The population of SNe/SNRs in the starburst galaxy Arp 220

Abstract

Context. 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). SNe and SNRs are thought to be the sites of relativistic particle acceleration powering star formation induced radio emission in galaxies, and are hence important for studies of for example the origin of the FIR–radio correlation. Aims. In this work we aim for a self-consistent analysis of a large collection of Arp 220 continuum VLBI data sets. With more data and improved consistency in calibration and imaging, we aim to detect more sources and improve source classifications with respect to previous studies. Furthermore, we aim to increase the number of sources with robust size estimates, to analyse the compact source luminosity function (LF), and to search for a luminosity–diameter (LD) relation within Arp 220. Methods. Using new and archival VLBI data spanning 20 years, we obtained 23 high-resolution radio images of Arp 220 at wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtained estimates of flux densities and sizes of detected sources. The sources were classified in groups according to their observed lightcurves, spectra and sizes. We fitted a multi-frequency supernova light-curve model to the object brightest at 6 cm to estimate explosion properties for this object. Results. We detect radio continuum emission from 97 compact sources and present flux densities and sizes for all analysed observation epochs. The positions of the sources trace the star forming disks of the two nuclei known from lower-resolution studies. We find evidence for a LD-relation within Arp 220, with larger sources being less luminous. We find a compact source LF n(L)∝Lβ with β = −2.19 ± 0.15, similar to SNRs in normal galaxies, and we argue that there are many relatively large and weak sources below our detection threshold. The brightest (at 6 cm) object 0.2195+0.492 is modelled as a radio SN with an unusually long 6 cm rise time of 17 years. Conclusions. 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 SNe, for example type IIn, and correspond to few percent of the total number of SNe in Arp 220. Assuming all IIns reach these luminosities, and no confusion with other SNe types, our data are 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. However, colliding SN shocks and the production of secondary electrons through cosmic ray (CR) protons colliding with the dense ISM may cause weak sources to radiate much longer than assumed in this work. This could potentially explain the remaining fraction of the smooth synchrotron component. Future, deeper observations of Arp 220 will probe the sources with lower luminosities and larger sizes. This will further constrain the evolution of SNe/SNRs in extreme environments and the presence of AGN activity.

A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from High-resolution Radio Observations

“…The Galactic SNR Cas A and Arp 220 are shown for comparison. Arp 220 has a SFR of ∼ 220 M yr −1 (Varenius et al 2016) and all of the radio SNRs/SNe have L 9GHz L 5GHz < 10 21 W Hz −1 (Varenius et al 2019). Right: 9 GHz luminosity densities of the compact radio sources versus the expected cumulative luminosity from a population of SNRs/SNe in the host galaxies ( §5.3).…”

MUSE view of Arp220: Kpc-scale multi-phase outflow and evidence for positive feedback

“…Arp220 experiences a powerful SB at each of the nuclei, which results in the IR prominence (L IR ∼ 5.6 × 10 45 erg s −1 ; Nardini et al 2010) as well as multiple radio SNe and SN remnants at each of the nuclei (Lonsdale et al 2006;Varenius et al 2019). There is still no convincing direct evidence, from radio to hard X-ray wavelengths, for AGN in Arp 220 (e.g.…”

“…König et al 2012), compact SCs (e.g. Wilson et al 2006;Varenius et al 2019), diffuse emission, and (probably) inflows and outflows (Arribas et al 2001;Colina et al 2004). The Hα and [N II] lines, the brightest features in the Arp220 optical spectra, generally show double peaks and prominent blue and/or red wings.…”

“…In order to investigate the possible origin of the outflow, we compared our inferred values of the total outflow energetics with the expected kinetic and momentum power ascribed to stellar processes. The expected rate of energy injection from SN explosions is ∼10 44 erg s −1 , and it has been derived assuming the standard SN energy, K SN = 10 51 erg, and considering the Arp220 SN rate of 4 yr −1 (Varenius et al 2019). This expected kinetic power is very similar to that derived assuming a proportionality between K and the SFR (Veilleux et al 2005), KSF ∼ 1.8 × 10 44 erg s −1 , taking into account the Arp220 SFR of 250 M yr −1 (Nardini et al 2010).…”

Core-collapse supernova subtypes in luminous infrared galaxies

“…The effect of a missing population of CCSNe becomes even more prominent in luminous (10 11 L < L IR < 10 12 L ) and ultraluminous (L IR > 10 12 L ) infrared galaxies (LIRGs and ULIRGs, respectively) which are highly obscured and star-forming galaxies; it is also very common that these objects are closely paired or merging galaxies (Sanders et al 1988). Radio observations have shown the existence of a rich population of radio SNe (and SN remnants) in ULIRGs (e.g., Parra et al 2007;Varenius et al 2019), LIRGs (e.g., Pérez-Torres et al 2009;Ulvestad 2009), and starburst galaxies (e.g., Fenech et al 2008;Mattila et al 2013), which have remained undetected by optical surveys. In particular, LIRGs are relatively rare in the local Universe; however, they dominate the cosmic star formation history and the resulting CCSN rate beyond z ∼ 1 (e.g., Magnelli et al 2011), both of which peak at z ∼ 2 (Madau & Dickinson 2014).…”