23 research outputs found
Observational evidence of a slow downfall of star formation efficiency in massive galaxies during the last 10 Gyr
In this paper we study the causes of the reported mass-dependence of the
slope of SFR-M* relation, the so-called "Main Sequence" of star-forming
galaxies, and discuss its implication on the physical processes that shaped the
star formation history of massive galaxies over cosmic time. We use the CANDELS
near-IR imaging from the Hubble Space Telescope to perform the bulge-to-disk
decomposition of distant galaxies and measure for the first time the slope of
the SFR-Mdisk relation at z=1. We find that this relation follows very closely
the shape of the SFR-M* correlation, still with a pronounced flattening at the
high-mass end. This is clearly excluding, at least at z=1, the secular growth
of quiescent bulges in star-forming galaxies as the main driver for the change
of slope of the Main Sequence. Then, by stacking the Herschel data available in
the CANDELS field, we estimate the total gas mass and the star formation
efficiency at different positions on the SFR-M* relation. We find that the
relatively low SFRs observed in massive galaxies (M* > 5e10 Msun) are caused by
a decreased star formation efficiency, by up to a factor of 3 as compared to
lower stellar mass galaxies, and not by a reduced gas content. The trend at the
lowest masses is likely linked to the dominance of atomic over molecular gas.
We argue that this stellar-mass-dependent SFE can explain the varying slope of
the Main Sequence since z=1.5, hence over 70% of the Hubble time. The drop of
SFE occurs at lower masses in the local Universe (M* > 2e10 Msun) and is not
present at z=2. Altogether this provides evidence for a slow downfall of the
star formation efficiency in massive Main Sequence galaxies. The resulting loss
of star formation is found to be rising starting from z=2 to reach a level
comparable to the mass growth of the quiescent population by z=1. We finally
discuss the possible physical origin of this phenomenon.Comment: 21 pages, 15 figures, accepted for publication in A&
On the dust temperatures of high redshift galaxies
Dust temperature is an important property of the interstellar medium (ISM) of
galaxies. It is required when converting (sub)millimeter broadband flux to
total infrared luminosity (L_IR), and hence star formation rate, in high-z
galaxies. However, different definitions of dust temperatures have been used in
the literature, leading to different physical interpretations of how ISM
conditions change with, e.g., redshift and star formation rate. In this paper,
we analyse the dust temperatures of massive (M* > 10^10 Msun) z=2-6 galaxies
with the help of high-resolution cosmological simulations from the Feedback in
Realistic Environments (FIRE) project. At z~2, our simulations successfully
predict dust temperatures in good agreement with observations. We find that
dust temperatures based on the peak emission wavelength increase with redshift,
in line with the higher star formation activity at higher redshift, and are
strongly correlated with the specific star formation rate. In contrast, the
mass-weighted dust temperature does not strongly evolve with redshift over
z=2-6 at fixed IR luminosity but is tightly correlated with L_IR at fixed z.
The mass-weighted temperature is important for accurately estimating the total
dust mass. We also analyse an 'equivalent' dust temperature for converting
(sub)millimeter flux density to total IR luminosity, and provide a fitting
formula as a function of redshift and dust-to-metal ratio. We find that
galaxies of higher equivalent (or higher peak) dust temperature ('warmer dust')
do not necessarily have higher mass-weighted temperatures. A 'two-phase'
picture for interstellar dust can explain the different scaling relations of
the various dust temperatures.Comment: 26 pages, 15 figures, accepted for publication in MNRA
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
AGN Emission Line Diagnostics and the Mass-Metallicity Relation up to Redshift z~2: the Impact of Selection Effects and Evolution
Emission line diagnostic diagrams probing the ionization sources in galaxies,
such as the Baldwin-Phillips-Terlevich (BPT) diagram, have been used
extensively to distinguish AGN from purely star-forming galaxies. Yet, they
remain poorly understood at higher redshifts. We shed light on this issue with
an empirical approach based on a z~0 reference sample built from ~300,000 SDSS
galaxies, from which we mimic selection effects due to typical emission line
detection limits at higher redshift. We combine this low-redshift reference
sample with a simple prescription for luminosity evolution of the global galaxy
population to predict the loci of high-redshift galaxies on the BPT and
Mass-Excitation (MEx) diagnostic diagrams. The predicted bivariate
distributions agree remarkably well with direct observations of galaxies out to
z~1.5, including the observed stellar mass-metallicity (MZ) relation evolution.
As a result, we infer that high-redshift star-forming galaxies are consistent
with having "normal" ISM properties out to z~1.5, after accounting for
selection effects and line luminosity evolution. Namely, their optical line
ratios and gas-phase metallicities are comparable to that of low-redshift
galaxies with equivalent emission-line luminosities. In contrast, AGN
narrow-line regions may show a shift toward lower metallicities at higher
redshift. While a physical evolution of the ISM conditions is not ruled out for
purely star-forming galaxies, and may be more important starting at z>2, we
find that reliably quantifying this evolution is hindered by selections
effects. The recipes provided here may serve as a basis for future studies
toward this goal. Code to predict the loci of galaxies on the BPT and MEx
diagnostic diagrams, and the MZ relation as a function of emission line
luminosity limits, is made publicly available.Comment: Main article: 15 pages, 7 figures; Appendix: 13 pages, 11 figures.
Revisions: Paper now accepted for publication in the Astrophysical Journal
(same scientific content as previous arXiv version). IDL routines to make
empirical predictions on the BPT, MEx, and M-Z plane are now released at
https://sites.google.com/site/agndiagnostics/home/me
On the dust temperatures of high-redshift galaxies
Dust temperature is an important property of the interstellar medium (ISM) of galaxies. It is required when converting (sub)millimetre broad-band flux to total infrared luminosity (LIR), and hence star formation rate, in high-redshift galaxies. However, different definitions of dust temperatures have been used in the literature, leading to different physical interpretations of how ISM conditions change with, e.g. redshift and star formation rate. In this paper, we analyse the dust temperatures of massive (â Mstar>1010Mââ ) z = 2â6 galaxies with the help of high-resolution cosmological simulations from the Feedback in Realistic Environments (fire) project. At z ⌠2, our simulations successfully predict dust temperatures in good agreement with observations. We find that dust temperatures based on the peak emission wavelength increase with redshift, in line with the higher star formation activity at higher redshift, and are strongly correlated with the specific star formation rate. In contrast, the mass-weighted dust temperature, which is required to accurately estimate the total dust mass, does not strongly evolve with redshift over z = 2â6 at fixed IR luminosity but is tightly correlated with LIR at fixed zâ . We also analyse an âequivalentâ dust temperature for converting (sub)millimetre flux density to total IR luminosity, and provide a fitting formula as a function of redshift and dust-to-metal ratio. We find that galaxies of higher equivalent (or higher peak) dust temperature (âwarmer dustâ) do not necessarily have higher mass-weighted temperatures. A âtwo-phaseâ picture for interstellar dust can explain the different scaling relations of the various dust temperatures
The Herschel view of the dominant mode of galaxy growth from z=4 to the present day
We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS-North, GOODS-South, UDS and COSMOS obtained within the GOODS-Herschel and CANDELS-Herschel key programs. The picture provided by 10497 individual far-infrared detections is supplemented by the stacking analysis of a mass-complete sample of 62361 star-forming galaxies from the CANDELS-HST H band-selected catalogs and from two deep ground-based Ks band-selected catalogs in the GOODS-North and the COSMOS-wide fields, in order to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called "scatter stacking" that may be easily generalized to other experiments. We demonstrate that galaxies of all masses from z=4 to 0 follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the logSFR-logM* relation with evidence for a flattening of the main sequence at high masses (log(M*/Msun) > 10.5) that becomes less prominent with increasing redshift and almost vanishes by z~2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies. Within the main sequence, we measure a non varying SFR dispersion of 0.3 dex. The specific SFR (sSFR=SFR/M*) of star-forming galaxies is found to continuously increase from z=0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and show that more than 2/3 of present-day stars must have formed in a regime dominated by the main sequence mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 Gyr
Revealing environmental dependence of molecular gas content in a distant X-ray cluster at z=2.51
We present a census of the molecular gas properties of galaxies in the most distant known X-ray cluster, CLJ1001, at z=2.51, using deep observations of CO(1-0) with JVLA. In total 14 cluster members with Mâ>1010.5Mâ are detected, including all the massive star-forming members within the virial radius, providing the largest galaxy sample in a single cluster at z>2 with CO(1-0) measurements. We find a large variety in the gas content of these cluster galaxies, which is correlated with their relative positions (or accretion states), with those closer to the cluster core being increasingly gas-poor. Moreover, despite their low gas content, the galaxies in the cluster center exhibit an elevated star formation efficiency (SFE=SFR/Mgas) compared to field galaxies, suggesting that the suppression on the SFR is likely delayed compared to that on the gas content. Their gas depletion time is around tdepâŒ400 Myrs, comparable to the cluster dynamical time. This implies that they will likely consume all their gas within a single orbit around the cluster center, and form a passive cluster core by zâŒ2. This result is one of the first direct pieces of evidence for the influence of environment on the gas reservoirs and SFE of z>2 cluster galaxies, thereby providing new insights into the rapid formation and quenching of the most massive galaxies in the early universe
DISCOVERY OF A GALAXY CLUSTER WITH A VIOLENTLY STARBURSTING CORE AT z=2.506
We report the discovery of a remarkable concentration of massive galaxies with extended X-ray emission at z(spec) = 2.506, which contains 11 massive (M-* greater than or similar to 10(11) M-circle dot) galaxies in the central 80 kpc region (11.6 sigma overdensity). We have spectroscopically confirmed 17 member galaxies with 11 from CO and the remaining ones from Ha. The X-ray luminosity, stellar mass content, and velocity dispersion all point to a collapsed, cluster-sized dark matter halo with mass M-200c = 10(13.9 +/- 0.2) M-circle dot, making it the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive galaxies classified as quiescent. The star formation rate (SFR) in the 80 kpc core reaches similar to 3400 M-circle dot yr(-1) with a. gas depletion time of similar to 200 Myr, suggesting that we caught this cluster in rapid build-up of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst fraction (similar to 25%, compared to 3%-5% in the field). The presence of both a collapsed, cluster-sized halo and a predominant population of massive SFGs suggests that this structure could represent an important transition phase between protoclusters and mature clusters. It provides evidence that the main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster, providing new insights into massive cluster formation at early epochs. The large integrated stellar mass at such high redshift challenges our understanding of massive cluster formation.Peer reviewe