54 research outputs found
Star formation quenching in massive galaxies
Understanding how and why star formation turns off in massive galaxies is a
major challenge for studies of galaxy evolution. Many theoretical explanations
have been proposed, but a definitive consensus is yet to be reached.Comment: Comment published in Nature Astronomy on 3rd September 2018. The full
text is publicly available at this link: https://rdcu.be/5KbA. Authors'
version, 4 pages and 1 figur
Flame: A Flexible Data Reduction Pipeline for Near-Infrared and Optical Spectroscopy
We present flame, a pipeline for reducing spectroscopic observations obtained
with multi-slit near-infrared and optical instruments. Because of its flexible
design, flame can be easily applied to data obtained with a wide variety of
spectrographs. The flexibility is due to a modular architecture, which allows
changes and customizations to the pipeline, and relegates the
instrument-specific parts to a single module. At the core of the data reduction
is the transformation from observed pixel coordinates (x, y) to rectified
coordinates (lambda, gamma). This transformation consists in the polynomial
functions lambda(x,y) and gamma(x,y) that are derived from arc or sky emission
lines and slit edge tracing, respectively. The use of 2D transformations allows
one to wavelength calibrate and rectify the data using just one interpolation
step. Furthermore, the gamma(x,y) transformation includes also the spatial
misalignment between frames, which can be measured from a reference star
observed simultaneously with the science targets. The misalignment can then be
fully corrected during the rectification, without having to further resample
the data. Sky subtraction can be performed via nodding and/or modeling of the
sky spectrum; the combination of the two methods typically yields the best
results. We illustrate the pipeline by showing examples of data reduction for a
near-infrared instrument (LUCI at the Large Binocular Telescope) and an optical
one (LRIS at the Keck telescope).Comment: 17 pages, 10 figures, published in MNRAS. The pipeline is available
at https://github.com/siriobelli/flam
Velocity Dispersions and Dynamical Masses for a Large Sample of Quiescent Galaxies at z > 1: Improved Measures of the Growth in Mass and Size
We present Keck LRIS spectroscopy for a sample of 103 massive galaxies with
redshifts 0.9 < z < 1.6. Of these, 56 are quiescent with high signal-to-noise
absorption line spectra, enabling us to determine robust stellar velocity
dispersions for the largest sample yet available beyond a redshift of 1.
Together with effective radii measured from deep Hubble Space Telescope images,
we calculate dynamical masses and address key questions relating to the
puzzling size growth of quiescent galaxies over 0 < z < 2. We examine the
relationship between stellar and dynamical masses at high redshift, finding
that it closely follows that determined locally. We also confirm the utility of
the locally-established empirical calibration which enables high-redshift
velocity dispersions to be estimated photometrically, and we determine its
accuracy to be 35%. To address recent suggestions that progenitor bias - the
continued arrival of recently-quenched larger galaxies - can largely explain
the size evolution of quiescent galaxies, we examine the growth at fixed
velocity dispersion assuming this quantity is largely unaffected by the merger
history. We demonstrate that significant size and mass growth have clearly
occurred in individual systems. Parameterizing the relation between mass and
size growth over 0 < z < 1.6 as R \propto M^alpha, we find alpha = 1.6 +- 0.3,
in agreement with theoretical expectations from simulations of minor mergers.
Relaxing the assumption that the velocity dispersion is unchanging, we examine
growth assuming a constant ranking in galaxy velocity dispersion. This approach
is applicable only to the large-dispersion tail of the distribution, but yields
a consistent growth rate of alpha = 1.4 +- 0.2. Both methods confirm that
progenitor bias alone is insufficient to explain our new observations and that
quiescent galaxies have grown in both size and stellar mass over 0 < z < 1.6.Comment: Updated to match the published versio
Stellar populations from spectroscopy of a large sample of quiescent galaxies at z > 1: Measuring the contribution of progenitor bias to early size growth
We analyze the stellar populations of a sample of 62 massive (log Mstar/Msun
> 10.7) galaxies in the redshift range 1 < z < 1.6, with the main goal of
investigating the role of recent quenching in the size growth of quiescent
galaxies. We demonstrate that our sample is not biased toward bright, compact,
or young galaxies, and thus is representative of the overall quiescent
population. Our high signal-to-noise ratio Keck LRIS spectra probe the
rest-frame Balmer break region which contains important absorption line
diagnostics of recent star formation activity. We obtain improved measures of
the various stellar population parameters, including the star-formation
timescale tau, age and dust extinction, by fitting templates jointly to both
our spectroscopic and broad-band photometric data. We identify which quiescent
galaxies were recently quenched and backtrack their individual evolving
trajectories on the UVJ color-color plane finding evidence for two distinct
quenching routes. By using sizes measured in the previous paper of this series,
we confirm that the largest galaxies are indeed among the youngest at a given
redshift. This is consistent with some contribution to the apparent growth from
recent arrivals, an effect often called progenitor bias. However, we calculate
that recently-quenched objects can only be responsible for about half the
increase in average size of quiescent galaxies over a 1.5 Gyr period,
corresponding to the redshift interval 1.25 < z < 2. The remainder of the
observed size evolution arises from a genuine growth of long-standing quiescent
galaxies.Comment: Accepted for publication in the Astrophysical Journal, 14 pages, 11
figure
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. II - Star Formation Histories and Galaxy Quenching
We investigate the stellar populations for a sample of 24 quiescent galaxies
at 1.5 < z < 2.5 using deep rest-frame optical spectra obtained with Keck
MOSFIRE. By fitting templates simultaneously to the spectroscopic and
photometric data, and exploring a variety of star formation histories, we
obtain robust measurements of median stellar ages and residual levels of star
formation. After subtracting the stellar templates, the stacked spectrum
reveals the Halpha and [NII] emission lines, providing an upper limit on the
ongoing star formation rate of 0.9 +/- 0.1 Msun/yr. By combining the MOSFIRE
data to our sample of Keck LRIS spectra at lower redshift, we analyze in a
consistent manner the quiescent population at 1 < z < 2.5. We find a tight
relation (with a scatter of 0.13 dex) between the stellar age and the
rest-frame U-V and V-J colors, which can be used to estimate the age of
quiescent galaxies given their colors. Applying this age--color relation to
large, photometric samples, we are able to model the number density evolution
for quiescent galaxies of various ages. We find evidence for two distinct
quenching paths: a fast quenching that produces compact post-starburst systems,
and a slow quenching of larger galaxies. Fast quenching accounts for about a
fifth of the growth of the red sequence at z~1.4, and half at z~2.2. We
conclude that fast quenching is triggered by dramatic events such as gas-rich
mergers, while slow quenching is likely caused by a different physical
mechanism.Comment: 28 pages, 15 figures, accepted in Ap
Discovery of a Strongly Lensed Massive Quiescent Galaxy at z=2.636: Spatially Resolved Spectroscopy and Indications of Rotation
We report the discovery of RG1M0150, a massive, recently quenched galaxy at
z=2.636 that is multiply imaged by the cluster MACSJ0150.3-1005. We derive a
stellar mass of log M_*=11.49+0.10-0.16 and a half-light radius of R_e,maj
=1.8+-0.4 kpc. Taking advantage of the lensing magnification, we are able to
spatially resolve a remarkably massive yet compact quiescent galaxy at z>2 in
ground-based near-infrared spectroscopic observations using Magellan/FIRE and
Keck/MOSFIRE. We find no gradient in the strength of the Balmer absorption
lines over 0.6 R_e - 1.6 R_e, which are consistent with an age of 760 Myr. Gas
emission in [NII] broadly traces the spatial distribution of the stars and is
coupled with weak Halpha emission (log [NII]/Halpha = 0.6+-0.2), indicating
that OB stars are not the primary ionizing source. The velocity dispersion
within the effective radius is sigma_e = 271+-41 km/s. We detect rotation in
the stellar absorption lines for the first time beyond z~1. Using a
two-integral Jeans model that accounts for observational effects, we measure a
dynamical mass of log M_dyn =11.24+-0.14 and V/sigma=0.70+-0.21. This is a high
degree of rotation considering the modest observed ellipticity of 0.12+-0.08,
but it is consistent with predictions from dissipational merger simulations
that produce compact remnants. The mass of RG1M0150 implies that it is likely
to become a slowly rotating elliptical. If it is typical, this suggests that
the progenitors of massive ellipticals retain significant net angular momentum
after quenching which later declines, perhaps through accretion of satellites.Comment: Accepted to ApJ Letters; updated to include revisions from the
referee process, including an improved Fig.
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I - Evolution of Structural and Dynamical Properties
We present deep near-infrared spectra for a sample of 24 quiescent galaxies
in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at
the W. M. Keck Observatory. In conjunction with a similar dataset we obtained
in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic
and structural properties for 80 quiescent galaxies, the largest
homogeneously-selected sample to date spanning 3 Gyr of early cosmic history.
Analysis of our Keck spectra together with measurements derived from associated
HST images reveals increasingly larger stellar velocity dispersions and smaller
sizes to redshifts beyond z~2. By classifying our sample according to Sersic
indices, we find that among disk-like systems the flatter ones show a higher
dynamical to stellar mass ratio compared to their rounder counterparts which we
interpret as evidence for a significant contribution of rotational motion. For
this subset of disk-like systems, we estimate that V/sigma, the ratio of the
circular velocity to the intrinsic velocity dispersion, is a factor of two
larger than for present-day disky quiescent galaxies. We use the velocity
dispersion measurements also to explore the redshift evolution of the dynamical
to stellar mass ratio, and to measure for the first time the physical size
growth rate of individual systems over two distinct redshift ranges, finding a
faster evolution at earlier times. We discuss the physical origin of this
time-dependent growth in size in the context of the associated reduction of the
systematic rotation.Comment: Updated to match the published versio
A Pilot Survey for CIII] Emission in the Reionization Era: Gravitationally-Lensed z Galaxies in the Frontier Fields Cluster Abell 2744
We report results of a search for CIII] 1907,1909 {\AA}
emission using Keck's MOSFIRE spectrograph in a sample of 7
candidates () lensed by the Hubble Frontier Field cluster Abell 2744.
Earlier work has suggested the promise of using the CIII] doublet for redshift
confirmation of galaxies in the reionization era given
(1216 {\AA}) is likely attenuated by the neutral intergalactic medium.
The primary challenge of this approach is the feasibility of locating CIII]
emission without advanced knowledge of the spectroscopic redshift. With an
integration time of 5 hours in the H-band, we reach a median flux
limit (in between the skylines) of ergs cm
sec but no convincing CIII] emission was found. We also incorporate
preliminary measurements from two other CLASH/HFF clusters in which, similarly,
no line was detected, but these were observed to lesser depth. Using the known
distribution of OH emission and the photometric redshift likelihood
distribution of each lensed candidate, we present statistical upper limits on
the mean total CIII] rest-frame equivalent width for our sample.
For a signal/noise ratio of 5, we estimate the typical CIII] doublet rest-frame
equivalent width is, with 95\% confidence, {\AA}. Although consistent
with the strength of earlier detections in brighter objects at ,
our study illustrates the necessity of studying more luminous or
strongly-lensed examples prior to the launch of the James Webb Space Telescope.Comment: 7 pages, 4 figures, 1 table; accepted for publication in ApJ Letter
MOSFIRE Absorption Line Spectroscopy of z > 2 Quiescent Galaxies: Probing a Period of Rapid Size Growth
Using the MOSFIRE near-infrared multi-slit spectrograph on the Keck 1
Telescope, we have secured high signal-to-noise ratio absorption line spectra
for six massive galaxies with redshift 2 < z < 2.5. Five of these galaxies lie
on the red sequence and show signatures of passive stellar populations in their
rest-frame optical spectra. By fitting broadened spectral templates we have
determined stellar velocity dispersions and, with broad-band HST and Spitzer
photometry and imaging, stellar masses and effective radii. Using this enlarged
sample of galaxies we confirm earlier suggestions that quiescent galaxies at z
> 2 have small sizes and large velocity dispersions compared to local galaxies
of similar stellar mass. The dynamical masses are in very good agreement with
stellar masses (log Mstar/Mdyn = -0.02 +/- 0.03), although the average
stellar-to-dynamical mass ratio is larger than that found at lower redshift
(-0.23 +/- 0.05). By assuming evolution at fixed velocity dispersion, not only
do we confirm a surprisingly rapid rate of size growth but we also consider the
necessary evolutionary track on the mass-size plane and find a slope alpha =
dlogR / dlogM > ~2 inconsistent with most numerical simulations of minor
mergers. Both results suggest an additional mechanism may be required to
explain the size growth of early galaxies.Comment: Updated to match the published versio
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