252 research outputs found
A Window On The Earliest Star Formation: Extreme Photoionization Conditions of a High-Ionization, Low-Metallicity Lensed Galaxy at z~2
We report new observations of SL2SJ021737-051329, a lens system consisting of
a bright arc at z=1.84435, magnified ~17x by a massive galaxy at z=0.65.
SL2SJ0217 is a low-mass (M <10^9 M*), low-metallicity (Z~1/20 Z*) galaxy, with
extreme star-forming conditions that produce strong nebular UV emission lines
in the absence of any apparent outflows. Here we present several notable
features from rest-frame UV Keck/LRIS spectroscopy: (1) Very strong narrow
emission lines are measured for CIV 1548,1550, HeII 1640, OIII] 1661,1666,
SiIII] 1883,1892, and CIII] 1907,1909. (2) Double-peaked LyA emission is
observed with a dominant blue peak and centered near the systemic velocity. (3)
The low- and high-ionization absorption features indicate very little or no
outflowing gas along the sightline to the lensed galaxy. The relative emission
line strengths can be reproduced with a very high-ionization, low-metallicity
starburst with binaries, with the exception of He \ii, which indicates an
additional ionization source is needed. We rule out large contributions from
AGN and shocks to the photoionization budget, suggesting that the emission
features requiring the hardest radiation field likely result from extreme
stellar populations that are beyond the capabilities of current models.
Therefore, SL2S0217 serves as a template for the extreme conditions that are
important for reionization and thought to be more common in the early Universe.Comment: 28 pages, 16 figures, 8 tables, re-submitted to ApJ, comments welcom
FIREWORKS U38-to-24 micron photometry of the GOODS-CDFS: multi-wavelength catalog and total IR properties of distant Ks-selected galaxies
We present a Ks-selected catalog, dubbed FIREWORKS, for the Chandra Deep
Field South (CDFS) containing photometry in U_38, B_435, B, V, V_606, R, i_775,
I, z_850, J, H, Ks, [3.6 um], [4.5 um], [5.8 um], [8.0 um], and the MIPS [24
um] band. The imaging has a typical Ks limit of 24.3 mag (5 sigma, AB) and
coverage over 113 arcmin^2 in all bands and 138 arcmin^2 in all bands but H. We
cross-correlate our catalog with the 1 Ms X-ray catalog by Giacconi et al.
(2002) and with all available spectroscopic redshifts to date. We find and
explain systematic differences in a comparison with the 'z_850 + Ks'-selected
GOODS-MUSIC catalog that covers ~90% of the field. We exploit the U38-to-24
micron photometry to determine which Ks-selected galaxies at 1.5<z<2.5 have the
brightest total IR luminosities and which galaxies contribute most to the
integrated total IR emission. The answer to both questions is that red galaxies
are dominating in the IR. This is true no matter whether color is defined in
the rest-frame UV, optical, or optical-to-NIR. We do find however that among
the reddest galaxies in the rest-frame optical, there is a population of
sources with only little mid-IR emission, suggesting a quiescent nature.Comment: Accepted for publication in the Astrophysical Journal, 20 pages, 10
figures, reference to website correcte
What are the Progenitors of Compace, Massive, Quiescent Galaxies at z (equals) 2.3? The Population of Massive Galaxies at z (greater than) 3 From NMBS AND CANDELS
Using public data from the NEWFIRM Medium-Band Survey (NMBS) and the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS), we investigate the population of massive galaxies at z > 3. The main aim of this work is to identify the potential progenitors of z 2 compact, massive, quiescent galaxies (CMQGs), furthering our understanding of the onset and evolution of massive galaxies. Our work is enabled by high-resolution images from CANDELS data and accurate photometric redshifts, stellar masses, and star formation rates (SFRs) from 37-band NMBS photometry. The total number of massive galaxies at z > 3 is consistent with the number of massive, quiescent galaxies (MQGs) at z 2, implying that the SFRs for all of these galaxies must be much lower by z 2. We discover four CMQGs at z > 3, pushing back the time for which such galaxies have been observed. However, the volume density for these galaxies is significantly less than that of galaxies at z 1010.6M; these galaxies are likely to become members of the massive, quiescent, compact galaxy population at z 2. We evolve the stellar masses and SFRs of each individual z > 3 galaxy adopting five different star formation histories (SFHs) and studying the resulting population of massive galaxies at z = 2.3. We find that declining or truncated SFHs are necessary to match the observed number density of MQGs at z 2, whereas a constant delayed-exponential SFH would result in a number density significantly smaller than observed. All of our assumed SFHs imply number densities of CMQGs at z 2 that are consistent with the observed number density. Better agreement with the observed number density of CMQGs at z 2 is obtained if merging is included in the analysis and better still if star formation quenching is assumed to shortly follow the merging event, as implied by recent models of the formation of MQGs
JWST/NIRSpec Measurements of the Relationships Between Nebular Emission-line Ratios and Stellar Mass at z~3-6
We analyze the rest-optical emission-line ratios of star-forming galaxies at
2.7<=z<6.5 drawn from the Cosmic Evolution Early Release Science (CEERS)
Survey, and their relationships with stellar mass (M_*). Our analysis includes
both line ratios based on the [NII]6583 feature -- [NII]6583/Ha,
([OIII]5007/Hb)/([NII]6583/Ha) (O3N2), and [NII]6583/[OII]3727 -- and those
those featuring alpha elements -- [OIII]5007/Hb, [OIII]5007/[OII]3727 (O_32),
([OIII]4959,5007+[OII]3727)/Hb (R_23), and [NeIII]3869/[OII]3727. Given the
typical flux levels of [NII]6583 and [NeIII]3869, which are undetected in the
majority of individual CEERS galaxies at 2.7<=z<6.5, we construct composite
spectra in bins of M_* and redshift. Using these composite spectra, we compare
the relationships between emission-line ratios and M_* at 2.7<=z<6.5 with those
observed at lower redshift. While there is significant evolution towards higher
excitation (e.g., higher [OIII]5007/Hb, O_32, O3N2), and weaker nitrogen
emission (e.g., lower [NII]6583/Ha and [NII]6583/[OII]3727) between z~0 and
z~3, we find in most cases that there is no significant evolution in the
relationship between line ratio and M_* beyond z~3. The [NeIII]3869/[OII]3727
ratio is anomalous in showing evidence for significant elevation at 4.0<=z<6.5
at fixed mass, relative to z~3.3. Collectively, however, our empirical results
suggest that there is no significant evolution in the mass-metallicity
relationship at 2.7<=z<6.5. Representative galaxy samples and metallicity
calibrations based on existing and upcoming JWST/NIRSpec observations will be
required to translate these empirical scaling relations into ones tracing
chemical enrichment and gas cycling, and distinguish among the descriptions of
star-formation feedback in simulations of galaxy formation at z>3.Comment: 10 pages, 4 figures, ApJL, in pres
EAZY: A Fast, Public Photometric Redshift Code
We describe a new program for determining photometric redshifts, dubbed EAZY.
The program is optimized for cases where spectroscopic redshifts are not
available, or only available for a biased subset of the galaxies. The code
combines features from various existing codes: it can fit linear combinations
of templates, it includes optional flux- and redshift-based priors, and its
user interface is modeled on the popular HYPERZ code. A novel feature is that
the default template set, as well as the default functional forms of the
priors, are not based on (usually highly biased) spectroscopic samples, but on
semi-analytical models. Furthermore, template mismatch is addressed by a novel
rest-frame template error function. This function gives different wavelength
regions different weights, and ensures that the formal redshift uncertainties
are realistic. We introduce a redshift quality parameter, Q_z, that provides a
robust estimate of the reliability of the photometric redshift estimate.
Despite the fact that EAZY is not "trained" on spectroscopic samples, the code
(with default parameters) performs very well on existing public datasets. For
K-selected samples in CDF-South and other deep fields we find a 1-sigma scatter
in dz/(1+z) of 0.034, and we provide updated photometric redshift catalogs for
the FIRES, MUSYC, and FIREWORKS surveys.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 11
figures. Code available at http://www.astro.yale.edu/eazy
JWST/NIRSpec Balmer-line Measurements of Star Formation and Dust Attenuation at z~3-6
We present an analysis of the star-formation rates (SFRs) and dust
attenuation properties of star-forming galaxies at drawn from
the Cosmic Evolution Early Release Science (CEERS) Survey. Our analysis is
based on {\it JWST}/NIRSpec Micro-Shutter Assembly (MSA)
spectroscopic observations covering approximately m. Our primary
rest-frame optical spectroscopic measurements are H/H Balmer
decrements, which we use as an indicator of nebular dust attenuation. In turn,
we use Balmer decrements to obtain dust-corrected H-based SFRs (i.e.,
SFR(H)). We construct the relationship between SFR(H) and
stellar mass () in three bins of redshift (, , and ), which represents the first time the star-forming
main sequence has been traced at these redshifts using direct spectroscopic
measurements of Balmer emission as a proxy for SFR. In tracing the relationship
between SFR(H) and back to such early times (), it is
essential to use a conversion factor between H and SFR that accounts
for the subsolar metallicity prevalent among distant galaxies. We also use
measured Balmer decrements to investigate the relationship between dust
attenuation and stellar mass out to . The lack of significant redshift
evolution in attenuation at fixed stellar mass, previously confirmed using
Balmer decrements out to , appears to hold out to . Given
the rapidly evolving gas, dust, and metal content of star-forming galaxies at
fixed mass, this lack of significant evolution in attenuation provides an
ongoing challenge to explain.Comment: 9 pages, 4 figures, ApJ, in pres
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
HST Grism Observations of a Gravitationally Lensed Redshift 10 Galaxy
We present deep spectroscopic observations of a Lyman-break galaxy candidate
(hereafter MACS1149-JD) at with the Space
Telescope () WFC3/IR grisms. The grism observations were taken at
4 distinct position angles, totaling 34 orbits with the G141 grism, although
only 19 of the orbits are relatively uncontaminated along the trace of
MACS1149-JD. We fit a 3-parameter (, F160W mag, and Ly equivalent
width) Lyman-break galaxy template to the three least contaminated grism
position angles using an MCMC approach. The grism data alone are best fit with
a redshift of ( confidence), in
good agreement with our photometric estimate of
( confidence). Our analysis
rules out Lyman-alpha emission from MACS1149-JD above a equivalent
width of 21 \AA{}, consistent with a highly neutral IGM. We explore a scenario
where the red /IRAC color of the galaxy
previously pointed out in the literature is due to strong rest-frame optical
emission lines from a very young stellar population rather than a 4000 \AA{}
break. We find that while this can provide an explanation for the observed IRAC
color, it requires a lower redshift (), which is less preferred
by the imaging data. The grism data are consistent with both
scenarios, indicating that the red IRAC color can still be explained by a 4000
\AA{} break, characteristic of a relatively evolved stellar population. In this
interpretation, the photometry indicate that a Myr stellar
population is already present in this galaxy only after
the Big Bang.Comment: Accepted to ApJ. This is the accepted versio
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