Abstract
Aims. We use high-resolution continuum images obtained with the Atacama Large Millimeter Array (ALMA) to probe the surface density of star formation in z ~ 2 galaxies and study the different physical properties between galaxies within and above the star-formation main sequence of galaxies. Methods. We use ALMA images at 870 μm with 0.2 arcsec resolution in order to resolve star formation in a sample of eight star-forming galaxies at z ~ 2 selected among the most massive Herschel galaxies in the GOODS-South field. This sample is supplemented with eleven galaxies from the public data of the 1.3 mm survey of the Hubble Ultra-Deep Field, HUDF. We derive dust and gas masses for the galaxies, compute their depletion times and gas fractions, and study the relative distributions of rest-frame ultraviolet (UV) and far-infrared (FIR) light. Results. ALMA reveals systematically dense concentrations of dusty star formation close to the center of the stellar component of the galaxies. We identify two different starburst regimes: (i) the classical population of starbursts located above the SFR-M⋆ main sequence, with enhanced gas fractions and short depletion times and (ii) a sub-population of galaxies located within the scatter of the main sequence that experience compact star formation with depletion timescales typical of starbursts of ~150 Myr. In both starburst populations, the FIR and UV are distributed in distinct regions and dust-corrected star formation rates (SFRs) estimated using UV-optical-near-infrared data alone underestimate the total SFR. Starbursts hidden in the main sequence show instead the lowest gas fractions of our sample and could represent the last stage of star formation prior to passivization. Being Herschel-selected, these main sequence galaxies are located in the high-mass end of the main sequence, hence we do not know whether these “starbursts hidden in the main sequence” also exist below 1011 M⊙. Active galactic nuclei (AGNs) are found to be ubiquitous in these compact starbursts, suggesting that the triggering mechanism also feeds the central black hole or that the active nucleus triggers star formation.
Highlights
We present the properties of a sample of 19 z ∼ 2 star-forming galaxies located in the GOODS-South field with high-angular-resolution imaging and photometry from the UV to the millimeter range, combining data from the ground, HST, Spitzer, Herschel, and Atacama Large Millimeter Array (ALMA): the ALMA data combine information from deep integrations at 870 μm on 8 z ∼ 2 sources selected among the brightest Herschel sources with data on 11 galaxies coming from a published 1.3 mm survey of the Hubble Ultra-Deep Field, Hubble Ultra Deep Field (HUDF) (Dunlop et al 2017; Rujopakarn et al 2016)
Compact star-formation in z ∼ 2 massive galaxies: while dusty star-formation is resolved in all ALMA galaxies, we find a common point among these massive z ∼ 2 star-forming galaxies: their star formation appears to be concentrated towards the mass center of the galaxies and to be 1.45 ± 1.0 times more compact than at 1.6 μm, that is, the observed HST–WFC3 H-band
Minor contribution of kiloparsec-size clumps of star formation: kiloparsec-size clumps of star formation seen in the UV do not contribute a large fraction of the total star formation rates (SFRs) measured in these massive z ∼ 2 galaxies
Summary
During the 6 billion years that passed between a redshift of z ∼ 2.5 and 0.5, galaxies formed 75% of their present stellar mass (see Fig. 11 of Madau & Dickinson 2014) following a star-formation mode in which most of the ultraviolet (UV) starlight was absorbed by interstellar dust and re-radiated in the mid- to far-infrared (MIR and FIR, respectively, see e.g., Le Floc’h et al 2005; Magnelli et al 2009, 2013; Burgarella et al 2013; Madau & Dickinson 2014 and references therein). This evolution from a local population of rare violent merger-driven local (U)LIRGs to a common population of secularly evolving star-forming galaxies at z ∼ 2 is for the most part a natural result of the fast rise of the gas fraction of (U)LIRGs with increasing redshift (see e.g., Daddi et al 2010a; Tacconi et al 2010, 2018; Magdis et al 2012b) This change in the nature of (U)LIRGs as a function of cosmic time can be seen in the framework of the global evolution of the correlation between the SFR and stellar mass followed by star-forming galaxies, the so-called “star-formation main sequence” (MS, hereafter). The resulting sample of 19 galaxies at z ∼ 2 is described below
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