[Paper Review] The population of SNe/SNRs in the starburst galaxy Arp 220. A self-consistent analysis of 20 years of VLBI monitoring

E. Varenius, J. E. Conway+7 more|LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas)|Feb 15, 2017

Astrophysics and Cosmic Phenomena42 references4 citations

TL;DR

This study presents a self-consistent 20-year VLBI analysis of Arp 220, detecting 97 compact radio sources and finding their luminosity function follows $n(L)\propto L^{-2.19\pm0.15}$, consistent with SNRs in normal galaxies. It reveals a luminosity-diameter relation and identifies two populations: luminous, compact SNe (likely Type IIn) and larger, less luminous SNRs, with fainter sources potentially contributing significantly via cosmic-ray re-acceleration.

ABSTRACT

The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellent laboratory for studies of extreme astrophysical environments. For 20 years, Very Long Baseline Interferometry (VLBI) has been used to monitor a population of compact sources thought to be supernovae (SNe), supernova remnants (SNRs) and possibly active galactic nuclei (AGNs). Using new and archival VLBI data spanning 20 years, we obtain 23 high-resolution radio images of Arp 220 at wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtain estimates of flux densities and sizes of all detected sources. We detect radio continuum emission from 97 compact sources and present flux densities and sizes for all analysed observation epochs. We find evidence for a LD-relation within Arp 220, with larger sources being less luminous. We find a compact source LF $n(L)\propto L^β$ with $β=-2.19\pm0.15$, similar to SNRs in normal galaxies. Based on simulations we argue that there are many relatively large and weak sources below our detection threshold. The observations can be explained by a mixed population of SNe and SNRs, where the former expand in a dense circumstellar medium (CSM) and the latter interact with the surrounding interstellar medium (ISM). Nine sources are likely luminous, type IIn SNe. This number of luminous SNe correspond to few percent of the total number of SNe in Arp 220 which is consistent with a total SN-rate of 4 yr$^{-1}$ as inferred from the total radio emission given a normal stellar initial mass function (IMF). Based on the fitted luminosity function, we argue that emission from all compact sources, also below our detection threshold, make up at most 20\% of the total radio emission at GHz frequencies.

Motivation & Objective

• To perform a self-consistent analysis of 20 years of VLBI data on Arp 220 to improve source detection and classification beyond previous studies.
• To increase the number of compact sources with robust size estimates for better characterization of the SN/SNR population.
• To derive the compact source luminosity function (LF) and investigate a potential luminosity-diameter (LD) relation in the extreme environment of Arp 220.
• To assess the contribution of unresolved or faint sources to the total radio emission, particularly considering non-thermal particle acceleration mechanisms.

Proposed method

• Acquisition and reprocessing of 23 high-resolution VLBI images spanning 18 cm to 2 cm wavelengths from 20 years of monitoring.
• Model-fitting to visibility data to estimate flux densities and sizes of compact sources, using consistent calibration and imaging techniques across all datasets.
• Classification of sources based on lightcurves, spectral indices, and morphological sizes to distinguish between SNe, SNRs, and possible AGN candidates.
• Fitting a multi-frequency supernova light-curve model to the brightest 6 cm source to estimate explosion parameters such as explosion energy and circumstellar medium density.
• Statistical analysis of the luminosity function and investigation of selection effects to assess the reliability of the luminosity-diameter relation.
• Evaluation of potential AGN contributions through search for radio jets and analysis of image noise for non-Gaussian features, including consideration of stacking and tapering techniques for future sensitivity improvements.

Experimental results

Research questions

• RQ1 What is the true number and distribution of compact radio sources in Arp 220, and how do they evolve over 20 years of VLBI monitoring?
• RQ2 What is the shape of the compact source luminosity function in Arp 220, and how does it compare to those in normal galaxies?
• RQ3 Is there a luminosity-diameter relation among the compact sources, and what does it imply about their evolutionary state?
• RQ4 To what extent do faint or undetected sources contribute to the total radio emission, and could mechanisms like cosmic-ray re-acceleration explain the missing flux?
• RQ5 Could an underlying AGN population be hidden in the data due to lack of spatial resolution or episodic activity, and how might future observations constrain this?

Key findings

• The study detects 97 compact radio sources in Arp 220, with a luminosity function best described by $n(L)\propto L^{-2.19\pm0.15}$, consistent with SNRs in quiescent galaxies.
• The spatial distribution of sources traces the star-forming disks of the two nuclei, indicating a strong correlation with ongoing star formation.
• Evidence for a luminosity-diameter relation is found, where larger sources are less luminous, though selection effects complicate precise quantification.
• The observed source population is best explained by two distinct groups: very luminous, compact SNe (likely Type IIn) and larger, less luminous SNRs interacting with the ISM.
• The number of very luminous SNe detected is consistent with expectations from a standard initial mass function and the galaxy’s total star formation rate, though age estimates remain uncertain.
• Extrapolating the luminosity function below detection limits suggests these sources contribute at most 20% of the total radio flux at GHz frequencies, implying that secondary cosmic-ray processes may account for the remainder.