20 research outputs found
How to Measure Galaxy Star Formation Histories. II. Nonparametric Models
Nonparametric star formation histories (SFHs) have long promised to be the
`gold standard' for galaxy spectral energy distribution (SED) modeling as they
are flexible enough to describe the full diversity of SFH shapes, whereas
parametric models rule out a significant fraction of these shapes {\it a
priori}. However, this flexibility is not fully constrained even with
high-quality observations, making it critical to choose a well-motivated prior.
Here, we use the SED-fitting code \texttt{Prospector} to explore the effect of
different nonparametric priors by fitting SFHs to mock UV-IR photometry
generated from a diverse set of input SFHs. First, we confirm that
nonparametric SFHs recover input SFHs with less bias and return more accurate
errors than do parametric SFHs. We further find that, while nonparametric SFHs
robustly recover the overall shape of the input SFH, the primary determinant of
the size and shape of the posterior star formation rate (SFR) as a function of
time is the choice of prior, rather than the photometric noise. As a practical
demonstration, we fit the UV-IR photometry of 6000 galaxies from the GAMA
survey and measure inter-prior scatters in mass (0.1 dex), SFR (0.8 dex), and mass-weighted ages (0.2 dex), with the bluest
star-forming galaxies showing the most sensitivity. An important distinguishing
characteristic for nonparametric models is the characteristic timescale for
changes in SFR(t). This difference controls whether galaxies are assembled in
bursts or in steady-state star formation, corresponding respectively to
(feedback-dominated/accretion-dominated) models of galaxy formation and to
(larger/smaller) confidence intervals derived from SED-fitting. High-quality
spectroscopy has the potential to further distinguish between these proposed
models of SFR(t).Comment: replacing with ApJ accepted versio
How to Measure Galaxy Star Formation Histories. I. Parametric Models
Parametric models for galaxy star-formation histories (SFHs) are widely used,
though they are known to impose strong priors on physical parameters. This has
consequences for measurements of the galaxy stellar-mass function (GSMF),
star-formation-rate density (SFRD) and star-forming main sequence (SFMS). We
investigate the effects of the exponentially declining, delayed exponentially
declining, lognormal and double power law SFH models using BAGPIPES. We
demonstrate that each of these models imposes strong priors on specific
star-formation rates (sSFRs), potentially biasing the SFMS, and also imposes a
strong prior preference for young stellar populations. We show that stellar
mass, SFR and mass-weighted age inferences from high-quality mock photometry
vary with the choice of SFH model by at least 0.1, 0.3 and 0.2 dex
respectively. However the biases with respect to the true values depend more on
the true SFH shape than the choice of model. We also demonstrate that
photometric data cannot discriminate between SFH models, meaning it is
important to perform independent tests to find well-motivated priors. We
finally fit a low-redshift, volume-complete sample of galaxies from the Galaxy
and Mass Assembly (GAMA) Survey with each model. We demonstrate that our
stellar masses and SFRs at redshift, are consistent with other
analyses. However, our inferred cosmic SFRDs peak at , approximately
6 Gyr later than direct observations suggest, meaning our mass-weighted ages
are significantly underestimated. This makes the use of parametric SFH models
for understanding mass assembly in galaxies challenging. In a companion paper
we consider non-parametric SFH models.Comment: 20 pages, 12 figures, ApJ accepte
A massive quiescent galaxy at redshift 4.658
A. C. Carnall thanks the Leverhulme Trust for their support via a Leverhulme Early Career Fellowship. R. J. McLure, J. S. Dunlop, D. J. McLeod, V. Wild, R. Begley, C. T. Donnan and M. L. Hamadouche acknowledge the support of the Science and Technology Facilities Council. F. Cullen acknowledges support from a UKRI Frontier Research Guarantee Grant (grant reference EP/X021025/1). A. Cimatti acknowledges support from the grant PRIN MIUR 2017 - 20173ML3WW 001.The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics (1; 2; 3; 4; 5). The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years (6; 7; 8). Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang (9; 10), however, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of M∗ = 3.8 ± 0.2 × 1010 M⊙, which formed over a ≃ 200 Myr period before this galaxy quenched its star formation activity at z=6.5+0.2−0.5, when the Universe was ≃ 800 million years old. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies.PostprintPeer reviewe
HST Imaging of the Ionizing Radiation from a Star-forming Galaxy at z = 3.794
We report on the HST detection of the Lyman-continuum (LyC) radiation emitted
by a galaxy at redshift z=3.794, dubbed Ion1 (Vanzella et al. 2012). The LyC
from Ion1 is detected at rest-frame wavelength 820890 \AA with HST
WFC3/UVIS in the F410M band ( magnitude (AB), peak SNR =
4.17 in a circular aperture with radius r = 0.12'') and at 700830 \AA
with the VLT/VIMOS in the U-band ( magnitude (AB), peak SNR
= 6.7 with a r = 0.6'' aperture). A 20-hr VLT/VIMOS spectrum shows low- and
high-ionization interstellar metal absorption lines, the P-Cygni profile of CIV
and Ly in absorption. The latter spectral feature differs from what
observed in known LyC emitters, which show strong Ly emission. An HST
far-UV color map reveals that the LyC emission escapes from a region of the
galaxy that is bluer than the rest, presumably because of lower dust
obscuration. The F410M image shows that the centroid of the LyC emission is
offset from the centroid of the non-ionizing UV emission by 0.12''0.03'',
corresponding to 0.850.21 kpc (physical), and that its morphology is
likely moderately resolved. These morphological characteristics favor a
scenario where the LyC photons produced by massive stars escape from low HI
column-density "cavities" in the ISM, possibly carved by stellar winds and/or
supernova. We also collect the VIMOS U-band images of a sample of 107
Lyman-break galaxies with spectroscopic redshifts at , i.e.
sampling the LyC, and stack them with inverse-variance weights. No LyC emission
is detected in the stacked image, resulting in a 32.5 magnitude (AB) flux limit
(1) and an upper limit of absolute LyC escape fraction . LyC emitters like Ion1 are very likely at the bright-end of the LyC
luminosity function.Comment: 24 pages, 13 figures, accepted for publication in Ap
Cosmic Vine: A z=3.44 Large-Scale Structure Hosting Massive Quiescent Galaxies
We report the discovery of a large-scale structure at z=3.44 revealed by JWST
data in the EGS field. This structure, dubbed "Cosmic Vine", consists of 20
galaxies with spectroscopic redshifts at and six galaxy
overdensities with consistent photometric redshifts, making up a vine-like
structure extending over a ~4x0.2 pMpc^2 area. The two most massive galaxies
(M*~10^10.9 Msun) of the Cosmic Vine are found to be quiescent with
bulge-dominated morphologies (). Comparisons with simulations suggest
that the Cosmic Vine would form a cluster with halo mass >10^14 Msun at z=0,
and the two massive galaxies are likely forming the brightest cluster galaxies
(BCGs). The results unambiguously reveal that massive quiescent galaxies can
form in growing large-scale structures at z>3, thus disfavoring the
environmental quenching mechanisms that require a virialized cluster core.
Instead, as suggested by the interacting and bulge-dominated morphologies, the
two galaxies are likely quenched by merger-triggered starburst or AGN feedback
before falling into a cluster core. Moreover, we found that the observed
specific star formation rates of massive quiescent galaxies in z>3 dense
environments are two orders of magnitude lower than that of the BCGs in the
TNG300 simulation. This discrepancy potentially poses a challenge to the models
of massive cluster galaxy formation. Future studies comparing a large sample
with dedicated cluster simulations are required to solve the problem.Comment: Submitted to A&
The stellar metallicities of massive quiescent galaxies at 1.0 < z < 1.3 from KMOS+VANDELS
We present a rest-frame UV-optical stacked spectrum representative of massive
quiescent galaxies at . The stack
is constructed using VANDELS survey data, combined with new KMOS observations.
We apply two independent full-spectral-fitting approaches, measuring a total
metallicity, [Z/H]= with Bagpipes, and [Z/H]= with
Alf, a fall of dex compared with the local Universe. We also
measure an iron abundance, [Fe/H] =, a fall of dex
compared with the the local Universe. We measure the alpha enhancement via the
magnesium abundance, obtaining [Mg/Fe]=0.12, consistent with
similar-mass galaxies in the local Universe, indicating no evolution in the
average alpha enhancement of log quiescent galaxies over
the last Gyr. This suggests the very high alpha enhancements recently
reported for several bright quiescent galaxies are due to their
extreme masses, log, rather than being typical
of the population. The metallicity evolution we observe with
redshift (falling [Z/H], [Fe/H], constant [Mg/Fe]) is consistent with recent
studies. We recover a mean stellar age of Gyr,
corresponding to a formation redshift, z_\rm{form}=2.4^{+0.6}_{-0.3}. Recent
studies have obtained varying average formation redshifts for
massive quiescent galaxies, and, as these studies report consistent
metallicities, we identify different star-formation-history models as the most
likely cause. Larger spectroscopic samples from upcoming ground-based
instruments will provide precise constraints on ages and metallicities at
. Combining these with precise JWST quiescent-galaxy
stellar-mass functions will provide an independent test of formation redshifts
derived from spectral fitting.Comment: 16 pages, 3 figures, accepted for publication in Ap
High-velocity outflows in massive post-starburst galaxies at z > 1
We investigate the prevalence of galactic-scale outflows in post-starburst (PSB) galaxies at high redshift (1 1010M⊙) PSBs at z > 1, there is clear evidence for a strong blue-shifted component to the Mg ii absorption feature, indicative of high-velocity outflows (vout∼1150±160kms−1) in the interstellar medium. We conclude that such outflows are typical in massive PSBs at this epoch, and potentially represent the residual signature of a feedback process that quenched these galaxies. Using full spectral fitting, we also obtain a typical stellar velocity dispersion σ* for these PSBs of ∼200kms−1, which confirms they are intrinsically massive in nature (dynamical mass Md∼1011M⊙). Given that these high-z PSBs are also exceptionally compact (re ∼ 1–2kpc) and spheroidal (Sérsic index n ∼ 3), we propose that the outflowing winds may have been launched during a recent compaction event (e.g. major merger or disc collapse) that triggered either a centralized starburst or active galactic nuclei (AGN) activity. Finally, we find no evidence for AGN signatures in the optical spectra of these PSBs, suggesting they were either quenched by stellar feedback from the starburst itself, or that if AGN feedback is responsible, the AGN episode that triggered quenching does not linger into the post-starburst phase.Publisher PDFPeer reviewe
RELICS: Properties of z>5.5 Galaxies Inferred from Spitzer and Hubble Imaging Including A Candidate z~6.8 Strong [OIII] Emitter
We present constraints on the physical properties (including stellar mass,
age, and star formation rate) of 207 galaxy candidates
from the Reionization Lensing Cluster Survey (RELICS) and companion
Spitzer-RELICS surveys. We measure photometry using T-PHOT and perform spectral
energy distribution fitting using EAY and BAGPIPES. Of the 207 candidates
for which we could successfully measure Spitzer fluxes, 23 were demoted to
likely low redshift (). Among the remaining high redshift candidates, we
find intrinsic stellar masses between and
, and rest-frame UV absolute magnitudes between
and mag. While our sample is mostly comprised of
galaxies, there are a number of brighter objects in the
sample, extending to . The galaxies in our sample span
approximately four orders of magnitude in stellar mass and star-formation
rates, and exhibit ages ranging from maximally young to maximally old. We
highlight 11 galaxies which have detections in Spitzer/IRAC imaging and
redshift estimates , several of which show evidence for some
combination of evolved stellar populations, large contributions of nebular
emission lines, and/or dust. Among these is PLCKG287+32-2013, one of the
brightest candidates known (AB mag 24.9) with a Spitzer 3.6m flux
excess suggesting strong [OIII] + H- emission (1000\AA\ rest-frame
equivalent width). We discuss the possible uses and limits of our sample and
present a public catalog of Hubble 0.4--1.6m + Spitzer 3.6m and
4.5m photometry along with physical property estimates for all 207 objects
in the sample. Because of their apparent brightnesses, high redshifts, and
variety of stellar populations, these objects are excellent targets for
follow-up with James Webb Space Telescope.Comment: 20 pages, 9 figure
Spectroscopic verification of very luminous galaxy candidates in the early universe
During the first 500 million years of cosmic history, the first stars and
galaxies formed and seeded the cosmos with heavy elements. These early galaxies
illuminated the transition from the cosmic "dark ages" to the reionization of
the intergalactic medium. This transitional period has been largely
inaccessible to direct observation until the recent commissioning of JWST,
which has extended our observational reach into that epoch. Excitingly, the
first JWST science observations uncovered a surprisingly high abundance of
early star-forming galaxies. However, the distances (redshifts) of these
galaxies were, by necessity, estimated from multi-band photometry. Photometric
redshifts, while generally robust, can suffer from uncertainties and/or
degeneracies. Spectroscopic measurements of the precise redshifts are required
to validate these sources and to reliably quantify their space densities,
stellar masses, and star formation rates, which provide powerful constraints on
galaxy formation models and cosmology. Here we present the results of JWST
follow-up spectroscopy of a small sample of galaxies suspected to be amongst
the most distant yet observed. We confirm redshifts z > 10 for two galaxies,
including one of the first bright JWST-discovered candidates with z = 11.4, and
show that another galaxy with suggested z ~ 16 instead has z = 4.9, with strong
emission lines that mimic the expected colors of more distant objects. These
results reinforce the evidence for the rapid production of luminous galaxies in
the very young Universe, while also highlighting the necessity of spectroscopic
verification for remarkable candidates.Comment: Submitted to Natur
JWST reveals a possible galaxy merger in triply-lensed MACS0647JD
MACS0647JD is a triply-lensed galaxy originally discovered with
the Hubble Space Telescope. Here we report new JWST imaging, which clearly
resolves MACS0647JD as having two components that are either merging
galaxies or stellar complexes within a single galaxy. Both are very small, with
stellar masses and radii . The brighter
larger component "A" is intrinsically very blue (), likely due
to very recent star formation and no dust, and is spatially extended with an
effective radius . The smaller component "B" appears redder
(), likely because it is older () with mild dust
extinction (), and a smaller radius . We
identify galaxies with similar colors in a high-redshift simulation, finding
their star formation histories to be out of phase. With an estimated stellar
mass ratio of roughly 2:1 and physical projected separation ,
we may be witnessing a galaxy merger 400 million years after the Big Bang. We
also identify a candidate companion galaxy C away, likely
destined to merge with galaxies A and B. The combined light from galaxies A+B
is magnified by factors of 8, 5, and 2 in three lensed images JD1, 2, and
3 with F356W fluxes , , (AB mag 25.1, 25.6, 26.6).
MACS0647JD is significantly brighter than other galaxies recently discovered
at similar redshifts with JWST. Without magnification, it would have AB mag
27.3 (). With a high confidence level, we obtain a photometric
redshift of based on photometry measured in 6 NIRCam filters
spanning , out to rest-frame. JWST NIRSpec
observations planned for January 2023 will deliver a spectroscopic redshift and
a more detailed study of the physical properties of MACS0647JD.Comment: 27 pages, 14 figures, submitted to Natur