71 research outputs found
Modeling the red sequence: Hierarchical growth yet slow luminosity evolution
We explore the effects of mergers on the evolution of massive early-type
galaxies by modeling the evolution of their stellar populations in a
hierarchical context. We investigate how a realistic red sequence population
set up by z~1 evolves under different assumptions for the merger and star
formation histories, comparing changes in color, luminosity and mass. The
purely passive fading of existing red sequence galaxies, with no further
mergers or star formation, results in dramatic changes at the bright end of the
luminosity function and color-magnitude relation. Without mergers there is too
much evolution in luminosity at a fixed space density compared to observations.
The change in color and magnitude at a fixed mass resemble that of a passively
evolving population that formed relatively recently, at z~2. Mergers amongst
the red sequence population ("dry mergers") occurring after z=1 build up mass,
counteracting the fading of the existing stellar populations to give smaller
changes in both color and luminosity for massive galaxies. By allowing some
galaxies to migrate from the blue cloud onto the red sequence after z=1 through
gas-rich mergers, younger stellar populations are added to the red sequence.
This manifestation of the progenitor bias increases the scatter in age and
results in even smaller changes in color and luminosity between z=1 and z=0 at
a fixed mass. The resultant evolution appears much slower, resembling the
passive evolution of a population that formed at high redshift (z~3-5) and is
in closer agreement with observations. Measurements of the luminosity and color
evolution alone are not sufficient to distinguish between the purely passive
evolution of an old population and cosmologically motivated hierarchical
growth, although these scenarios have very different implications for the mass
growth of early-type galaxies over the last half of cosmic history.Comment: 14 pages, 4 figures. Final version accepted for publication in ApJ
(2012, ApJ 753, 44
High Redshift Massive Quiescent Galaxies are as Flat as Star Forming Galaxies: The Flattening of Galaxies and the Correlation with Structural Properties in CANDELS/3D-HST
We investigate the median flattening of galaxies at in all five
CANDELS/3D-HST fields via the apparent axis ratio . We separate the sample
into bins of redshift, stellar-mass, s\'ersic index, size, and UVJ determined
star-forming state to discover the most important drivers of the median
(). Quiescent galaxies at are
rounder than those at lower masses, consistent with the hypothesis that they
have grown significantly through dry merging. The massive quiescent galaxies at
higher redshift become flatter, and are as flat as star forming massive
galaxies at , consistent with formation through direct
transformations or wet mergers. We find that in quiescent galaxies,
correlations with and , and are driven by the
evolution in the s\'ersic index (), consistent with the growing accumulation
of minor mergers at lower redshift. Interestingly, does not drive these
trends fully in star-forming galaxies. Instead, the strongest predictor of
in star-forming galaxies is the effective radius, where larger galaxies are
flatter. Our findings suggest that is tracing bulge-to-total ()
galaxy ratio which would explain why smaller/more massive star-forming galaxies
are rounder than their extended/less massive analogues, although it is unclear
why s\'ersic index correlates more weakly with flattening for star forming
galaxies than for quiescent galaxies.Comment: 13 pages, 11 figures, accepted to Ap
The Evolution of the Fractions of Quiescent and Star-forming Galaxies as a Function of Stellar Mass Since z=3: Increasing Importance of Massive, Dusty Star-forming Galaxies in the Early Universe
Using the UltraVISTA DR1 and 3D-HST catalogs, we construct a
stellar-mass-complete sample, unique for its combination of surveyed volume and
depth, to study the evolution of the fractions of quiescent galaxies,
moderately unobscured star-forming galaxies, and dusty star-forming galaxies as
a function of stellar mass over the redshift interval . We
show that the role of dusty star-forming galaxies within the overall galaxy
population becomes more important with increasing stellar mass, and grows
rapidly with increasing redshift. Specifically, dusty star-forming galaxies
dominate the galaxy population with at . The ratio of dusty and non-dusty star-forming galaxies as
a function of stellar mass changes little with redshift. Dusty star-forming
galaxies dominate the star-forming population at , being a factor of 3-5 more common,
while unobscured star-forming galaxies dominate at . At , red
galaxies dominate the galaxy population at all redshift , either because
they are quiescent (at late times) or dusty star-forming (in the early
universe).Comment: 7 pages, 4 figures, 1 table. Accepted by Astrophysical Journal
Letters after minor revisio
The Number Density Evolution of Extreme Emission Line Galaxies in 3D-HST: Results from a Novel Automated Line Search Technique for Slitless Spectroscopy
The multiplexing capability of slitless spectroscopy is a powerful asset in
creating large spectroscopic datasets, but issues such as spectral confusion
make the interpretation of the data challenging. Here we present a new method
to search for emission lines in the slitless spectroscopic data from the 3D-HST
survey utilizing the Wide-Field Camera 3 on board the Hubble Space Telescope.
Using a novel statistical technique, we can detect compact (extended) emission
lines at 90% completeness down to fluxes of 1.5 (3.0) times 10^{-17}
erg/s/cm^2, close to the noise level of the grism exposures, for objects
detected in the deep ancillary photometric data. Unlike previous methods, the
Bayesian nature allows for probabilistic line identifications, namely redshift
estimates, based on secondary emission line detections and/or photometric
redshift priors. As a first application, we measure the comoving number density
of Extreme Emission Line Galaxies (restframe [O III] 5007 equivalent widths in
excess of 500 Angstroms). We find that these galaxies are nearly 10 times more
common above z~1.5 than at z<0.5. With upcoming large grism surveys such as
Euclid and WFIRST as well as grisms featuring prominently on the NIRISS and
NIRCam instruments on James Webb Space Telescope, methods like the one
presented here will be crucial for constructing emission line redshift catalogs
in an automated and well-understood manner.Comment: 16 pages, 14 Figures; Accepted to Ap
Ages of massive galaxies at from 3D-HST rest-frame optical spectroscopy
We present low-resolution near-infrared stacked spectra from the 3D-HST
survey up to and fit them with commonly used stellar population
synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar
Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz,
Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the
unambiguous detection of many emission and absorption features, and thus a
first systematic exploration of the rest-frame optical spectra of galaxies up
to . We select massive galaxies (), we
divide them into quiescent and star-forming via a rest-frame color-color
technique, and we median-stack the samples in 3 redshift bins between
and . We find that stellar population models fit the observations well
at wavelengths below rest-frame, but show systematic residuals
at redder wavelengths. The FSPS-C3K model generally provides the best fits
(evaluated with a statistics) for quiescent galaxies, while BC03
performs the best for star-forming galaxies. The stellar ages of quiescent
galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at
to 1.5 Gyr at , with an uncertainty of a factor of 2
caused by the unknown metallicity. On average the stellar ages are half the age
of the Universe at these redshifts. We show that the inferred evolution of ages
of quiescent galaxies is in agreement with fundamental plane measurements,
assuming an 8 Gyr age for local galaxies. For star-forming galaxies the
inferred ages depend strongly on the stellar population model and the shape of
the assumed star-formation history.Comment: 13 pages, 15 figures, accepted for publication in Ap
The mass, colour, and structural evolution of today's massive galaxies since z~5
In this paper, we use stacking analysis to trace the mass-growth, colour
evolution, and structural evolution of present-day massive galaxies
() out to . We utilize the exceptional depth
and area of the latest UltraVISTA data release, combined with the depth and
unparalleled seeing of CANDELS to gather a large, mass-selected sample of
galaxies in the NIR (rest-frame optical to UV). Progenitors of present-day
massive galaxies are identified via an evolving cumulative number density
selection, which accounts for the effects of merging to correct for the
systematic biases introduced using a fixed cumulative number density selection,
and find progenitors grow in stellar mass by since
. Using stacking, we analyze the structural parameters of the progenitors
and find that most of the stellar mass content in the central regions was in
place by , and while galaxies continue to assemble mass at all radii,
the outskirts experience the largest fractional increase in stellar mass.
However, we find evidence of significant stellar mass build up at
probing an era of significant mass assembly in
the interiors of present day massive galaxies. We also compare mass assembly
from progenitors in this study to the EAGLE simulation and find qualitatively
similar assembly with at . We identify as a
distinct epoch in the evolution of massive galaxies where progenitors
transitioned from growing in mass and size primarily through in-situ star
formation in disks to a period of efficient growth in consistent with
the minor merger scenario.Comment: 19 pages, 14 figures, accepted for publicatio
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