AGN in dusty Starbursts at z=2: feedback still to kick in
We investigate a sample of 152 dusty sources at 1.5 < z < 2.5 to understand the connection of enhanced Star-Formation-Rate (SFR) and Black-Hole-Accretion-Rate (BHAR). The sources are Herschel-selected, having stellar masses M∗>10^10 Msun and SFR (∼100−1000M/yr) elevated(>4) above the star-forming "main sequence", classifying them as Starbursts (SB). Through a multiwavelength fitting approach (including a dusty torus component), we divided the sample into active SBs (dominated by an AGN
... by an AGN emission, SBs-AGN,~23% of the sample) and purely star-forming SBs (SBs-SFR). We visually inspected their HST/UV-restframe maps: SBs-SFR are generally irregular and composite systems;∼50% of SBs-AGN are instead dominated by regular compact morphologies. We then found archival ALMA continuum counterparts for 33 galaxies (12 SBs-AGN and 21 SBs-SFR). For these sources we computed dust masses, and, with standard assumptions, we also guessed total molecular gas-masses. SBs turn to be gas rich systems (fgas=Mgas/(Mgas+M∗) ~ 20%−70%), and the gas fractions of the two SB classes are very similar (fgas= 43±4% and fgas= 42±2%).Our results show that SBs are consistent with a mixture of: 1) highly star-forming merging systems (dominating the SBs-SFR), and 2) primordial galaxies, rapidly growing their M∗ together with their Black Hole (mainly the more compact SBs-AGN). Anyway, feedback effects have not reduced their fgas yet. Indeed, SBs at z=2, with relatively low bolometric AGN luminosities in the range 10^44 < Lbol(AGN) < 10^46 (compared to bright optical and X-ray quasars), are still relatively far from the epoch when the AGN feedback will quench the SFR in the host and will substantially depress the gas fractions.