Starbursts in and out of the star-formation main sequence

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 starformation 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 starforming 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 UV and far-infrared 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 far infrared and UV are distributed in distinct regions and dust-corrected star formation rates estimated using UVoptical-NIR data alone underestimate the total star formation rate. 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 10 11 M ⊙ . Active galactic nuclei 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.

KASHz: No evidence for ionised outflows instantaneously suppressing star formation in moderate luminosity AGN at z ∼ 1.4–2.6

“…The RA and Dec of this central position in then determined by the position of the source in the corresponding broad-band images (HST or UKIRT). As with many previous studies (Miller et al 2008;Hsu et al 2014;Dunlop et al 2017;Elbaz et al 2018;Scholtz et al 2018) we noticed a systematic offset between the optical astrometric frame (e.g., in HST) and the radio astrometric frame (e.g., from VLA or ALMA) in the CDFS field. This affects six of our eight targets in our sample which lie in this field.…”

“…We could not find previous work which clearly quantifies the spatial offsets between Hα emission and FIR continuum for high-z galaxies to compare to. However, offsets between optical continuum and dust continuum have previously been reported in a qualitative way in several works (e.g., Hodge et al 2016;Chen et al 2017;Elbaz et al 2018) In Figure 9 we compare the half-light radii of Hα and FIR emission. These are calculated as described in § 3.1.4 and 3.2 and the values are provided in Table 4.…”

ALMA Reveals the Molecular Gas Properties of Five Star-forming Galaxies across the Main Sequence at 3

“…The horizontal continuous line marks the location of the MS, the dashed (dotted) lines indicate the locii 4x (10x) times above or below the MS. The grey area emphasizes the location of the main sequence, according to the classical definition by Rodighiero et al (2011;, and Elbaz et al (2018).…”

HST/WFC3 grism observations of z ∼ 1 clusters: evidence for evolution in the mass–size relation of quiescent galaxies from post-starburst galaxies

“…A galaxy undergoing a starburst usually lies systematically above the star-forming main sequence at fixed redshift, with SFRs 4 (e.g. Noeske et al 2007;Rodighiero et al 2011;Elbaz et al 2018). Towards higher redshifts, the intercept of the star-forming main sequence (SFR-stellar mass relation for star-forming galaxies) increases (Noeske et al 2007;Whitaker et al 2012b).…”