Nicole Vogt » High Redshift Spiral Galaxy Morphologies
The figure shown below displays a range of data for four high redshift disk galaxies. The first row contains an HST+WFPC2 I814 image for each galaxy. The horizontal bars show the (one arcsecond) width of the slit which was placed across the major axis of each galaxy for spectroscopy. Each plot is anotated with the observed I184 magnitude (top left), the observed V606 - I814 color (top right), the absolute blue magnitude, M_B (lower right), and the redshift (below image).
The second row contains a Keck+LRIS image of the [OII]3727 doublet (the two lines are separated by 2.7 angstroms in the rest-frame and at high redshifts the observed separation becomes even greater, so we have no difficulty in resolving the doublet within our spectra. The x axis shows the distribution of flux across the major axis of the galaxy (the physical scale is the same as in the I814 image above), and the y-axis shows the distribution in wavelength or velocity space. The circular velocity is noted below each image, in parentheses where the value is complicated by unusual factors.
The third row shows a fit to the optical spectrum, with the observed size along the x axis and the distribution in velocity along the y-axis. Blue points denote the strong [OII] flux, and for the lowest redshift case the [OIII]5007 line is shown in red as well. The black line indicates a fit to the velocity profile of each galaxy. The fitting process is fairly complex, and takes into the account the similarity in size scale of the galaxy, the spectroscopic slit, and the seeing disk at the time of the observations.
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The first galaxy, reading from left to right, is a large, grand design spiral. The [OII] flux can be traced well across the disk, terminated early on the left side by a slit which was shortened to allow the simultaeous observation of a close neighboring object on the sky. This galaxy is representative of the bulk of our sample of high redshift galaxies, with luminosity, size, morphology and mass analogous to those found in local spirals today.
The remaining three galaxies have been chosen because they display unusual characteristics. The second galaxy, at a similar redshift to the first, has an I morphology with the appearance of individual "pearls on a string" rather than an inclined disk, and the flux is not well fit by an exponential disk. The [OII] spectrum is best fit by assuming that there are three separate objects along the slit, each with a velocity profile about its own center, rather than as a single potential. This galaxy is offset from the local Tully-Fisher relation (see previous figure for details).
The third galaxy, at redshift z = 0.9, is a high surface brightness outlier from the Tully-Fisher relation. The observed velocity profile is smooth, and fit well by a standard set of models (characterizing the intrinsic velocity profile with a steep inner region, then leveling off to a near-constant terminal velocity beyond the elbow point of the curve). The disk appears exponential in I, though the I morphology is slightly clumpy.
The fourth galaxy has a very distorted appearance. The main portion of the object is curved in an arclike structure, with a small jutting extension to the right which, though faint, appears in both V and I and is traced in the [OII] spectrum. The relative strength of the two [OII] emission lines in the central region is extremely high, indicative of a region of very high density. The velocity profile indicates a circular velocity of roughly 80 km/s, assuming that the galaxy is extended straight along the major axis. When the arclike curve is taken into account, the profile can be fit well with a circular velocity of a mere 25 km/s.