1,145 research outputs found
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 km/s accords with a recently discovered,
large radial velocity stream at km/s towards the Galactic longitude
of . 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
and two dimensionful parameters. We set = 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 . We summarize the observational
evidence in support of the model. The evidence is consistent with
= 0.18 in Concordance Cosmology, equivalent to = 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
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