(U)LIRGs

Local (U)LIRGs as testbeds for high-z star-forming galaxy studies

A major science goal for the SKA and its pathfinders is the observation of high-z galaxies to study galaxy formation and evolution across cosmic time. This ambitious SKA goal can only be achieved if we can properly interpret the diagnostic tools that are provided by radio continuum observations: we must be confident that the empirically observed radio to infrared correlation remains valid (or its variations can be calibrated) for a wide range of galaxy types and redshifts (e.g., Beswick et al. 2008, MNRAS 385, 1143). We must be also sure that we can reliably convert IR luminosities into star-formation rates, which requires a stellar initial mass function (IMF) that is stable with time, or whose variations are understood.

An important way to address the above crucial questions is via high-angular continuum observations of local Luminous and Ultra-Luminous Infra-Red Galaxies (LIRGs, Lir > 1e11 Lsun and ULIRGs, Lir > 1e12 LSun), since they are thought to represent the nearby cousins of the far away, high-z star forming galaxies. Most LIRGs are interacting galaxies, and essentially all ULIRGs appear to be advanced merger systems and may represent an important stage in the formation of quasi-stellar objects (e.g., Yuan et al. 2010, ApJ 709, 884) and, in general, of the co-evolution of a SMBH and a starburst. One of the milestones reached during the previous project was the finding that the radial distribution of CCSNe in central regions of starburst galaxies is consistent with the existence of a nuclear disk that would be responsible for driving the gas from the inner ~100 pc down to central pc (Herrero-Illana, Pérez-Torres & Alberdi 2012, Letters to A&A, 540, L5), suggesting also a coevolution between the central SMBH and star-formation activity.

Disentangling the dominant heating mechanism and the population of compact radio sources in the central regions of (U)LIRGs

Most of the luminosity of (U)LIRGs seems to originate in small regions of a few hundred pársec, and is dominated by IR emission from warm dust grains. The question still under debate is whether the dust heating mechanism is mainly an AGN, or a starburst, or both, and what is the precise nature of the compact sources (AGN, young SNe, SNRs, ultra-compact H II regions, micro-quasars,...). Multi-frequency, high-angular observations, in particular in radio, can contribute in an essential way to solve this issue, since they yield the required resolution to pinpoint the AGN, as well as other compact sources, and radio is not hampered by extinction, unlike the case of optical observations. The IAA group has been leading several case studies in the past years that demonstrate the usefulness of high-angular radio observations, e.g., Arp 299-A (Pérez-Torres et al. 2009, A&A 507, L17; Pérez-Torres et al. 2010, A&A 519, L5; Romero-Cañizales et al. 2011, MNRAS 415, 2688); NGC7469 (Alberdi et al. 2006, ApJ 638, 938; Pérez-Torres et al. 2009, MNRAS 399, 1641); (Romero-Cañizales, Pérez-Torres & Alberdi 2012, MNRAS 422, 510 Romero- Cañizales, Pérez-Torres, Alberdi et al. 2012, A&A 543, 72).

In the framework of the previous project, other milestone reached in this field was the un- obscured, direct estimate of the CCSN rate for Arp 299A (Bondi, Pérez-Torres, Herrero-Illana and Alberdi 2012, A&A, 539, 134), thanks to our VLBI monitoring (PI: Pérez-Torres), which is still ongoing. The value obtained (≥1.2 SN/yr) is larger than expected from the empirical relations linking far-infrared emission and star-formation and supernova rates. We also obtained EVN observations of some of the farthest and brightest ULIRGs in the local Universe (PI: Pérez-Torres), and published the results obtained for the cases of IRAS 23365+3604 (Romero-Cañizales, Pérez-Torres and Alberdi 2012, A&A, 422, 510), and IC883 (Romero-Cañizales, Pérez-Torres, Alberdi et al. 2012, A&A, 543, 72), and the multi- frequency study of the central kpc region of the LIRG NGC 1614 (Herrero-Illana, Pérez- Torres, Alonso-Herrero et al. 2014, ApJ, 786, 156), which essentially rules out the possibility of an AGN at its center. In this paper, we also presented a new method for disentangling the thermal and non-thermal radio contributions. Also a new way to correct for the extinction, towards this galaxy in particular, but of general applicability, was developed by our PhD student Herrero-Illana (Xu et al. – includes Herrero-Illana - 2014, ApJ, 799, 11).

While the study of single objects is necessary for a detailed understanding of the physics of individual sources, to gain an overall understanding of the (U)LIRG phenomenon in the local universe --mandatory for similar investigations at high-z to be of any use—a study on large, statistically significant sample is needed. To this end, the IAA group leads the eMERLIN legacy project LIRGI (Luminous Infrared Galaxy Inventory; PIs J. Conway and M.A. Pérez- Torres), for which 353 hr have been granted to observe a sample of 42 LIRGs and ULIRGs in the nearby (D < 250 Mpc) universe. We also lead the VLBI effort of LIRGI (PI: Pérez- Torres), namely, we submitted several EVN proposals during the previous project, which were accepted by the EVN programme committee. 30 out of the 42 LIRGI sources are being observed (by now, 18 already observed and with their data correlated).

The hidden population of core-collapse supernovae in local (U)LIRGs

It has been found that there was a mismatch between the expected CCSN rate and the actual observed rate by a factor of two (Horiuchi et al. 2011, ApJ). Under the previous project, we already advocated for the use of high-angular ( 0.3”) radio observations as a way to obtain an accurate estimate of the true CCSN rate, and hence of the massive SFR, in the central regions of nearby U/LIRGs, where the extinction makes impossible to see any SN in the optical, or even in the near-IR (e.g., the central 100 pc of Arp 299A – Pérez-Torres et al. 2009, A&A 507, L17; 2010, A&A 519, L5; Bondi et al. 2012, A&A 539, A134). We

continued our collaboration with the group of Dr. Mattila (Univ. of Turku), which has been using -for a decade now- high-angular resolution NIR observations (~0.1") which are also useful to probe the hidden supernova population in the circumnuclear regions of (U)LIRGs, where extinction at near-IR wavelengths is significantly less than in the central 200 pc. The combined monitoring of radio and NIR observations is thus an extremely powerful tool for a direct estimation of the true CCSN rate in (U)LIRGs, and has resulted in a very relevant milestone. Namely, a quantitative estimate of the fraction of missed CCSNe in the local universe (Mattila et al. – includes Pérez-Torres, 2012, ApJ, 756, 111).