Background Information, MgII:

Steidel, C. C. 1992, PASP, 104, 843

• QSOs are found easily at z > 4; this facilitates line-of-sight research of gas in galaxies, "as a function of time", for more than "90% the age of the Universe."
• Studying gas dynamics with QAL is free of bias in the data; the spectra comes from an effective random distribution of sources in the sky. Also bias-free because the observations are not limited by luminosity for detection. Galaxies don't need to be luminous to be detected, they only need to be in the path of the "pencil beam" from the high-z QSO.
• Detectable MgII gas is characterized by being optically thick in the HI Lyman limit. [Lanzetta et al. 1987; Bergeron and Stasinska 1986; Steidel and Sargent 1992]
• Theory (photoionization models) and observation confirm that high HI column density systems are host to both MgII and LLS absorption systems.
• MgII is detectable in ~25% "of all heavy-element systems."
• How do we know that heavy-element systems trace galaxies? Galaxies cluster on velocity scales of ∆v<1000 km s^-1, the same scale as for heavy-element systems. [Sargent et al. 1988]
• Spitzer and Bahcall (1969) first suggested that QSO absorption systems are found in the halos of intervening/intercepted galaxies. This is supported by similar velocity spreads of absorbers with galaxy velocity dispersion.
• These peeps (Bergeron and Boisse 1991; Steidel and Dickinson 1992) positively identified a sample of z<1 (moderate redshift) galaxies to contain MgII absorbing systems. This was a sample of relatively bright (0.1 < L/L^* < 3) galaxies covering a broad range of "spectral type and star-formation rate." (which indicates it was not biased in any way). This lead to the conclusions (in 1992) that for z<1 galaxies contain "an extended gas-rich halo" and "ordinary galaxies" (those that are optically thick LLS) are primarily responsible for QSO absorption signatures.
• C.C.S. was one of the first (NOT TRUE-- LOOK UP W SARGENT 1980 w PETER YOUNG? ABOUT LALPHA-FOREST AND GAMMA-DEPENDENCE) to propose the paramatrization of N(z) ∝ (1 + z) ^γ. ( γ as a power-law index suggesting no evolution in the N(z) for W_r > 0.3 Å ) Evolution depends critically on W_r. Inspection of gradually increased W_r cutoff demonstrates that "systems become weaker with time." W_r becomes smaller as z → 0.
• What physically makes the W_r ↓ as z → 0? this will be important!! Could be that the "number of individual absorbing subcomponents and/or their spread in velocity" decreases as z → 0. The number of intercepted absorbing halo clouds and their respective spread in velocity decreases with increasing chronological time. [cf. York et al 1986; Lanzetta et al 1987]
• In the redshift range z ~ 0.5 to z ~ 2, the filling factor of individual clouds in the halo doubles, while the halo size itself stays constant (WHAT'S THE DIFF??).
• Interesting to note: "How much extreme halo gas is left by the present epoch?"

Churchill, C.W., Mellon, R.R., Charlton, J.C., Jannuzi, B.T., Kirhakos, S., Steidel, C.C., & Schneider, D.P. 2000a, ApJS, 130, 91 [PAPER I]

• "MgII-absorbing galaxies at intermediate redshifts have multiphase gaseous structures."←(Main point from paper)
• The W_r > 0.3 Å bin of MgII absorbers (typically) traces galaxies (halos?). [Bergeron & Boisse 1991; Steidel 1995]
• "Metal-line absorption in quasar spectra arises in extended gaseous envelopes surrounding intervening galaxies." [Bahcall & Spitzer 1969]
• CWC et al first questioned the conditions of low- to high-ionization extended gaseous galaxy halos selected by MgII. What are the relationships (VP fitting parameters) that describe the physical nature of the gas? Further--> How do these relationships change, would they change, why would they change, with time?
• The absorption strengths of a sample of ions with ionization potentials less than ~25eV (associated with ~CII, CaII not included) in a halo cloud correlate with W_r(MgII) at the 3σ level. (97% level) This is shown by Spearman-Kendall nonparametric rank correlation tests. [Isobe, Feigelson, & Nelson 1986; LaValley, Isobe, & Feigelson 1992]
• "Equivalent widths are a better measure of the overall kinematic spread of the gas than column densities" because "most of the low-ionizations transitions are saturated." [e.g., Petitjean & Bergeron 1990, 1994]
• MgII systems, as with LLS, are optically thick at the Lyman limit for neutral hydrogren. This corresponds to average HI column densities of N(HI) ≥ 2 × 10¹⁷ cm^-2. [Bergeron & Stasinska 1986; SS92]
• The strong MgII absorbers (W_r > 0.6 Å) found in HI-rich and nondamped systems have a comoving number density that evolves very closely with "the strongest MgII absorbers." [SS92]
• SiII, III, and IV can be studied in parallel with MgII / can be used to study MgII. SiII and MgII are always found in the same samples as Si and Mg are both α-process elements, and have very similar ionization potentials and "transitions with similar oscillator strengths."
• High-ionization species (mostly CIV and SiIV) have been found in intermediate-redshift MgII-selected galaxies. These ions are common in the Galaxy [e.g., Savage & Sembach 1996; Savage et al. 1997], so this suggests that the MgII-absorbing galaxies could be precursors to locally observed galaxies (including the Galaxy/ our Galaxy) with similar "multiphase interstellar media and halos." [Dahlem 1998; Churchill et al. 1999a]
• "How is the structure of CIV related to that seen in MgII?"
• It would be very useful to quantify the "relative kinematics of the low- and high-ionization gas."

Petitjean, P., & Bergeron, J. 1990, A&A, 231, 309

• The redshift distribution of MgII absorbers is non-evolving for W_min > 0.3 Å.
  
• P&B first pointed to the correlation of the number of subsystems with the equivalent width of an MgII absorber.
• The sample of MgII absorbers is saturated with weak equivalent widths (W_r > 0.6 Å). The sample is dominated by small systems at this bin.
• The lower the W_min distribution cutoff, the lower the probability for evolution in γ.
• All LLS trace MgII absorbers.
• "LLS of low or mixed ionization level with singly ionized elements (CII, MgII, SiII, FeII) are always present at z~2."
• P&B first suggested that a larger W_r from MgII traces a larger column density.
• MgII clouds tend to cluster on a scale of ∆v≤ 200 km s^-1. [Sargent et al., 1988b]
• There is a larger number density of clouds in each system in the disk "than in the spherical case."
• There are "two distinct populations" of clouds traced by MgII (in galaxy halos): "clouds within galaxies and galaxy pairs" (galaxy pairs? wtf?)
• The distribution of absorbing systems vs. velocity separation corresponds to much fewer systems having a large separation.
• The velocity distribution is linearly proportional to the number of subcomponents of an absorber.
• The highest density of clustering of absorbers localizes around the lowest velocity separations.
• At intermediate z, the number of subsystems scales linearly with W_r,tot(Å).
• The N(MgII) & N(HI) distributions are not the same; the cosmological evolutions of the two species differ for W_r(MgII)> 0.6Å. γ -dependence also!
• FeII column densities are typically similar to MgII
• CaII is minorly present in systems where the N(MgII)/N(FeII) ratio is vastly greater than unity.
• The fact that there are fewer weak systems at higher redshifts suggests that z ~ 1.5-2 MgII systems "are almost always multiple."
• "The cross-section of gaseous regions with high density and/or high column density should strongly increase with increasing redshift in the range z= 0-2.5, whereas the overall size of the gaseous halo (that probed by LLS), is non evolving."

Lanzetta, K.M., 1992, PASP, 104, 835

• For z~0.5, MgII absorbers from QSO absorption lines trace galactic "conditions at earlier epochs."
• Spitzer & Bahcall (1969) first suggested that MgII (heavy element?) absorption in QSO spectra originates from "extended gaseous halos" of normal galaxies.
• The size of absorbing halos is ~ 50 h^-1 kpc.
• The covering factor of the halos of intervening galaxies is ~ unity.
• "Galactic fountain" model for halo-enrichment.
• "The envisioned halos are not static remnants of the initial collapse of galaxies but rather are dynamic entities that must be continually replenished and refueled, perhaps by 'galactic fountain' mechanisms or by accretion o intergalactic material. Under the halo model, the epoch of formation must be 'early' in the sense that more or less normal galaxies must exist with extended gaseous halos at redshifts of at least z≳1."
• QSO absorption lines are [nearly] unbiased in their selection because "one line of sight through many absorbers is equivalent to many lines of sight through one absorber," hence this [marginally] reduces difficulty with observational limitations [of galaxy halos].

Steidel, C.C., Kollmeier, J.A., Shapley, A.E., Churchill, C.W., Dickinson, M., & Pettini, M. 2002, ApJ, 570, 526

• Intermediate-redshift spiral galaxies commonly have a rotating gaseous halo whose kinematic rotation is much stronger than radial infall or outflow, "even for gas well away from the galactic plane."

SS92

• The mean number density of MgII absorbers per unit redshift range decreases

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