839 research outputs found
On the Reported Death of the MACHO Era
We present radial velocity measurements of four wide halo binary candidates
from the sample in Chaname & Gould (2004; CG04) which, to date, is the only
sample containing a large number of such candidates. The four candidates that
we have observed have projected separations >0.1 pc, and include the two widest
binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that
three of the four CG04 candidates are genuine, including the one with the
largest separation. The fourth candidate, however, is spurious at the 5-sigma
level. In the light of these measurements we re-examine the implications for
MACHO models of the Galactic halo. Our analysis casts doubt on what MACHO
constraints can be drawn from the existing sample of wide halo binaries.Comment: 6 Pages, 4 Figures, Accepted for MNRAS Letter
M/L and Color Evolution for A Deep Sample of M* Cluster Galaxies at z~1: The Formation Epoch and the Tilt of the Fundamental Plane
We have measured velocity dispersions for a sample of 36 galaxies with J <
21.2 or Mr < -20.6 mag in MS1054-03, a massive cluster of galaxies at z = 0.83.
Our data are of uniformly high quality down to our selection limit, our 16-hour
exposures typically yielding errors of only \delta(dispersion)~10% for L* and
fainter galaxies. By combining our measurements with data from the literature,
we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived
sizes, colors and surface brightnesses. This sample is complete for the typical
L* galaxy at z~1, unlike most previous z~1 cluster samples which are complete
only for the massive cluster members (>1e11 M_sun). We find no evidence for a
change in the tilt of the fundamental plane (FP). Nor do we find evidence for
evolution in the slope of the color-dispersion relation and M/L_B-dispersion
relations; measuring evolution at a fixed dispersion should minimize the impact
of size evolution found in other work. The M/L_B at fixed dispersion evolves by
\Delta log10 M/L_B=-0.50 +/- 0.03 between z=0.83 and z=0.02 or d(log10
M/L_B)=-0.60 +/- 0.04 dz, and we find \Delta (U-V)_z=-0.24 +/- 0.02 mag at
fixed dispersion in the rest-frame, matching the expected evolution in M/L_B
within 2.25 standard deviations. The implied formation redshift from both the
color and M/L_B evolution is z*=2.0 +/- 0.2 +/- 0.3 (sys), during the epoch in
which the cosmic star-formation activity peaked, with the systematic
uncertainty showing the dependence of z* on the assumptions we make about the
stellar populations. The lack of evolution in either the tilt of the FP or in
the M/L- and color-dispersion relations imply that the formation epoch depends
weakly on mass, ranging from z*=2.3 +1.3 -0.3 at 300 km/s to z*=1.7 +0.3 -0.2
at 160 km/s and implies that the IMF similarly varies slowly with galaxy mass.Comment: revised; typos corrected, references updated, and other cosmetic
changes to meet ApJ format ApJ accepted, 22 pages in emulate ApJ format, 8
color figures, 1 b/w figur
Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)
We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83)
confirming that at least six of the nine merging galaxy pairs identified by van
Dokkum et al. (1999) are indeed bound systems: they have projected separations
of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the
remaining three pairs, we were unable to obtain redshifts of both constituent
galaxies. To identify a more objective sample of merging systems, we select
bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121
confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of
the cluster population. The (B-K_s) color-magnitude diagram shows that the pair
galaxies are as red as the E/S0 members and have a homogeneous stellar
population. The red pair galaxies span a large range in luminosity and internal
velocity dispersion to include some of the brightest, most massive members
(L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members
by z~0.7. These results combined with MS1054's high merger fraction and
reservoir of likely future mergers indicates that most, if not all, of its
early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.Comment: accepted by ApJ Letters; high resolution version of Fig. 2 available
at http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms1054mgrs.ps.g
The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) I: The Sample and the Spectra
We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies
Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises
medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright
gravitationally lensed galaxies at redshifts of 1.68z3.6, obtained with
the MagE spectrograph on the Magellan telescopes. The spectra cover the
observed-frame wavelength range \AA ; the average
spectral resolving power is R=3300. The median spectrum has a signal-to-noise
ratio of per resolution element at 5000 \AA . As such, the MEGaSaURA
spectra have superior signal-to-noise-ratio and wavelength coverage compared to
what COS/HST provides for starburst galaxies in the local universe. This paper
describes the sample, the observations, and the data reduction. We compare the
measured redshifts for the stars, the ionized gas as traced by nebular lines,
and the neutral gas as traced by absorption lines; we find the expected bulk
outflow of the neutral gas, and no systemic offset between the redshifts
measured from nebular lines and the redshifts measured from the stellar
continuum. We provide the MEGaSaURA spectra to the astronomical community
through a data release.Comment: Resubmitted to AAS Journals. Data release will accompany journal
publication. v2 addresses minor comments from refere
Weak homology of elliptical galaxies
We start by studying a small set of objects characterized by photometric
profiles that have been pointed out to deviate significantly from the standard
R^{1/4} law. For these objects we confirm that a generic R^{1/n} law, with n a
free parameter, can provide superior fits (the best-fit value of n can be lower
than 2.5 or higher than 10), better than those that can be obtained by a pure
R^{1/4} law, by an R^{1/4}+exponential model, and by other dynamically
justified self--consistent models. Therefore, strictly speaking, elliptical
galaxies should not be considered homologous dynamical systems. Still, a case
for "weak homology", useful for the interpretation of the Fundamental Plane of
elliptical galaxies, could be made if the best-fit parameter n, as often
reported, correlates with galaxy luminosity L, provided the underlying
dynamical structure also follows a systematic trend with luminosity. We
demonstrate that this statement may be true even in the presence of significant
scatter in the correlation n(L). Preliminary indications provided by a set of
"data points" associated with a sample of 14 galaxies suggest that neither the
strict homology nor the constant stellar mass--to--light solution are a
satisfactory explanation of the observed Fundamental Plane (abridged).Comment: 34 pages, 11 figures, accepted by Astronomy and Astrophysic
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