Spectral Classification of Galaxies Along the Hubble Sequence

We develop a straightforward and quantitative two-step method for spectroscopically classifying galaxies from the low signal-to-noise (S/N) optical spectra typical of galaxy redshift surveys. First, using \chi^2-fitting of characteristic templates to the object spectrum, we determine the relative contributions of the old stellar component, the young stellar component, and various emission line spectra. Then, we classify the galaxy by comparing the relative strengths of the components with those of galaxies of known morphological type. In particular, we use the ratios of (1) the emission line to absorption line contribution, (2) the young to old stellar contribution, and (3) the oxygen to hydrogen emission line contribution. We calibrate and test the method using published morphological types for 32 galaxies from the long-slit spectroscopic survey of Kennicutt (1992) and for 304 galaxies from a fiber spectroscopic survey of nearby galaxy clusters. From an analysis of a sample of long-slit spectra of spiral galaxies in two galaxy clusters, we conclude that the majority of the galaxies observed in the fiber survey are sufficiently distant that their spectral classification is unaffected by aperture bias. Our spectral classification is consistent with the morphological classification to within one type (e.g. E to S0 or Sa to Sb) for \gtsim 80% of the galaxies. Disagreements between the spectral and morphological classifications of the remaining galaxies reflect a divergence in the correspondence between spectral and morphological types, rather than a problem with the data or method.Comment: 13 pages, uuencoded gzip'ed ps-file that includes 8 of 9 Figures, accepted for publication in A

Kinematics of Tidal Debris from Omega Centauri's Progenitor Galaxy

We present the kinematic properties of a tidally disrupted dwarf galaxy in the Milky Way, based on the hypothesis that its central part once contained the most massive Galactic globular cluster, omega Cen. Dynamical evolution of a self-gravitating progenitor galaxy that follows the present-day and likely past orbits of omega Cen is calculated numerically and the kinematic nature of their tidal debris is analyzed, combined with randomly generated stars comprising spheroidal halo and flat disk components. We show that the retrograde rotation of the debris stars at $\sim -100$ km/s accords with a recently discovered, large radial velocity stream at $\sim 300$ km/s towards the Galactic longitude of $\sim 270^\circ$. These stars also contribute, only in part, to a reported retrograde motion of the outer halo at the North Galactic Pole. The prospects for future debris searches and the implications for the early evolution of the Galaxy are briefly presented.Comment: 14 pages, 3 figures, accepted for publication in ApJ Letter

The Caustic Ring Model of the Milky Way Halo

We present a proposal for the full phase space distribution of the Milky Way halo. The model is axially and reflection symmetric and its time evolution is self-similar. It describes the halo as a set of discrete dark matter flows with stated densities and velocity vectors everywhere. We first discuss the general conditions under which the time evolution of a cold collisionless self-gravitating fluid is self-similar, and show that symmetry is not necessary for self-similarity. When spherical symmetry is imposed, the model is the same as described by Fillmore and Goldreich, and by Bertschinger, twenty-three years ago. The spherically symmetric model depends on one dimensionless parameter $\epsilon$ and two dimensionful parameters. We set $\epsilon$ = 0.3, a value consistent with the slope of the power spectrum of density perturbations on galactic scales. The dimensionful parameters are determined by the Galactic rotation velocity (220 km/s) at the position of the Sun and by the age of the Galaxy (13.7 Gyr). The properties of the outer caustics are derived in the spherically symmetric model. The structure of the inner halo depends on the angular momentum distribution of the dark matter particles. We assume that distribution to be axial and reflection symmetric, and dominated by net overall rotation. The inner caustics are rings whose radii are determined in terms of a single additional parameter $j_{\rm max}$. We summarize the observational evidence in support of the model. The evidence is consistent with $j_{\rm max}$ = 0.18 in Concordance Cosmology, equivalent to $j_{\rm max,old}$ = 0.26 in Einstein - de Sitter cosmology. We give formulas to estimate the flow densities and velocity vectors anywhere in the Milky Way halo. The properties of the first forty flows at the location of the Earth are listed.Comment: 35 pages, 6 figure

The Environment of ``E+A'' Galaxies

The violent star formation history of ``E+A'' galaxies and their detection almost exclusively in distant clusters is frequently used to link them to the ``Butcher-Oemler effect'' and to argue that cluster environment influences galaxy evolution. From 11113 spectra in the Las Campanas Redshift Survey, we have obtained a unique sample of 21 nearby ``E+A" galaxies. Surprisingly, a large fraction (about 75%) of these ``E+A''s lie in the field. Therefore, interactions with the cluster environment, in the form of the ICM or cluster potential, are not essential for ``E+A'' formation. If one mechanism is responsible for ``E+A''s, their existence in the field and the tidal features in at least 5 of the 21 argue that galaxy-galaxy interactions and mergers are that mechanism. The most likely environments for such interactions are poor groups, which have lower velocity dispersions than clusters and higher galaxy densities than the field. In hierarchical models, groups fall into clusters in greater numbers at intermediate redshifts than they do today. Thus, the Butcher-Oemler effect may reflect the typical evolution of galaxies in groups and in the field rather than the influence of clusters on star formation in galaxies. This abstract is abridged.Comment: 39 uuencoded, compressed pages (except Fig 1), complete preprint at ftp://ociw.edu/pub/aiz/eplusa.ps, ApJ, submitte

The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies

We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17 edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which must be emitted at these flux levels by any accumulation of hydrogen due to the presence of the extragalactic UV background and any other, local source of UV flux. We present results from the individual galaxy spectra and a stacked composite. We detect H-alpha in many cases well beyond R_25 and sometimes as far as 2 R_25. The combination of sensitivity, spatial resolution, and kinematic resolution of this technique thus provides a powerful complement to 21-cm observations. Kinematics in the outer disk are generally disk-like (flat rotation curves, small velocity dispersions) at all radii, and there is no evidence for a change in the velocity dispersion with radius. We place strong limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25. These results suggest that thin disks extend well beyond R_25; however, we also find a few puzzling anomalies. In ESO 323-G033 we find two emission regions that have velocities close to the systemic velocity rather than the expected rotation velocity. These low relative velocities are unlikely to be simply due to projection effects and so suggest that these regions are not on disk-plane, circular orbits. In MCG-01-31-002 we find emission from gas with a large velocity dispersion that is co-rotating with the inner disk.Comment: 18 pages, 14 figures, accepted for publication in Ap