1,439 research outputs found
Dynamical Models of Elliptical Galaxies in z=0.5 Clusters: I. Data-Model Comparison and Evolution of Galaxy Rotation
We present spatially resolved stellar rotation velocity and velocity
dispersion profiles form Keck/LRIS absorption-line spectra for 25 galaxies,
mostly visually classified ellipticals, in three clusters at z=0.5. We
interpret the kinematical data and HST photometry using oblate axisymmetric
two-integral f(E,Lz) dynamical models based on the Jeans equations. This yields
good fits, provided that the seeing and observational characteristics are
carefully modeled. The fits yield for each galaxy the dynamical M/L and a
measure of the galaxy rotation rate. Paper II addresses the implied M/L
evolution. Here we study the rotation-rate evolution by comparison to a sample
of local elliptical galaxies of similar present-day luminosity. The brightest
galaxies in the sample all rotate too slowly to account for their flattening,
as is also observed at z=0. But the average rotation rate is higher at z=0.5
than locally. This may be due to a higher fraction of misclassified S0 galaxies
(although this effect is insufficient to explain the observed strong evolution
of the cluster S0 fraction with redshift). Alternatively, dry mergers between
early-type galaxies may have decreased the average rotation rate over time. It
is unclear whether such mergers are numerous enough in clusters to explain the
observed trend quantitatively. Disk-disk mergers may affect the comparison
through the so-called progenitor bias, but this cannot explain the direction of
the observed rotation-rate evolution. Additional samples are needed to
constrain possible environmental dependencies and cosmic variance in galaxy
rotation rates. Either way, studies of the internal stellar dynamics of distant
galaxies provide a valuable new approach for exploring galaxy evolution.Comment: ApJ, submitted; 17 pages formatted with emulateap
Dynamical Models of Elliptical Galaxies in z=0.5 Clusters: II. Mass-to-Light Ratio Evolution without Fundamental Plane Assumptions
We study M/L evolution of early-type galaxies using dynamical modeling of
resolved internal kinematics. This makes fewer assumptions than Fundamental
Plane (FP) studies and provides a powerful new approach for studying galaxy
evolution. We focus on the sample of 25 galaxies in clusters at z=0.5 modeled
in Paper I. For comparison we compile and homogenize M/L literature data for 60
nearby galaxies that were modeled in comparable detail. The nearby sample obeys
log(M/L)_B = Z + S log(sigma_eff/[200 km/s]), with Z = 0.896 +/- 0.010, S =
0.992 +/- 0.054, and sigma_eff the effective velocity dispersion. The z=0.5
sample follows a similar relation but with lower zeropoint. The implied M/L
evolution is Delta log(M/L) / Delta z = -0.457 +/- 0.046(random) +/-
0.078(systematic), consistent with passive evolution following high-redshift
formation. This agrees with the FP results for this sample by van Dokkum & van
der Marel. This confirms that FP evolution tracks M/L evolution, which is an
important verification of the assumptions that underly FP studies. However,
while we find more FP evolution for galaxies of low sigma_eff (or low mass),
the dynamical M/L evolution instead shows little trend with sigma_eff. We argue
that this difference can be plausibly attributed to a combination of two
effects: (a) evolution in structural galaxy properties other than M/L; and (b)
the neglect of rotational support in studies of FP evolution. The results leave
the question open whether the low-mass galaxies in the sample have younger
population ages than the high-mass galaxies. This highlights the general
importance in the study of population ages for complementing dynamical
measurements with broad-band colors or spectroscopic population diagnostics.Comment: ApJ, submitted; 17 pages formatted with emulateap
Keck Spectroscopy of Dwarf Elliptical Galaxies in the Virgo Cluster
Keck spectroscopy is presented for four dwarf elliptical galaxies in the
Virgo Cluster. At this distance, the mean velocity and velocity dispersion are
well resolved as a function of radius between 100 to 1000 pc, allowing a clear
separation between nuclear and surrounding galaxy light. We find a variety of
dispersion profiles for the inner regions of these objects, and show that none
of these galaxies is rotationally flattened.Comment: 4 pages, 2 figures, to appear in the proceedings of the Yale
Cosmology Workshop "The Shapes of Galaxies and their Halos", (ed. P.
Natarjan
Spatially Resolved Stellar Kinematics of Field Early-Type Galaxies at z=1: Evolution of the Rotation Rate
We use the spatial information of our previously published VLT/FORS2
absorption line spectroscopy to measure mean stellar velocity and velocity
dispersion profiles of 25 field early-type galaxies at a median redshift z=0.97
(full range 0.6<z<1.2). This provides the first detailed study of early-type
galaxy rotation at these redshifts. From surface brightness profiles from HST
imaging we calculate two-integral oblate axisymmetric Jeans equation models for
the observed kinematics. Fits to the data yield for each galaxy the degree of
rotational support and the mass-to-light ratio M/L_Jeans. S0 and Sa galaxies
are generally rotationally supported, whereas elliptical galaxies rotate less
rapidly or not at all. Down to M(B)=-19.5 (corrected for luminosity evolution),
we find no evidence for evolution in the fraction of rotating early-type (E+S0)
galaxies between z=1 (63+/-11%) and the present (61+/-5%). We interpret this as
evidence for little or no change in the field S0 fraction with redshift. We
compare M/L_Jeans with M/L_vir inferred from the virial theorem and globally
averaged quantities and assuming homologous evolution. There is good agreement
for non-rotating (mostly E) galaxies. However, for rotationally supported
galaxies (mostly S0) M/L_Jeans is on average ~40% higher than M/L_vir. We
discuss possible explanations and the implications for the evolution of M/L
between z=1 and the present and its dependence on mass.Comment: To appear in ApJ 683 (9 pages, 7 figures). Minor changes included to
match published versio
Internal Dynamics, Structure and Formation of Dwarf Elliptical Galaxies: II. Rotating Versus Non-Rotating Dwarfs
We present spatially-resolved internal kinematics and stellar chemical
abundances for a sample of dwarf elliptical (dE) galaxies in the Virgo Cluster
observed with Keck/ESI. We find that 4 out of 17 dEs have major axis rotation
velocities consistent with rotational flattening, while the remaining dEs have
no detectable major axis rotation. Despite this difference in internal
kinematics, rotating and non-rotating dEs are remarkably similar in terms of
their position in the Fundamental Plane, morphological structure, stellar
populations, and local environment. We present evidence for faint underlying
disks and/or weak substructure in a fraction of both rotating and non-rotating
dEs, but a comparable number of counter-examples exist for both types which
show no evidence of such structure. Absorption-line strengths were determined
based on the Lick/IDS system (Hbeta, Mgb, Fe5270, Fe5335) for the central
region of each galaxy. We find no difference in the line-strength indices, and
hence stellar populations, between rotating and non-rotating dE galaxies. The
best-fitting mean age and metallicity for our 17 dE sample are 5 Gyr and Fe/H =
-0.3 dex, respectively, with rms spreads of 3 Gyr and 0.1 dex. The majority of
dEs are consistent with solar alpha/Fe abundance ratios. By contrast, the
stellar populations of classical elliptical galaxies are, on average, older,
more metal rich, and alpha-enhanced relative to our dE sample. The local
environments of both dEs types appear to be diverse in terms of their proximity
to larger galaxies in real or velocity space within the Virgo Cluster. Thus,
rotating and non-rotating dEs are remarkably similar in terms of their
structure, stellar content, and local environments, presenting a significant
challenge to theoretical models of their formation. (abridged)Comment: 33 pages, 12 figures. To appear in the October 2003 Astronomical
Journal. See http://www.ucolick.org/~mgeha/geha_dE.ps.gz for version with
high resolution figure
An Intermediate-Mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations
We present dynamical models for the massive globular cluster G1. The goal is
to measure or place a significant upper limit on the mass of any central black
hole. Whether or not globular clusters contain central massive black holes has
important consequences for a variety of studies. We use new kinematic data
obtained with Keck and new photometry from the Hubble Space Telescope. The Keck
spectra allow us to obtain kinematics out to large radii that are required to
pin down the mass-to-light ratio of the dynamical model and the orbital
structure. The Hubble Space Telescope observations give us a factor of two
better spatial resolution for the surface brightness profile. By fitting
non-parametric, spherical, isotropic models we find a best-fit black hole mass
of 1.7(+-0.3)e4 Msun. Fully general axisymmetric orbit-based models give
similar results, with a black hole mass of 1.8(+-0.5)e4 Msun. The no-black hole
model has Delta_chi^2=5 (marginalized over mass-to-light ratio), implying less
than 3% significance. We have taken into account any change in the
mass-to-light ratio in the center due to stellar remnants. These results are
consistent with our previous estimate in Gebhardt, Rich & Ho (2002), and
inconsistent with the analysis of Baumgardt et al. (2003) who claim that G1
does not show evidence for a black hole. These new results make G1 the best
example of a cluster that contains an intermediate-mass black hole.Comment: accepted for publication in the Astrophysical Journa
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