Physics of ULIRGs with MUSE and ALMA: The PUMA project

Abstract

We analyze new high-resolution (400 pc) ∼220 GHz continuum and CO(2–1) Atacama Large Millimeter Array (ALMA) observations of a representative sample of 23 local (z < 0.165) ultra-luminous infrared systems (ULIRGs; 34 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, rcont, 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 (LIR). The good agreement, within a factor of 2, between the observed ∼220 GHz fluxes and the extrapolation of the infrared gray-body as well as the small contributions from synchrotron and free–free emission support this assumption. The cold molecular gas emission sizes, rCO, are between 60 and 700 pc and are similar in advanced mergers and early interacting systems. On average, rCO are ∼2.5 times larger than rcont. Using these measurements, we derived the nuclear LIR and cold molecular gas surface densities (ΣLIR = 1011.5 − 1014.3 L⊙ kpc−2 and ΣH2 = 102.9 − 104.2 M⊙ pc−2, respectively). Assuming that the LIR is produced by star formation, the median ΣLIR corresponds to ΣSFR = 2500 M⊙ yr−1 kpc−2. This ΣSFR implies extremely short depletion times, ΣH2/ΣSFR < 1–15 Myr, and unphysical star formation efficiencies > 1 for 70% of the sample. Therefore, this favors the presence of an obscured active galactic nucleus (AGN) in these objects that could dominate the LIR. We also classify the ULIRG nuclei in two groups: (a) compact nuclei (rcont < 120 pc) with high mid-infrared excess emission (ΔL6−20 μm/LIR) found in optically classified AGN; and (b) nuclei following a relation with decreasing ΔL6−20 μm/LIR for decreasing rcont. The majority, 60%, of the nuclei in interacting systems lie in the low-rcont end (<120 pc) of this relation, while this is the case for only 30% of the mergers. 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.

ALMA Spatially Resolved Dense Molecular Gas Survey of Nearby Ultraluminous Infrared Galaxies

“…Since probed physical scale in the original beam spectra is largely different among different (U)LIRGs (Table 3), we compare molecular emission line fluxes with the same physical scale for further discussion. We choose 1 kpc, because we can investigate dense molecular emission line properties at energetically dominant ULIRGs' nuclei (e.g., Soifer et al 2000;Diaz-Santos et al 2010;Imanishi et al 2011;Pereira-Santaella et al 2021), with minimum contaminations from spatially extended ( a few kpc) star-formation in the host galaxies. We modify the beam to 1 kpc for all molecular lines in ULIRGs if their beam sizes are smaller than 1 kpc, using the CASA task "imsmooth", and then extract 1 kpc beam-sized spectra, which are overplotted as red dotted lines in Figure 3.…”

“…These relations were derived by observing galaxies with a wide (six orders of magnitude) infrared luminosity range (Zhang et al 2014;Tan et al 2018) and were largely determined by nearby ULIRGs which dominate the high luminosity part. Although the HCN and HCO + J=4-3 line data of nearby ULIRGs (Zhang et al 2014;Tan et al 2018) were taken with large apertures of single dish telescopes, we regard that the bulk of the observed J=4-3 luminosities come from nuclear regions, because (1) nearby ULIRGs are usually energetically dominated by compact (∼1 kpc) nuclear regions, with small contributions from spatially extended ( a few kpc) star-forming regions in the host galaxies (e.g., Soifer et al 2000;Diaz-Santos et al 2010;Imanishi et al 2011;Pereira-Santaella et al 2021), and (2) when ALMA high-spatial-resolution data are available, the HCN and HCO + J=4-3 emission of nearby (U)LIRGs are confirmed to be spatially compact (Imanishi et al 2018b). Our J=2-1 data roughly follow the updated J=4-3 relation by Tan et al (2018).…”

“…However, there remain many nearby (U)LIRGs that have no significant AGN signatures even in the hard X-rays and infrared. It is important to distinguish whether (1) they contain no energetically important AGNs, or (2) they do, but are elusive even in hard Xrays and infrared owing to extremely large extinction (e.g., Downes & Eckart 2007;Matsushita et al 2009;Scoville et al 2017;Pereira-Santaella et al 2021). Observations at wavelengths of even lower extinction can provide useful information.…”

Physics of ULIRGs with MUSE and ALMA: The PUMA project

“…A more comprehensive study of physical and kinematic properties of the interstellar medium (ISM) of the archetypical ULIRG Arp 220 was presented in Perna et al (2020, as part of the PUMA project). In Pereira-Santaella et al (2021;Paper II hereinafter) we instead analysed the ∼220 GHz and CO(2−1) ALMA observations to constrain the hidden energy source of ULIRGs, providing evidence for the ubiquitous presence of obscured AGN that could dominate their IR emission.…”

Stellar and black hole assembly in z<0.3 infrared-luminous mergers: intermittent starbursts vs. super-Eddington accretion

“…are rare, but almost invariably harbour one or both of rapid accretion onto a supermassive black hole (SMBH) and high rates of star formation (Genzel et al 1998;Farrah et al 2003;Armus et al 2007;Nardini et al 2009;Pereira-Santaella et al 2021). The inferred star formation and SMBH accretion rates make these systems viable sites for assembling significant stellar and black hole mass.…”