401 research outputs found
An alternate approach to measure specific star formation rates at 2<z<7
We trace the specific star formation rate (sSFR) of massive star-forming
galaxies () from to 7. Our method
is substantially different from previous analyses, as it does not rely on
direct estimates of star formation rate, but on the differential evolution of
the galaxy stellar mass function (SMF). We show the reliability of this
approach by means of semi-analytical and hydrodynamical cosmological
simulations. We then apply it to real data, using the SMFs derived in the
COSMOS and CANDELS fields. We find that the sSFR is proportional to
at , in agreement with other observations but in
tension with the steeper evolution predicted by simulations from to 2.
We investigate the impact of several sources of observational bias, which
however cannot account for this discrepancy. Although the SMF of high-redshift
galaxies is still affected by significant errors, we show that future
large-area surveys will substantially reduce them, making our method an
effective tool to probe the massive end of the main sequence of star-forming
galaxies.Comment: ApJ accepte
A Method to Distinguish Quiescent and Dusty Star-forming Galaxies with Machine Learning
Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z > 1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high- and low-redshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample
Hubble Frontier Field Clusters and their Parallel Fields: Photometric and Photometric Redshift Catalogs
We present a multi-band analysis of the six Hubble Frontier Field clusters
and their parallel fields, producing catalogs with measurements of source
photometry and photometric redshifts. We release these catalogs to the public
along with maps of intracluster light and models for the brightest galaxies in
each field. This rich data set covers a wavelength range from 0.2 to 8 ,
utilizing data from the Hubble Space Telescope, Keck Observatories, Very Large
Telescope array, and Spitzer Space Telescope. We validate our products by
injecting into our fields and recovering a population of synthetic objects with
similar characteristics as in real extragalactic surveys. The photometric
catalogs contain a total of over 32,000 entries with 50\% completeness at a
threshold of for unblended sources, and
for blended ones, in the IR-Weighted detection band.
Photometric redshifts were obtained by means of template fitting and have an
average outlier fraction of 10.3\% and scatter when compared
to spectroscopic estimates. The software we devised, after being tested in the
present work, will be applied to new data sets from ongoing and future surveys.Comment: Accepted in ApJS, 26 pages, 20 figure
Galaxy evolution in the VIMOS public extragalactic redshift survey (VIPERS)
In this work I present the first measurements of the galaxy stellar mass function (GSMF) from the first public release of the VIPERS catalogue, containing ∼55,000 objects.
First, I present the survey design, its scientific goal, the redshift measurements and validation. Then, I provide details about the estimate of galaxy stellar masses, star formation rates, and other physical quantities. I derive the GSMF of different galaxy types (e.g. active and passive galaxies) and as a function of the environment (defined through the local galaxy density contrast). These estimates represent new observational evidence useful to characterise the mechanism of galaxy evolution
SILVERRUSH. V. Census of Lyα, [O III] λ5007, Hα, and [C II] 158 μm Line Emission with ~1000 LAEs at z = 4.9–7.0 Revealed with Subaru/HSC
We investigate Lyα, [O III] λ5007, Hα, and [C II] 158 μm emission from 1124 galaxies at z = 4.9–7.0. Our sample is composed of 1092 Lyα emitters (LAEs) at z = 4.9, 5.7, 6.6, and 7.0 identified by Subaru/Hyper-Suprime-Cam (HSC) narrowband surveys covered by Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH) and 34 galaxies at z = 5.148–7.508 with deep ALMA [C II] 158 μm data in the literature. Fluxes of strong rest-frame optical lines of [O III] and Hα (Hβ) are constrained by significant excesses found in the SPLASH 3.6 and 4.5 μm photometry. At z = 4.9, we find that the rest-frame Hα equivalent width and the Lyα escape fraction f_(Lyα) positively correlate with the rest-frame Lyα equivalent width EW^0_( Lyα). The f_(Lyα)-EW^0_( Lyα) correlation is similarly found at z ~ 0–2, suggesting no evolution of the correlation over z ≃ 0–5. The typical ionizing photon production efficiency of LAEs is log(ξ_(ion)/[Hz erg^(−1)]) ≃ 25.5, significantly (60%–100%) higher than those of LBGs at a given UV magnitude. At z = 5.7–7.0, there exists an interesting turnover trend that the [O III]/Hα flux ratio increases in EW^0_( Lyα) ≃ 0-30 Å and then decreases out to EW^0_( Lyα) ≃ 130 Å. We also identify an anticorrelation between a ratio of [C II] luminosity to star formation rate (L [C II]/SFR) and EW^0_( Lyα) at the >99% confidence level.. We carefully investigate physical origins of the correlations with stellar-synthesis and photoionization models and find that a simple anticorrelation between EW^0_( Lyα) and metallicity explains self-consistently all of the correlations of Lyα, Hα, [O III]/Hα, and [C II] identified in our study, indicating detections of metal-poor (~0.03 Z⊙) galaxies with EW^0_( Lyα) ≃ 200 Å
Brain MRI-to-PET Synthesis using 3D Convolutional Attention Networks
Accurate quantification of cerebral blood flow (CBF) is essential for the
diagnosis and assessment of a wide range of neurological diseases. Positron
emission tomography (PET) with radiolabeled water (15O-water) is considered the
gold-standard for the measurement of CBF in humans. PET imaging, however, is
not widely available because of its prohibitive costs and use of short-lived
radiopharmaceutical tracers that typically require onsite cyclotron production.
Magnetic resonance imaging (MRI), in contrast, is more readily accessible and
does not involve ionizing radiation. This study presents a convolutional
encoder-decoder network with attention mechanisms to predict gold-standard
15O-water PET CBF from multi-sequence MRI scans, thereby eliminating the need
for radioactive tracers. Inputs to the prediction model include several
commonly used MRI sequences (T1-weighted, T2-FLAIR, and arterial spin
labeling). The model was trained and validated using 5-fold cross-validation in
a group of 126 subjects consisting of healthy controls and cerebrovascular
disease patients, all of whom underwent simultaneous $15O-water PET/MRI. The
results show that such a model can successfully synthesize high-quality PET CBF
measurements (with an average SSIM of 0.924 and PSNR of 38.8 dB) and is more
accurate compared to concurrent and previous PET synthesis methods. We also
demonstrate the clinical significance of the proposed algorithm by evaluating
the agreement for identifying the vascular territories with abnormally low CBF.
Such methods may enable more widespread and accurate CBF evaluation in larger
cohorts who cannot undergo PET imaging due to radiation concerns, lack of
access, or logistic challenges.Comment: 19 pages, 14 figure
Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z~5
We measure a relation between the depth of four prominent rest-UV absorption
complexes and metallicity for local galaxies and verify it up to z~3. We then
apply this relation to a sample of 224 galaxies at 3.5 = 4.8) in
COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses
from SPLASH are available. The average galaxy population at z~5 and log(M/Msun)
> 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower
metallicities than at z~2, but comparable to z~3.5. We find galaxies with
weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and
therefore may represent an evolved sub-population of z~5 galaxies. We find a
correlation between metallicity and dust in good agreement with local galaxies
and an inverse trend between metallicity and star-formation rate (SFR)
consistent with observations at z~2. The relation between stellar mass and
metallicity (MZ relation) is similar to z~3.5, however, there are indications
of it being slightly shallower, in particular for the young, Ly-alpha emitting
galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is
expected in the case of a fast (exponential) build-up of stellar mass with an
e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust
production and metal enrichment as a function of mass could be more stochastic
in the first billion years of galaxy formation compared to later times.Comment: 20 pages, 13 figures, 4 tables; Submitted to Ap
The VIPERS Multi-Lambda Survey. II. Diving with massive galaxies in 22 square degrees since z = 1.5
We investigate the evolution of the galaxy stellar mass function (SMF) and
stellar mass density from redshift z=0.2 to z=1.5 of a <22-selected
sample with highly reliable photometric redshifts and over an unprecedentedly
large area. Our study is based on NIR observations carried out with WIRCam at
CFHT over the footprint of the VIPERS spectroscopic survey and benefits from
the high quality optical photometry from the CFHTLS and UV observations with
the GALEX satellite. The accuracy of our photometric redshifts is <
0.03 and 0.05 for the bright (22.5) samples,
respectively. The SMF is measured with ~760,000 galaxies down to =22 and
over an effective area of ~22.4 deg, the latter of which drastically
reduces the statistical uncertainties (i.e. Poissonian error & cosmic
variance). We point out the importance of a careful control of the photometric
calibration, whose impact becomes quickly dominant when statistical
uncertainties are reduced, which will be a major issue for future generation of
cosmological surveys with, e.g. EUCLID or LSST. By exploring the rest-frame
(NUV-r) vs (r-) color-color diagram separating star-forming and quiescent
galaxies, (1) we find that the density of very massive log() >
11.5 galaxies is largely dominated by quiescent galaxies and increases by a
factor 2 from z~1 to z~0.2, which allows for additional mass assembly via dry
mergers, (2) we confirm a scenario where star formation activity is impeded
above a stellar mass log() = 10.640.01, a value that
is found to be very stable at 0.2 < z < 1.5, (3) we discuss the existence of a
main quenching channel that is followed by massive star-forming galaxies, and
finally (4) we characterise another quenching mechanism required to explain the
clear excess of low-mass quiescent galaxies observed at low redshift.Comment: 22 pages, 20 figures. Accepted for publication in A&A. Version to be
publishe
The VIPERS Multi-Lambda Survey. I. UV and NIR Observations, multi-color catalogues and photometric redshifts
We present observations collected in the CFHTLS-VIPERS region in the
ultraviolet (UV) with the GALEX satellite (far and near UV channels) and the
near infrared with the CFHT/WIRCam camera (-band) over an area of 22 and
27 deg, respectively. The depth of the photometry was optimized to measure
the physical properties (e.g., SFR, stellar masses) of all the galaxies in the
VIPERS spectroscopic survey. The large volume explored by VIPERS will enable a
unique investigation of the relationship between the galaxy properties and
their environment (density field and cosmic web) at high redshift (0.5 < z <
1.2). In this paper, we present the observations, the data reductions and the
build-up of the multi-color catalogues. The CFHTLS-T0007 (gri-{\chi}^2) images
are used as reference to detect and measure the -band photometry, while
the T0007 u-selected sources are used as priors to perform the GALEX photometry
based on a dedicated software (EMphot). Our final sample reaches ~25
(at 5{\sigma}) and ~22 (at 3{\sigma}). The large spectroscopic sample
(~51,000 spectroscopic redshifts) allows us to highlight the robustness of our
star/galaxy separation, and the reliability of our photometric redshifts with a
typical accuracy 0.04 and a catastrophic failure rate {\eta} <
2% down to i~23. We present various tests on the band completeness and
photometric redshift accuracy by comparing with existing, overlapping deep
photometric catalogues. Finally, we discuss the BzK sample of passive and
active galaxies at high redshift and the evolution of galaxy morphology in the
(NUV-r) vs (r-K_s) diagram at low redshift (z < 0.25) thanks to the high image
quality of the CFHTLS. The images, catalogues and photometric redshifts for 1.5
million sources (down to 25 or 22) are released and
available at this URL: http://cesam.lam.fr/vipers-mls/Comment: 14 pages, 16 figures. Accepted for publication in A&A. Version to be
publishe
A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. redshift evolution. Our model
describes consistently the ensemble of galaxies including intrinsic properties
(age, metallicity, star-formation history), dust-attenuation, and optical
emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in
local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This
demonstrates the potential of using "local high-z analogs" to investigate the
spectroscopic properties and relations of galaxies in the re-ionization epoch.Comment: 18 pages, 11 figures, 3 table
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