Deciphering an evolutionary sequence of merger stages in infrared-luminous starburst galaxies atz∼ 0.7

Abstract

Based on optical/near-IR Magellan FIRE spectra of 25 starburst galaxies at 0.5 < z < 0.9, Calabrò et al. (2018) showed that their attenuation properties can be explained by a single-parameter sequence of total obscurations ranging from A V = 2 to A V = 30 towards the starburst core centers in a mixed stars and dust configuration. We investigate here the origin of this sequence for the same sample. We show that total attenuations anti-correlate with the starburst sizes in radio (3 GHz) with a significance larger than 5σ and a scatter of 0.26 dex. More obscured and compact starbursts also show enhanced N2 (=[NII]/Hα) ratios and larger line velocity widths that we attribute to an increasing shock contribution toward later merger phases, driven by deeper gravitational potential wells at the coalescence. Additionally, the attenuation is also linked to the equivalent width (EW) of hydrogen recombination lines, which is sensitive to the luminosity weighted age of the relatively unobscured stellar populations. Overall, the correlations among A V , radio size, line width, N2 and EW of Balmer/Paschen lines converge towards suggesting an evolutionary sequence of merger stages: all of these quantities are likely to be good time-tracers of the merger phenomenon, and their large spanned range appears to be characteristic of the different merger phases. Half of our sample at higher obscurations have radio sizes approximately 3 times smaller than early type galaxies at the same redshift, suggesting that, in analogy with local Ultraluminous Infrared galaxies (ULIRGs), these cores cannot be directly forming elliptical galaxies. Finally, we detect mid-IR AGN torus for half of our sample and additional X-ray emission for 6 starbursts; intriguingly, the latter have systematically more compact sizes, suggestive of emerging AGNs towards later merger stages, possibly precursors of a later QSO phase.

Discovery of Four Apparently Cold Dusty Galaxies at z = 3.62–5.85 in the COSMOS Field: Direct Evidence of Cosmic Microwave Background Impact on High-redshift Galaxy Observables

“…5.2) and none is Xray detected Jin et al (2018). Therefore, we speculate that the compact morphology in this very highly star-forming sample is a consequence of ongoing mergers, similarly to local ultra-luminous infrared galaxies (ULIRGs) (Soifer et al 2000;Juneau et al 2009) and high-z compact starbursts (Puglisi et al 2017;Calabrò et al 2019;Marrone et al 2018).…”

The Evolving AGN Duty Cycle in Galaxies Since z ∼ 3 as Encoded in the X-Ray Luminosity Function

“…Veilleux et al 2002). At intermediate redshifts (z∼0.7), Calabrò et al (2019) observed an increasing incidence of SF clumps when moving above the MS relation, which might indicate a prevalence of merger-induced clumpy star formation toward higher sSFRs. At z∼2, the morphological dichotomy seen in the local Universe becomes much less pronounced, since the fraction of irregular and disturbed morphologies is generally high, and spread out quite uniformly across the SFR-M plane (e.g.…”

Merger induced clump formation in distant infrared luminous starburst galaxies

“…In order to test this interpretation, we analyzed the subset of 19 starbursts, with available HST images and in our redshift range, that were presented in Calabrò et al (2018) as representatives of off-MS systems at 0.5 < z < 0.9 above a M * of 10 10 M . We showed that this sample comprises a sequence of different evolutionary merger stages, which can be traced by the equivalent width (EW) of Balmer or Paschen lines, and by the total attenuation (A V,tot ) toward the center of the starburst core in a mixed dust and stars configuration (Calabrò et al 2019).…”

“…In Fig. 14 we compare the clumpiness of this SB subset to A V,tot and the EW of Hα and Paβ lines, measured in (Calabrò et al 2019). In the first upper panel, we find no significant correlation between c and A V,tot , with a Spearman correlation coefficient R= −0.34 (p-value= 0.13) and angular coefficient of the best-fit line consistent with 0.…”

“…Following the results of previous simulation works, clumps can form with the increasing compressive turbulence modes and subsequent fragmentation induced by the merger during early-intermediate stages (Renaud et Fig. 14. Comparison between the clumpiness and the mixed-model attenuation A V,tot toward the center (upper plot), and the equivalent width (EW) of Hα (lower panel) for 21 starbursts in our same redshift range 0.5 < z < 0.9, and analyzed in Calabrò et al (2019). The corresponding IDs are added to each galaxy, while black crosses indicate X-ray detected sources.…”