The Atacama Large Millimetre/submillimetre Array (ALMA) is currently in the process of transforming our view of star-forming galaxies in the distant (z≳1) universe. Before ALMA, most of what we kne...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsos.200556

High-redshift star formation in the Atacama large millimetre/submillimetre array era.

Submitted to MNRAS; Comments welcome. Full catalogue will be made publicly available on acceptance of paper

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An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field : Source catalogue and properties

We report two secure (z = 3.775; 4.012) and one tentative (z ≈ 3.767) spectroscopic confirmations of massive and quiescent galaxies through K-band observations with Keck/MOSFIRE and Very Large Telescope/X-Shooter. The stellar continuum emission, absence of strong nebular emission lines, and lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M⋆/M⊙) ~ 11 and ongoing star formation rates placing these galaxies ≳ 1–2 dex below the main sequence at their redshifts. The adopted parameterization of the star formation history suggests that these sources experienced a strong (〈SFR〉 ~1200 - 3500 M⊙ yr⁻¹) and short (~50 Myr) burst of star formation, peaking ~150–500 Myr before the time of observation, all properties reminiscent of the characteristics of submillimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest submillimeter surveys detecting not only the most extreme starbursts but also more normal galaxies. We support these findings by further exploring the Illustris TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ~ 3–4 and SMGs at z ~ 4−5, with number densities and properties in agreement with the observations at z ~ 3 but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all of the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past, and, similarly, not all bright SMGs quench by z ~ 3, both fractions depending on the threshold assumed to define the SMGs themselves.

https://robots.iopscience.iop.org/article/10.3847/1538-4357/ab64dc/pdf

Quiescent galaxies 1.5 billion years after the Big Bang and their progenitors

We report two secure ($z=3.775, 4.012$) and one tentative ($z\approx3.767$) spectroscopic confirmations of massive and quiescent galaxies through $K$-band observations with Keck/MOSFIRE and VLT/X-Shooter. The stellar continuum emission, the absence of strong nebular emission lines and the lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of $\mathrm{log}(M_{\star}/M_\odot)\sim11$ and ongoing star formation rates placing these galaxies $\gtrsim 1-2$ dex below the main sequence at their redshifts. The adopted parametrization of the star formation history suggests that these sources experienced a strong ($\langle \rm SFR \rangle \sim 1200-3500\,M_\odot\,\mathrm{yr}^{-1}$) and short ($\sim 50$ Myr) burst of star formation, peaking $\sim 150-500$ Myr before the time of observation, all properties reminiscent of the characteristics of sub-millimeter galaxies (SMGs) at $z>4$. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest sub-mm surveys detecting not only the most extreme starbursts, but also more normal galaxies. We support these findings by further exploring the Illustris-TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at $z\sim3-4$ and SMGs at $z\sim4-5$, with number densities and properties in agreement with the observations at $z\sim3$, but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past and, similarly, not all bright SMGs quench by $z\sim3$, both fractions depending on the threshold assumed to define the SMGs themselves.

The Atacama Large Millimetre/submillimetre Array (ALMA) is currently in the process of transforming our view of star-forming galaxies in the distant ($z\gtrsim1$) universe. Before ALMA, most of what we knew about dust-obscured star formation in distant galaxies was limited to the brightest submillimetre sources$-$the so-called submillimetre galaxies (SMGs)$-$and even the information on those sources was sparse, with resolved (i.e., sub-galactic) observations of the obscured star formation and gas reservoirs typically restricted to the most extreme and/or strongly lensed sources. Starting with the beginning of early science operations in 2011, the last nine years of ALMA observations have ushered in a new era for studies of high-redshift star formation. With its long baselines, ALMA has allowed observations of distant dust-obscured star formation with angular resolutions comparable to$-$or even far surpassing$-$the best current optical telescopes. With its bandwidth and frequency coverage, it has provided an unprecedented look at the associated molecular and atomic gas in these distant galaxies through targeted follow-up and serendipitous detections/blind line scans. Finally, with its leap in sensitivity compared to previous (sub-)millimetre arrays, it has enabled the detection of these powerful dust/gas tracers much further down the luminosity function through both statistical studies of color/mass-selected galaxy populations and dedicated deep fields. We review the main advances ALMA has helped bring about in our understanding of the dust and gas properties of high-redshift ($z\gtrsim1$) star-forming galaxies during these first nine years of its science operations, and we highlight the interesting questions that may be answered by ALMA in the years to come.

High-redshift star formation in the ALMA era

We report the results of a search for serendipitous [CII] 157.74$\mu$m emitters at $z\sim4.4$-$4.7$ using the Atacama Large Millimeter/submillimeter Array (ALMA). The search exploits the AS2UDS continuum survey, which covers ~50 arcmin$^2$ of the sky towards 695 luminous ($S_{870}\gtrsim1$mJy) submillimeter galaxies (SMGs), selected from the SCUBA-2 Cosmology Legacy Survey (S2CLS) 0.96deg$^2$ Ultra Deep Survey (UDS) field. We detect ten candidate line emitters, with an expected false detection rate of ten percent. All of these line emitters correspond to 870$\mu$m continuum-detected sources in AS2UDS. The emission lines in two emitters appear to be high-J CO, but the remainder have multi-wavelength properties consistent with [CII] from $z\simeq4.5$ galaxies. Using our sample, we place a lower limit of $>5\times10^{-6}$Mpc$^{-3}$ on the space density of luminous ($L_{\rm IR} \simeq 10^{13}$Lsun) SMGs at $z=4.40$-$4.66$, suggesting $\ge7$percent of SMGs with $S_{870\mu{\rm m}}\gtrsim1$mJy lie at $4<z<5$. From stacking the high-resolution ($\sim0.15"$ full-width half maximum) ALMA $870\mu$m imaging, we show that the [CII] line emission is more extended than the continuum dust emission, with an average effective radius for the [CII] of $r_{\rm e} = 1.7^{+0.1}_{-0.2}$kpc compared to $r_{\rm e} = 1.0\pm0.1$kpc for the continuum (rest-frame $160\mu$m). By fitting the far-infrared photometry for these galaxies from $100$-$870\mu$m, we show that SMGs at $z\sim4.5$ have a median dust temperature of $T_{\rm d}=55\pm4$K. This is systematically warmer than $870\mu$m-selected SMGs at $z\simeq2$, which typically have temperatures around $35$K. These $z\simeq4.5$ SMGs display a steeper trend in the luminosity-temperature plane than $z\le2$ SMGs. We discuss the implications of this result in terms of the selection biases of high redshift starbursts in far-infrared/submillimeter surveys.

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An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Identifying candidate z~4.5 [CII] emitters

We analyse the multi-frequency radio spectral properties of $41$ 6GHz-detected ALMA-identified, submillimetre galaxies (SMGs), observed at 610MHz, 1.4GHz, 6GHz with GMRT and the VLA. Combining high-resolution ($\sim0.5''$) 6GHz radio and ALMA $870\,\mu$m imaging (tracing rest-frame $\sim20$GHz, and $\sim250\,\mu$m dust continuum), we study the far-infrared/radio correlation via the logarithmic flux ratio $q_{\rm IR}$, measuring $\langle q_{\rm IR}\rangle=2.20\pm 0.06$ for our sample. We show that the high-frequency radio sizes of SMGs are $\sim1.9\pm 0.4\times$ ($\sim2$-$3$kpc) larger than those of the cool dust emission, and find evidence for a subset of our sources being extended on $\sim 10$kpc scales at 1.4GHz. By combining radio flux densities measured at three frequencies, we can move beyond simple linear fits to the radio spectra of high-redshift star-forming galaxies, and search for spectral curvature, which has been observed in local starburst galaxies. At least a quarter (10/41) of our sample show evidence of a spectral break, with a median $\langle\alpha^{1.4\,{\rm GHz}}_{610\,{\rm GHz}}\rangle=-0.60\pm 0.06$, but $\langle\alpha^{6\,{\rm GHz}}_{1.4\,{\rm GHz}}\rangle=-1.06\pm 0.04$ -- a high-frequency flux deficit relative to simple extrapolations from the low-frequency data. We explore this result within this subset of sources in the context of age-related synchrotron losses, showing that a combination of weak magnetic fields ($B\sim35\,\mu$G) and young ages ($t_{\rm SB}\sim40$--$80\,$Myr) for the central starburst can reproduce the observed spectral break. Assuming these represent evolved (but ongoing) starbursts and we are observing these systems roughly half-way through their current episode of star formation, this implies starburst durations of $\lesssim100$Myr, in reasonable agreement with estimates derived via gas depletion timescales.

F. An

Papers

An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Identifying candidate z~4.5 [CII] emitters

Radio spectra and sizes of ALMA-identified submillimetre galaxies: evidence of age-related spectral curvature and cosmic ray diffusion?