Star Formation in Luminous & Ultra-luminous Galaxies

Linden et al. present the first results of a high-resolution Karl G. Jansky Very Large Array imaging survey of luminous and ultra-luminous infrared galaxies (U/LIRGs) in the Great Observatories All-sky LIRG Survey (GOALS). From a full sample of 68 galaxies, the authors selected 25 luminous infrared galaxies (LIRGs) that show resolved extended emission at sufficient sensitivity to image individual regions of star formation activity beyond the nucleus. Ranging from 10 pc to 1 kpc, the resolution was adequate to isolate numerous massive star forming regions across the galaxies, including those in tidal tails and filaments. With wideband radio continuum observations that sampled frequencies from 3 to 33 GHz, the authors made extinction-free measurements of the luminosities and spectral indices of 48 individual star-forming regions having deprojected galactocentric radii that lie outside the 13.2 μm core of the galaxy.  The median 33 GHz thermal fraction and 3–33 GHz spectral index measured for these extranuclear regions is 65% ± 11% and –0.51 ± 0.13, respectively. These values are consistent with measurements made on matched spatial scales in normal star-forming galaxies, and suggests that these regions are more heavily dominated by thermal free–free emission relative to the centers of local U/LIRGs. The authors also find that the median star formation rate derived for these regions is ~1 M_⊙/year, and when placed on the sub-galactic star-forming main sequence of galaxies, they are offset from their host galaxies' globally-averaged specific star formation rates. The authors conclude that while nuclear starburst activity drives LIRGs above the star-forming main sequence of galaxies, extranuclear star formation still proceeds in a more extreme fashion relative to what is seen in local spiral galaxies.

Image: Multifrequency imaging of a typical galaxy, in the GOALS VLA survey, NGC 3110 at a distance of 70 Mpc (Linden et al. 2019).

Publication: S.T. Linden (Univ of Virginia) et al., A Very Large Array Survey of Luminous Extranuclear Star-forming Regions in Luminous Infrared Galaxies in GOALS, Astrophysical Journal, 881, 70 (13 August 2019).