Are local ULIRGs powered by AGN? The sub-kpc view of the 220 GHz continuum. PUMA II

Abstract

We analyze new high-resolution (400 pc) ∼220 GHz continuum and CO(2-1) ALMA observations of a representative sample of 22 local (z<0.165) ULIRG systems (32 individual nuclei) as part of the "Physics of ULIRGs with MUSE and ALMA" (PUMA) project. The deconvolved half-light radii of the ∼220 GHz continuum sources, r cont , are between <60 pc and 350 pc (median 80-100 pc). We associate these regions with the regions emitting the bulk of the infrared luminosity (L IR ). The good agreement, within a factor of 2, between the observed ∼220 GHz fluxes and the extrapolation of the infrared gray-body, and the small contributions from synchrotron and free-free emission support this assumption. The cold molecular gas emission sizes are between 60 and 700 pc and are, on average, ∼2.6 times larger than the continuum. Using these measurements, we derive the nuclear L IR and cold molecular gas surface densities (Σ L IR = 10 11.5 − 10 14.3 L kpc −2 and Σ H 2 = 10 2.9 − 10 4.2 M pc −2 , respectively). Assuming that the L IR is produced by star-formation, the median Σ L IR corresponds to Σ SFR = 2500 M yr −1 kpc −2 . This Σ SFR implies extremely short depletion times, Σ H 2 /Σ SFR <1-15 Myr, and unphysical star-formation efficiencies >1 for 70% of the sample. Therefore, this favors the presence of an obscured AGN in these objects that could dominate the L IR . We also classify the ULIRG nuclei in two groups: (a) compact nuclei (r cont <130 pc) with high mid-IR excess emission (∆L 6−20µm /L IR ) found in optically classified AGN; and (b) nuclei following a relation with decreasing ∆L 6−20µm /L IR for decreasing r cont . The majority, 65%, of the nuclei in interacting systems lie in the low-r cont end (<130 pc) of this relation, while only 25% of the mergers do so. This suggests that in the early stages of the interaction, the activity occurs in a very compact and dust-obscured region while, in more advanced merger stages, the activity is more extended, unless an optically detected AGN is present. Approximately two thirds of the nuclei have nuclear radiation pressures above the Eddington limit. This is consistent with the ubiquitous detection of massive outflows in local ULIRGs and supports the importance of the radiation pressure in the outflow launching process.

Compact molecular gas emission in local LIRGs among low- and high-z galaxies

“…On the other hand, merger (type 2) LIRGs show similar molecular size than rotating disks: thus interactions may have an important role in the compaction of the molecular size, although this requires further investigation. The work by Pereira-Santaella et al (2021) seems to support the aforementioned result: they derived still more compact molecular size for their sources with respect to our LIRGs. Their sources are more extreme in terms of SFR (∼340 M yr −1 ) and even show a more disturbed morphology.…”

“…Local Spiral (gray red-contour diamonds) values are taken from the EDGE-CALIFA survey(Bolatto et al 2017) while ETGs (blue contour down pointing triangles) are taken from the ATLAS 3D sample(Cappellari et al 2013a). ULIRGs (purple pentagons) values are derived from ALMA data(Pereira-Santaella et al 2021). The median values of each sample is shown according to the following color-code: purple for ULIRGs, light green for interacting and merger LIRGs, light blue for disky (i.e., RD and PD) LIRGs, orange for Spirals and dark blue for ETGs.…”