolution, of course! UNDER CONSTRUCTION...
Caption: NOT COMPLETE. Our current knowledge base of Mg II absorption stops at z = 2.2 and this is at a relatively low sensitivity level in low resolution spectra. The kinematics and cloud velocity splitting have been observed only from 0.4 < z < 1.4. In the future, infrared spectroscopy will allow us to observe Mg II absorption to high sensitivity out to z ~ 4. The years over which the data were obtained (or will be obtained) are given to the right of the survey legends.
This page maintained by Chris Churchill email@example.com: If you have comments or questions please feel free to email me.