Starburst in the Ultraluminous Galaxy Arp 220: Constraints from Observations of Radio Recombination Lines and Continuum

Abstract

We present observations of radio recombination lines (RRL) from the starburst galaxy Arp 220 at 8.1 GHz (H92a) and 1.4 GHz (H167a and H165a) and at 84 GHz (H42a), 96 GHz (H40a) and 207 GHz (H31a) using the Very Large Array and the IRAM 30 m telescope, respectively. RRLs were detected at all the frequencies except 1.4 GHz, where a sensitive upper limit was obtained. We also present contin- uum —ux measurements at these frequencies as well as at 327 MHz made with the VLA. The continuum spectrum, which has a spectral index a D (0.6 between 5 and 10 GHz, shows a break near 1.5 (S l P la) GHz, a prominent turnover below 500 MHz, and a —atter spectral index above 50 GHz. We show that a model with three components of ionized gas with diUerent densities and area covering factors can consis- tently explain both RRL and continuum data. The total mass of ionized gas in the three components is 3.2 ) 107 requiring 3 ) 105 O5 stars with a total Lyman continuum production rate M _ , N Lyc D 1.3 ) 1055 photons s~1. The ratio of the expected to observed Bra and Brc —uxes implies a dust extinction mag. The derived Lyman continuum photon production rate implies a continuous star forma- A V D 45 tion rate (SFR) averaged over the lifetime of OB stars of D240 yr~1. The Lyman continuum photon M _ production rate of D3% associated with the high-density H II regions implies a similar SFR at recent epochs (t 105 yr). An alternative model of high-density gas, which cannot be excluded on the basis of the available data, predicts 10 times higher SFR at recent epochs. If con—rmed, this model implies that star formation in Arp 220 consists of multiple starbursts of very high SFR (few times 103 yr~1) and M _ short duration (D105 yr). The similarity of IR excess, in Arp 220 to values observed in L IR /L Lya D 24, starburst galaxies shows that most of the high luminosity of Arp 220 is due to the ongoing starburst rather than to a hidden active galactic nucleus (AGN). A comparison of the IR excesses in Arp 220, the Galaxy, and M33 indicates that the starburst in Arp 220 has an initial mass function that is similar to that in normal galaxies and has a duration longer than 107 yr. If there was no infall of gas during this period, then the star formation efficiency (SFE) in Arp 220 is D50%. The high SFR and SFE in Arp 220 is consistent with their known dependences on mass and density of gas in star-forming regions of normal galaxies. Subject headings: galaxies: individual (Arp 220) ¨ galaxies: nucleigalaxies: starburst ¨ radio continuum: galaxiesradio lines: galaxies