895 research outputs found
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
The Evolution of Rest-Frame K-band Properties of Early-Type Galaxies from z=1 to the Present
We measure the evolution of the rest-frame K-band Fundamental Plane from z=1
to the present by using IRAC imaging of a sample of early-type galaxies in the
Chandra Deep Field-South at z~1 with accurately measured dynamical masses. We
find that evolves as , which is
slower than in the B-band (). In the B-band
the evolution has been demonstrated to be strongly mass dependent. In the
K-band we find a weaker trend: galaxies more massive than
evolve as ;
less massive galaxies evolve as . As
expected from stellar population models the evolution in is slower than
the evolution in . However, when we make a quantitative comparison, we
find that the single burst Bruzual-Charlot models do not fit the results well,
unless large dust opacities are allowed at z=1. Models with a flat IMF fit
better, Maraston models with a different treatment of AGB stars fit best. These
results show that the interpretation of rest-frame near-IR photometry is
severely hampered by model uncertainties and therefore that the determination
of galaxy masses from rest-frame near-IR photometry may be harder than was
thought before.Comment: 5 pages, 3 figures, Accepted for publication in ApJ
The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
We investigate the stellar structure of massive, quiescent galaxies at z~2,
based on Hubble Space Telescope/WFC3 imaging from the Early Release Science
program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol
and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work,
their half-light radii are <2 kpc, much smaller than equally massive galaxies
in the present-day universe. A significant subset of the sample appears highly
flattened in projection, which implies, considering viewing angle statistics,
that a significant fraction of the galaxies in our sample have pronounced
disks. This is corroborated by two-dimensional surface brightness profile fits.
We estimate that 65% +/- 15% of the population of massive, quiescent z~2
galaxies are disk-dominated. The median disk scale length is 1.5 kpc,
substantially smaller than the disks of equally massive galaxies in the
present-day universe. Our results provide strong observational evidence that
the much-discussed ultra-dense high-redshift galaxies should generally be
thought of as disk-like stellar systems with the majority of stars formed from
gas that had time to settle into a disk.Comment: published versio
Star Formation Histories in a Cluster Environment at z~0.84
We present a spectrophotometric analysis of galaxies belonging to the
dynamically young, massive cluster RX J0152.7-1357 at z~0.84, aimed at
understanding the effects of the cluster environment on the star formation
history (SFH) of cluster galaxies and the assembly of the red-sequence (RS). We
use VLT/FORS spectroscopy, ACS/WFC optical and NTT/SofI near-IR data to
characterize SFHs as a function of color, luminosity, morphology, stellar mass,
and local environment from a sample of 134 spectroscopic members. In order to
increase the signal-to-noise, individual galaxy spectra are stacked according
to these properties. Moreover, the D4000, Balmer, CN3883, Fe4383 and C4668
indices are also quantified. The SFH analysis shows that galaxies in the blue
faint-end of the RS have on average younger stars (Delta t ~ 2 Gyr) than those
in the red bright-end. We also found, for a given luminosity range, differences
in age (Delta t ~ 0.5 - 1.3 Gyr) as a function of color, indicating that the
intrinsic scatter of the RS may be due to age variations. Passive galaxies in
the blue faint-end of the RS are preferentially located in the low density
areas of the cluster, likely being objects entering the RS from the "blue
cloud". It is likely that the quenching of the star formation of these RS
galaxies is due to interaction with the intracluster medium. Furthermore, the
SFH of galaxies in the RS as a function of stellar mass reveals signatures of
"downsizing" in the overall cluster.Comment: 36 pages, 5 tables, 14 figures. Accepted for publication in The
Astrophysical Journa
Constraints on the delta H-2 diffusion rate in firn from field measurements at Summit, Greenland
We performed detailed 2H isotope diffusion measurements in the upper
3 m of firn at Summit, Greenland. Using a small snow gun, a thin snow
layer was formed from 2H-enriched water over a 6 Ă 6 m2 area. We
followed the diffusion process, quantified as the increase of the ÎŽ2H
diffusion length, over a 4-year period, by retrieving the layer
once per year by drilling a firn core and slicing it into 1 cm layers and
measuring the ÎŽ2H signal of these layers.
We compared our experimental findings to calculations based on the model by
Johnsen et al. (2000) and found substantial differences. The
diffusion length in our experiments increased much less over the years than
in the model. We discuss the possible causes for this discrepancy and
conclude that several aspects of the diffusion process in firn are still
poorly constrained, in particular the tortuosity
Dissecting the Red Sequence--II. Star Formation Histories of Early-Type Galaxies Throughout the Fundamental Plane
This analysis uses spectra of ~16,000 nearby SDSS quiescent galaxies to track
variations in galaxy star formation histories along and perpendicular to the
Fundamental Plane (FP). We sort galaxies by their FP properties (sigma, R_e,
and I_e) and construct high S/N mean galaxy spectra that span the breadth and
thickness of the FP. From these spectra, we determine mean luminosity-weighted
ages, [Fe/H], [Mg/H], and [Mg/Fe] based on single stellar population models
using the method described in Graves & Schiavon (2008). In agreement with
previous work, the star formation histories of early-type galaxies are found to
form a two-parameter family. The major trend is that mean age, [Fe/H], [Mg/H],
and [Mg/Fe] all increase with sigma. However, no stellar population property
shows any dependence on R_e at fixed sigma, suggesting that sigma and not
dynamical mass (M_dyn ~ sigma^2 R_e) is the better predictor of past star
formation history. In addition to the main trend with sigma, galaxies also show
a range of population properties at fixed sigma that are strongly correlated
with surface brightness residuals from the FP, such that higher surface
brightness galaxies have younger mean ages, higher [Fe/H], higher [Mg/H], and
lower [Mg/Fe] than lower-surface brightness galaxies. These latter trends are a
major new constraint on star-formation histories.Comment: 23 pages, 14 figures. Accepted to Ap
Dissecting the Red Sequence. IV. The Role of Truncation in the Two-Dimensional Family of Early-Type Galaxy Star Formation Histories
In the three-dimensional parameter space defined by velocity dispersion,
effective radius (R_e), and effective surface brightness (I_e), early-type
galaxies are observed to populate a two-dimensional fundamental plane (FP) with
finite thickness. In Paper III of this series, we showed that the thickness of
the FP is predominantly due to variations in the stellar mass surface density
(Sigma_*) inside the effective radius R_e. These variations represent
differences in the dark matter fraction inside R_e (or possibly differences in
the initial mass function) from galaxy to galaxy. This means that galaxies do
not wind up below the FP at lower surface brightness due to the passive fading
of their stellar populations; they are structurally different. Here, we show
that these variations in Sigma_* at fixed dynamical mass (M_dyn) are linked to
differences in the galaxy stellar populations, and therefore to differences in
their star formation histories. We demonstrate that the ensemble of stellar
population and Sigma_* variations through the FP thickness can be explained by
a model in which early-type galaxies at fixed M_dyn have their star formation
truncated at different times. The thickness of the FP can therefore be
interpreted as a sequence of truncation times. Galaxies below the FP have
earlier truncation times for a given M_dyn, resulting in lower Sigma_*, older
ages, lower metallicities in both [Fe/H] and [Mg/H], and higher [Mg/Fe]. We
show that this model is quantitatively consistent with simple expectations for
chemical enrichment in galaxies. We also present fitting functions for
luminosity-weighted age, [Fe/H], [Mg/H], and [Mg/Fe] as functions of the FP
parameters velocity dispersion, R_e, and I_e. These provide a new tool for
estimating the stellar population properties of quiescent early-type galaxies
for which high-quality spectra are not available.Comment: 21 pages, 9 figures. Accepted to Ap
The Nascent Red Sequence at z~2
We present new constraints on the evolution of the early-type galaxy
color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy
protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space
Telescope. This field contains a spectroscopically confirmed space-overdensity
of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio
galaxy MRC 1138-262. Using these NICMOS data we identify a significant
surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram
(when compared with deep NICMOS imaging from the HDF-N and UDF). The
optical-NIR colors of these prospective red-sequence galaxies indicate the
presence of on-going dust-obscured star-formation or recently formed (<~ 1.5
Gyr)stellar populations in a majority of the red galaxies. We measure the slope
and intrinsic scatter of the CMR for three different red galaxy samples
selected by a wide color cut, and using photometric redshifts both with and
without restrictions on rest-frame optical morphology. In all three cases both
the rest-frame slope and intrinsic color scatter are considerably higher
than corresponding values for lower redshift galaxy clusters. These results
suggest that while some relatively quiescent galaxies do exist in this
protocluster both the majority of the galaxy population and hence the
color-magnitude relation are still in the process of forming, as expected.Comment: 8 pages, 7 figures, accepted for publication in ApJ (to appear June
1, 2008, v679n2
Dynamical masses of early-type galaxies at z~2: Are they truly superdense?
We measured stellar velocity dispersions sigma and dynamical masses of 9
massive (M~10^11 Msun) early-type galaxies (ETG) from the GMASS sample at
redshift 1.4<z<2.0. The sigma are based on individual spectra for two galaxies
at z~1.4 and on a stacked spectrum for 7 galaxies with 1.6<z<2.0, with 202-h of
exposure at the ESO Very Large Telescope. We constructed detailed axisymmetric
dynamical models for the objects, based on the Jeans equations, taking the
observed surface brightness (from deep HST/ACS observations), PSF and slit
effects into account. Our dynamical masses M_Jeans agree within ~30% with
virial estimates M_vir=5*Re*sigma^2/G, although the latter tend to be smaller.
This suggests that sizes are not underestimated by more than a similar
fraction. Our M_Jeans also agrees within a factor <2 with the M_pop previously
derived using stellar population models and 11 bands photometry. This confirms
that the galaxies are intrinsically massive. The inferred mass-to-light ratios
M/L_U in the very age-sensitive rest frame U-band are consistent with passive
evolution in the past ~1 Gyr (formation redshift z_f~3). A 'bottom-light'
stellar Initial Mass Function (IMF) appears to be required to ensure close
agreement between M_Jeans and M_pop at z~2, as it does at z~0. The GMASS ETGs
are on average more dense than their local counterpart. However a few percent
of local ETGs of similar dynamical masses also have comparable sigma and mass
surface density Sigma_50 inside Re.Comment: 6 pages, 4 figures, LaTeX with emulateapj. Accepted for publication
in ApJ Letter
HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3
We study the structural evolution of massive galaxies by linking progenitors
and descendants at a constant cumulative number density of n_c=1.4x10^{-4}
Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic profiles
to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the
UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given
redshift, we selected the HST band that most closely samples a common
rest-frame wavelength so as to minimize systematics from color gradients in
galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of ~3 from
z~3 to z~0. The size evolution is complex: galaxies appear roughly constant in
size from z~3 to z~2 and then grow rapidly to lower redshifts. The evolution in
the surface mass density profiles indicates that most of the mass at r<2 kpc
was in place by z~2, and that most of the new mass growth occurred at larger
radii. This inside-out mass growth is therefore responsible for the larger
sizes and higher Sersic indices of the descendants toward low redshift. At z<2,
the effective radius evolves with the stellar mass as r_e M^{2.0}, consistent
with scenarios that find dissipationless minor mergers to be a key driver of
size evolution. The progenitors at z~3 were likely star-forming disks with
r_e~2 kpc, based on their low Sersic index of n~1, low median axis ratio of
b/a~0.52, and typical location in the star-forming region of the U-V versus V-J
diagram. By z~1.5, many of these star-forming disks disappeared, giving rise to
compact quiescent galaxies. Toward lower redshifts, these galaxies continued to
assemble mass at larger radii and became the local ellipticals that dominate
the high mass end of the mass function at the present epoch.Comment: 12 pages, 8 figures in main text + appendix. v2 reflects the version
that was accepted to ApJ after addressing the referee repor
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