Unveiling the Power Sources of ULIRGs with VLBA Radio SEDs

VLBA Large Program 24B-355

Background: Ultraluminous infrared galaxies (ULIRGs), powered by star-formation and active galactic nuclei (AGN), are some of the most extreme galaxies in the local Universe, but two mysteries remain regarding the interplay of starbursts and AGN activity in their dusty nuclei. Firstly, the number of detected supernovae (SNe) in ULIRGs is significantly lower than what the star-formation rate implies. Secondly, AGN fractions derived by optical, infrared, and X-ray surveys are inconsistent, raising questions about the true prevalence of AGN in ULIRGs. These two mysteries may be rooted in the limitations of shorter-wavelength studies to probe obscured sources at high resolutions. Milliarcsecond radio interferometers, however, are sensitive to synchotron emission from AGN and radio-SNe and can resolve these parsec-scale sources without contamination from diffuse emission.

In this program, we conduct a multi-band VLBA survey of local ULIRGs to identify and characterize AGNs and SNe. The VLBA is the only instrument that can provide the robust detections necessary to constrain the true SNe rate and AGN fraction of ULIRGs. These detections will establish the relative contributions of AGN activity and starbursts in ULIRGs, informing merger-driven models of nuclear feedback, AGN fueling, and resolve the origins of the ULIRGs' radio emission.

Science Questions:

• How prevalent are obscured AGN in the nuclei of U/LIRGs? Do they harbor unique properties compared to less obscured AGN?
• What are the relative energetic contributions of AGN activity and star formation to the radio power of the host galaxy?
• What is the estimated star formation rate based on the radio SNe detections? Are these consistent with far-infrared estimates?
• How do AGN and starbursts co-evolve in the final stages of mergers?

Sample: We select 13 ULIRGs from the GOALS sample that did not have previous extensive VLBI studies. All targets have declinations north of -20 degrees, ensuring visibility to all 10 VLBA dishes for > 4 hours per day.

A pilot study of two proposed of the targets (IRAS 07251−0248 and IRAS F23365+3604) was approved as a part of VLBA program 24B-355.

Observations: We observe each galaxy nucleus at 2cm (13 GHz, U-band), 6cm (5 GHz, C-band), and 21cm (1.4GHz, L-band). These quasi-simultaneous multi-band observations will also allow us to determine the cm-wave spectral shape of all detected sources which will distinguish between AGN cores, jets, and radio SNe.

Data Products: Images will be released in tandem with the publication of a catalog paper.