ABSTRACT We report the detection of C ii λ158 $\mu$m emission from a system of three closely separated sources in the Cosmic Evolution Survey (COSMOS) field at z ∼ 4.56 , as part of the Atacama Large ...Millimeter/submillimeter Array (ALMA) Large Program to INvestigate C ii at Early times (ALPINE). The two most luminous sources are closely associated, both spatially (1.6 arcsec ∼ 11 kpc) and in velocity (∼100 km s−1), while the third source is slightly more distant (2.8 arcsec ∼ 18 kpc, ∼300 km s−1). The second most luminous source features a slight velocity gradient, while no significant velocity gradient is seen in the other two sources. Using the observed C ii luminosities, we derive a total log$_{10}(\rm SFR_{C\,{\small II}}\, M_{\odot }\, yr^{-1})=2.8\pm 0.2$, which may be split into contributions of 59, 31, and 10 per cent from the central, east, and west sources, respectively. Comparison of these C ii detections to recent zoom-in cosmological simulations suggests an ongoing major merger. We are thus witnessing a system in a major phase of mass build-up by merging, including an ongoing major merger and an upcoming minor merger, which is expected to end up in a single massive galaxy by z ∼ 2.5.
We follow the galaxy stellar mass assembly by morphological and spectral type in the COSMOS 2 deg2 field. We derive the stellar mass functions and stellar mass densities from z = 2 to z = 0.2 using ...196,000 galaxies selected at F 3.6 μm > 1 μJy with accurate photometric redshifts (σ_{(z_phot-z_spec)/(1+z_spec)}=0.008 at i + < 22.5). Using a spectral classification, we find that z ~ 1 is an epoch of transition in the stellar mass assembly of quiescent galaxies. Their stellar mass density increases by 1.1 dex between z = 1.5-2 and z = 0.8-1 (Δt ~ 2.5 Gyr), but only by 0.3 dex between z = 0.8-1 and z ~ 0.1 (Δt ~ 6 Gyr). Then, we add the morphological information and find that 80%-90% of the massive quiescent galaxies (log M ∼ 11) have an elliptical morphology at z < 0.8. Therefore, a dominant mechanism links the shutdown of star formation and the acquisition of an elliptical morphology in massive galaxies. Still, a significant fraction of quiescent galaxies present a Spi/Irr morphology at low mass (40%-60% at log M∼ 9.5), but this fraction is smaller than predicted by semi-analytical models using a "halo quenching" recipe. We also analyze the evolution of star-forming galaxies and split them into "intermediate activity" and "high activity" galaxies. We find that the most massive "high activity" galaxies end their high star formation rate phase first. Finally, the space density of massive star-forming galaxies becomes lower than the space density of massive elliptical galaxies at z < 1. As a consequence, the rate of "wet mergers" involved in the formation of the most massive ellipticals must decline very rapidly at z < 1, which could explain the observed slow down in the assembly of these quiescent and massive sources. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc., under NASA contract NAS 5-26555. Also based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory under Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; and the Canada-France-Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the NRC and CADC of Canada, the CNRS of France, TERAPIX, and the University of Hawaii.
Aims. We investigate the global galaxy evolution over ~12 Gyr (0.05 ≤ z ≤ 4.5), from the far ultraviolet (FUV) luminosity function (LF), luminosity density (LD), and star formation rate density ...(SFRD), using the VIMOS-VLT Deep Survey (VVDS), a single deep galaxy redshift survey with a well controlled selection function. Methods. We combine the VVDS Deep (17.5 ≤ IAB ≤ 24.0) and Ultra-Deep (23.00 ≤ i'AB ≤ 24.75) redshift surveys, totalizing ~11 000 galaxies, to estimate the rest-frame FUV LF and LD, using a wide wavelength range of deep photometry (337 < λ < 2310 nm). We extract the dust attenuation of the FUV radiation, embedded in the well-constrained spectral energy distributions. We then derive the dust-corrected SFRD. Results. We find a constant and flat faint-end slope α in the FUV LF at z < 1.7. At z > 1.7, we set α steepening with (1 + z). The absolute magnitude M*FUV steadily brightens in the entire range 0 < z < 4.5, and at z > 2 it is on average brighter than in the literature, while φ∗ is on average smaller. The evolution of our total LD shows a peak at z ≃ 2, clearly present also when considering all sources of uncertainty. The SFRD history peaks as well at z ≃ 2. It first steadily rises by a factor of ~6 during 2 Gyr (from z = 4.5 to z = 2), and then decreases by a factor of ~12 during 10 Gyr down to z = 0.05. This peak is mainly produced by a similar peak within the population of galaxies with −21.5 ≤ MFUV ≤ − 19.5. As times goes by, the total SFRD is dominated by fainter and fainter galaxies. The mean dust attenuation of the global galaxy population rises fast by 1 mag during 2 Gyr from z ≃ 4.5 to z ~ 2, reaches slowly its maximum at z ≃ 1 (AFUV ≃ 2.2 mag), and then decreases by 1.1 mag during 7 Gyr down to z ≃ 0. Conclusions. We have derived the cosmic SFRD history and the total dust amount in galaxies over a continuous period of ~12 Gyr, using a single homogeneous spectroscopic redshift sample. The presence of a clear peak at z ≃ 2 and a fast rise at z > 2 of the SFRD is compelling for models of galaxy formation. This peak is mainly produced by bright galaxies (L ≳ L*z=2), requiring that significant gas reservoirs still exist at this epoch and are probably replenished by cold accretion and wet mergers, while feedback or quenching processes are not yet strong enough to lower the SF. The dust attenuation maximum is reached ~2 Gyr after the SFRD peak, implying a contribution from the intermediate-mass stars to the dust production at z < 2.
Context. The size of a galaxy encapsulates the signature of the different physical processes driving its evolution. The distribution of galaxy sizes in the Universe as a function of cosmic time is ...therefore a key to understand galaxy evolution. Aims. We aim to measure the average sizes and size distributions of galaxies as they are assembling before the peak in the comoving star formation rate density of the Universe to better understand the evolution of galaxies across cosmic time. Methods. We used a sample of ~1200 galaxies in the COSMOS and ECDFS fields with confirmed spectroscopic redshifts 2 ≤ zspec ≤ 4.5 in the VIMOS Ultra Deep Survey (VUDS), representative of star-forming galaxies with iAB ≤ 25. We first derived galaxy sizes by applying a classical parametric profile-fitting method using GALFIT. We then measured the total pixel area covered by a galaxy above a given surface brightness threshold, which overcomes the difficulty of measuring sizes of galaxies with irregular shapes. We then compared the results obtained for the equivalent circularized radius enclosing 100% of the measured galaxy light r100T ~2.2 to those obtained with the effective radius re,circ measured with GALFIT. Results. We find that the sizes of galaxies computed with our non-parametric approach span a wide range but remain roughly constant on average with a median value r100T ~2.2 kpc for galaxies with 2 <z< 4.5. This is in stark contrast with the strong downward evolution of re with increasing redshift, down to sizes of <1 kpc at z ~ 4.5. We analyze the difference and find that parametric fitting of complex, asymmetric, multicomponent galaxies is severely underestimating their sizes. By comparing r100T with physical parameters obtained through fitting the spectral energy distribution we find that the star-forming galaxies that are the largest at any redshift are, on average, more massive and form more stars. We discover that galaxies present more concentrated light profiles with increasing redshifts. We interpret these results as the signature of several, possibly different, evolutionary paths of galaxies in their early stages of assembly, including major and minor merging or star formation in multiple bright regions.
Context.
The star formation rate density (SFRD) evolution presents an area of great interest in the studies of galaxy evolution and reionization. The current constraints of SFRD at
z
> 5 are based ...on the rest-frame UV luminosity functions with the data from photometric surveys. The VIMOS UltraDeep Survey (VUDS) was designed to observe galaxies at redshifts up to ∼6 and opened a window for measuring SFRD at
z
> 5 from a spectroscopic sample with a well-controlled selection function.
Aims.
We establish a robust statistical description of the star-forming galaxy population at the end of cosmic HI reionization (5.0 ≤
z
≤ 6.6) from a large sample of 49 galaxies with spectroscopically confirmed redshifts. We determine the rest-frame UV and Ly
α
luminosity functions and use them to calculate SFRD at the median redshift of our sample
z
= 5.6.
Methods.
We selected a sample of galaxies at 5.0 ≤
z
spec
≤ 6.6 from the VUDS. We cleaned our sample from low redshift interlopers using ancillary photometric data. We identified galaxies with Ly
α
either in absorption or in emission, at variance with most spectroscopic samples in the literature where Ly
α
emitters (LAE) dominate. We determined luminosity functions using the 1/
V
max
method.
Results.
The galaxies in this redshift range exhibit a large range in their properties. A fraction of our sample shows strong Ly
α
emission, while another fraction shows Ly
α
in absorption. UV-continuum slopes vary with luminosity, with a large dispersion. We find that star-forming galaxies at these redshifts are distributed along the main sequence in the stellar mass vs. SFR plane, described with a slope
α
= 0.85 ± 0.05. We report a flat evolution of the specific SFR compared to lower redshift measurements. We find that the UV luminosity function is best reproduced by a double power law, while a fit with a Schechter function is only marginally inferior. The Ly
α
luminosity function is best fitted with a Schechter function. We derive a logSFRD
UV
(
M
⊙
yr
−1
Mpc
−3
) = −1.45
+0.06
−0.08
and logSFRD
Ly
α
(
M
⊙
yr
−1
Mpc
−3
) = −1.40
+0.07
−0.08
. The SFRD derived from the Ly
α
luminosity function is in excellent agreement with the UV-derived SFRD after correcting for IGM absorption.
Conclusions.
Our new SFRD measurements at a mean redshift of
z
= 5.6 are ∼0.2 dex above the mean SFRD reported in Madau & Dickinson (2014, ARA&A, 52, 415), but in excellent agreement with results from Bouwens et al. (2015a, ApJ, 803, 34). These measurements confirm the steep decline of the SFRD at
z
> 2. The bright end of the Ly
α
luminosity function has a high number density, indicating a significant star formation activity concentrated in the brightest LAE at these redshifts. LAE with equivalent width EW > 25 Å contribute to about 75% of the total UV-derived SFRD. While our analysis favors low dust content in 5.0 <
z
< 6.6, uncertainties on the dust extinction correction and associated degeneracy in spectral fitting will remain an issue, when estimating the total SFRD until future surveys extending spectroscopy to the NIR rest-frame spectral domain, such as with JWST.
Aims. The aim of this work is to constrain the evolution of the fraction of strong Lyα emitters among UV selected star-forming galaxies at 2 <z< 6, and to measure the stellar escape fraction of Lyα ...photons over the same redshift range. Methods. We exploit the ultradeep spectroscopic observations with VIMOS on the VLT collected by the VIMOS Ultra-Deep Survey (VUDS) to build an unique, complete, and unbiased sample of ~4000 spectroscopically confirmed star-forming galaxies at 2 <z< 6. Our galaxy sample includes UV luminosities brighter than M*FUV at 2 <z< 6, and luminosities down to one magnitude fainter than M*FUV at 2 <z< 3.5. Results. We find that 80% of the star-forming galaxies in our sample have EW0(Lyα) < 10 Å, and correspondingly fesc(Lyα) < 1%. By comparing these results with the literature, we conclude that the bulk of the Lyα luminosity at 2 <z< 6 comes from galaxies that are fainter in the UV than those we sample in this work. The strong Lyα emitters constitute, at each redshift, the tail of the distribution of the galaxies with extreme EW0(Lyα) and fesc(Lyα). This tail of large EW0(Lyα) and fesc(Lyα) becomes more important as the redshift increases, and causes the fraction of strong Lyα with EW0(Lyα) > 25 Å to increase from ~5% at z ~ 2 to ~30% at z ~ 6, with the increase being stronger beyond z ~ 4. We observe no difference, for the narrow range of UV luminosities explored in this work, between the fraction of strong Lyα emitters among galaxies fainter or brighter than M*FUV, although the fraction for the faint galaxies evolves faster, at 2 <z< 3.5, than for the bright ones. We do observe an anticorrelation between E(B − V) and fesc(Lyα): generally galaxies with high fesc(Lyα) also have small amounts of dust (and vice versa). However, when the dust content is low (E(B − V) < 0.05) we observe a very broad range of fesc(Lyα), ranging from 10-3 to 1. This implies that the dust alone is not the only regulator of the amount of escaping Lyα photons.
We obtained optical/near-IR rest-frame Magellan FIRE spectra (including Paβ and Paγ) of 25 starburst galaxies at 0.5 < z < 0.9, with average star formation rates (SFRs) seven times above the main ...sequence (MS). We find that Paschen-to-Balmer line ratios saturate around a constant value corresponding to AV ∼ 2-3 mag, while line-to-IR-luminosity ratios suggest a large range of more extreme obscurations and appear to be uncorrelated with the former. This behavior is not consistent with standard attenuation laws derived for local and distant galaxies, yet is remarkably consistent with observations of starburst cores in which young stars and dust are homogeneously mixed. This model implies AV = 2-30 mag attenuation to the center of starburst cores, with a median of ∼9 mag (a factor of 4000). X-ray hardness ratios for six AGNs in our sample and column densities derived from observed dust masses and radio sizes independently confirm this level of attenuation. In these conditions observed optical/near-IR emission comes from surface regions, while inner starburst cores are invisible. We thus attribute the high N ii/H ratios to widespread shocks from accretion, turbulence, and dynamic disturbances rather than to AGNs. The large range of optical depths demonstrates that substantial diversity is present within the starburst population, possibly connected to different merger phases or progenitor properties. The majority of our targets are, in fact, morphologically classified as mergers. We argue that the extreme obscuration provides in itself smoking gun evidence of their merger origin, and a powerful tool for identifying mergers at even higher redshifts.
ABSTRACT
We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations towards 27 low-redshift (0.02 < z < 0.2) star-forming galaxies taken from the Valparaíso ALMA/APEX Line Emission ...Survey. We perform stacking analyses of the 12CO(1–0), 13CO(1–0), and C18O(1–0) emission lines to explore the L′ 12CO(1–0)/L′ 13CO(1–0) hereafter L′ (12CO)/L′ (13CO) and L′ 13CO(1–0)/L′ C18O(1–0) hereafter L′ (13CO)/L′ (C18O) line luminosity ratio dependence as a function of different global galaxy parameters related to the star formation activity. The sample has far-IR luminosities of $10^{10.1\!-\!11.9}\,$ L⊙ and stellar masses of 109.8–10.9 M⊙ corresponding to typical star-forming and starburst galaxies at these redshifts. On average, we find an L′ (12CO)/L′ (13CO) line luminosity ratio value of 16.1 ± 2.5. Galaxies with pieces of evidence of possible merging activity tend to show higher L′ (12CO)/L′ (13CO) ratios by a factor of 2, while variations of this order are also found in galaxy samples with higher star formation rates (SFRs) or star formation efficiencies (SFEs). We also find an average L′ (13CO)/L′ (C18O) line luminosity ratio of 2.5 ± 0.6, which is in good agreement with those previously reported for starburst galaxies. We find that galaxy samples with high LIR, SFR, and SFE show low L′ (13CO)/L′ (C18O) line luminosity ratios with high L′ (12CO)/L′ (13CO) line luminosity ratios, suggesting that these trends are produced by selective enrichment of massive stars in young starbursts.
Ultra-deep imaging of small parts of the sky has revealed many populations of distant galaxies, providing insight into the early stages of galaxy evolution. Spectroscopic follow-up has mostly ...targeted galaxies with strong emission lines at z > 2 or concentrated on galaxies at z < 1. The populations of both quiescent and actively star-forming galaxies at 1 < z < 2 are still under-represented in our general census of galaxies throughout the history of the Universe. In the light of galaxy formation models, however, the evolution of galaxies at these redshifts is of pivotal importance and merits further investigation. In addition, photometry provides only limited clues about the nature and evolutionary status of these galaxies.We therefore designed a spectroscopic observing campaign of a sample of both massive, quiescent and star-forming galaxies z > 1.4. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies.
![The ALPINE-ALMA [C ii ] sur...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1745-3925_1745-3933/small "The ALPINE-ALMA [C ii ] survey: a triple merger at z ∼ 4.56")
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