1,824 research outputs found
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
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
Long range Coulomb forces and the behaviour of the chemical potential of electrons in metals at a second order phase transition
We give a general thermodynamic analyzis of the behaviour of the chemical
potential of electrons in metals at a second order phase transition, including
in our analysis the effect of long range Coulomb forces. It is shown, that this
chemical potential can have a kink at T, both for fixed sample volume and
fixed external pressure. The Coulomb term transfers the changes in chemical
potential of the electrons into an experimentally observable shift of the
surface potential if the sample is electrically connected to a ground
potential. VSGD.93.9.th1Comment: 6 pages, no figures. Revtex, version 2, Materials Science Center
Internal Report Number VSGD.93.9.th
The counterrotating core and the black hole mass of IC1459
The E3 giant elliptical galaxy IC1459 is the prototypical galaxy with a fast
counterrotating stellar core. We obtained one HST/STIS long-slit spectrum along
the major axis of this galaxy and CTIO spectra along five position angles. We
present self-consistent three-integral axisymmetric models of the stellar
kinematics, obtained with Schwarzschild's numerical orbit superposition method.
We study the dynamics of the kinematically decoupled core (KDC) in IC1459 and
we find it consists of stars that are well-separated from the rest of the
galaxy in phase space. The stars in the KDC counterrotate in a disk on orbits
that are close to circular. We estimate that the KDC mass is ~0.5% of the total
galaxy mass or ~3*10^9 Msun. We estimate the central black hole mass M_BH of
IC1459 independently from both its stellar and its gaseous kinematics. Some
complications probably explain why we find rather discrepant BH masses with the
different methods. The stellar kinematics suggest that M_BH = (2.6 +/-
1.1)*10^9 Msun (3 sigma error). The gas kinematics suggests that M_BH ~
3.5*10^8 Msun if the gas is assumed to rotate at the circular velocity in a
thin disk. If the observed velocity dispersion of the gas is assumed to be
gravitational, then M_BH could be as high as ~1.0*10^9 Msun. These different
estimates bracket the value M_BH = (1.1 +/- 0.3)*10^9 Msun predicted by the
M_BH-sigma relation. It will be an important goal for future studies to assess
the reliability of black hole mass determinations with either technique. This
is essential if one wants to interpret the correlation between the BH mass and
other global galaxy parameters (e.g. velocity dispersion) and in particular the
scatter in these correlations (believed to be only ~0.3 dex). [Abridged]Comment: 51 pages, LaTeX with 19 PostScript figures. Revised version, with
three new figures and data tables. To appear in The Astrophysical Journal,
578, 2002 October 2
Black Hole Masses and Host Galaxy Evolution of Radio-loud Active Galactic Nuclei
We report stellar velocity dispersion measurements for a sample of 28 AGN
host galaxies including our previous work. Using the mass-dispersion
() and the fundamental plane relations, we estimate the
black hole mass for a sample of 66 BL Lac objects and investigate the role of
black hole mass in the energetics of BL Lac objects. The black hole mass range
for different BL Lac spectral types is similar, . Neither X-ray nor radio luminosity correlates with black hole
mass. Low-frequency-peaked BL Lac objects have higher Eddington ratios on
average, because of either more beaming or higher intrinsic power. For the
black hole mass range , the radio
luminosity of BL Lac objects and flat-spectrum radio quasars spans over 4
orders of magnitude, with BL Lac objects being low-power AGNs. We also
investigate the evolution of host galaxies for 39 AGNs out to
with measuredstellar velocity dispersions. Comparing the mass-to-light ratio
evolution in the observed frame with population synthesis models, we find that
single burst star formation models with are
consistent with the observations. From our model, we estimated
the intrinsic mass-to-light ratio evolution in the Cousins band, , consistent with that of normal early
type galaxies.Comment: ApJ accepted, 22 pages, 11 figure
LMC Self-lensing from a new perspective
We present a new analysis on the issue of the location of the observed
microlensing events in direction of the Large Magellanic Cloud (LMC). This is
carried out starting from a recently drawn coherent picture of the geometrical
structure and dynamics of the LMC disk and by considering different
configurations for the LMC bar. In this framework it clearly emerges that the
spatial distribution of the events observed so far shows a near--far asymmetry.
This turns out to be compatible with the optical depth calculated for the LMC
halo objects. In this perspective, our main conclusion, supported by a
statistical analysis on the outcome of an evaluation of the microlensing rate,
is that self lensing can not account for all the observed events. Finally we
propose a general inequality to calculate quickly an upper limit to the optical
depth along a line of view through the LMC center.Comment: revised version (minor changes) Accepted for publication in A&
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
First-principles study of thin magnetic transition-metal silicide films on Si(001)
In order to combine silicon technology with the functionality of magnetic
systems, a number of ferromagnetic (FM) materials have been suggested for the
fabrication of metal/semiconductor heterojunctions. In this work, we present a
systematic study of several candidate materials in contact with the Si surface.
We employ density-functional theory calculations to address the thermodynamic
stability and magnetism of both pseudomorphic CsCl-like Si (=Mn, Fe, Co,
Ni) thin films and Heusler alloy MnSi (=Fe, Co, Ni) films on Si(001).
Our calculations show that Si-termination of the Si films is energetically
preferable during epitaxy since it minimizes the energetic cost of broken bonds
at the surface. Moreover, we can explain the calculated trends in thermodynamic
stability of the Si thin films in terms of the -Si bond-strength and the
3d orbital occupation. From our calculations, we predict that ultrathin
MnSi films are FM with sizable spin magnetic moments at the Mn atoms, while
FeSi and NiSi films are nonmagnetic. However, CoSi films display itinerant
ferromagnetism. For the MnSi films with Heusler-type structure, the MnSi
termination is found to have the highest thermodynamic stability. In the FM
ground state, the calculated strength of the effective coupling between the
magnetic moments of Mn atoms within the same layer approximately scales with
the measured Curie temperatures of the bulk MnSi compounds. In particular,
the CoMnSi/Si(001) thin film has a robust FM ground state as in the bulk,
and is found to be stable against a phase separation into CoSi/Si(001) and
MnSi/Si(001) films. Hence this material is of possible use in FM-Si
heterojunctions and deserves further experimental investigations.Comment: 13 pages, 8 figure
Chandra Detection of Massive Black Holes in Galactic Cooling Flows
Anticipating forthcoming observations with the Chandra X-ray telescope, we
describe the continuation of interstellar cooling flows deep into the cores of
elliptical galaxies. Interstellar gas within about r = 50 parsecs from the
massive black hole is heated to T > 1 keV and should be visible unless thermal
heating is diluted by non-thermal pressure. Since our flows are subsonic near
the massive black holes, distributed cooling continues within 300 pc from the
center. Dark, low mass stars formed in this region may be responsible for some
of the mass attributed to central black holes.Comment: 6 pages with 3 figures; accepted by Astrophysical Journal Letter
The M(BH)-Sigma Relation for Supermassive Black Holes
We investigate the differences in the M(BH)-sigma relation derived recently
by Ferrarese & Merritt (2000) and Gebhardt et al. (2000). The shallower slope
found by the latter authors (3.75 vs. 4.8) is due partly to the use of a
regression algorithm that ignores measurement errors, and partly to the value
of the velocity dispersion adopted for a single galaxy, the Milky Way. A
steeper relation is shown to provide a better fit to black hole masses derived
from reverberation mapping studies. Combining the stellar dynamical, gas
dynamical, and reverberation mapping mass estimates, we derive a best-fit
relation M(BH) = 1.30 (+/- 0.36) X 10^8 (sigma_c/200)^{4.72(+/- 0.36)}, where
M(BH) is in solar masses, and sigma in km/s.Comment: The Astrophysical Journal, in pres
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