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Register a new userStarburst in the Ultra-luminous Galaxy Arp 220 - Constraints from Observations of Radio Recombination Lines and Continuum
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Authors
K.R. Anantharamaiah
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We present observations of radio recombination lines from the starburst galaxy Arp 220 at 1.4, 8.1, 84, 96 and 207 GHz (sensitive upper limit for the 1.4 GHz line and firm detections at the other frequencies), and the radio continuum spectrum between 330 MHz and 207 GHz. We show that a model with three components of ionized gas with different densities and area covering factors can consistently explain both RRL and continuum data. The total mass of ionized gas in the three components is 3 x 10^7 M_sun requiring 3 x 10^5 O5 stars with a total Lyman continuum production rate (NLyc) of 1.3 x 10^{55} photons /s. These values imply a dust extinction A_V ~ 45 magnitudes and an SFR of ~240 M_sun/yr. The NLyc of ~3% associated with the high density HII regions implies similar SFR at recent epochs. The data is also consistent with multiple starbursts of very high SFR and short durations. The derived value of 24 for the IR-excess favours a starburst rather than an AGN as the origin of the observed FIR luminosity. (the abstract has been abridged)
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We present our new deep optical imaging and long-slit spectroscopy for Arp 220 that is the archetypical ULIRG in the local universe. Our sensitive Ha imaging has newly revealed large-scale, Ha absorption, i.e., post-starburst regions in this merger; one is found in the eastern superbubble and the other is in the two tidal tails that are clearly reveled in our deep optical imaging. The size of Ha absorption region in the eastern bubble is 5 kpc x 7.5 kpc and the observed Ha equivalent widths are ~2 A +- 0.2 A. The sizes of the northern and southern Ha-absorption tidal tails are ~5 kpc x 10 kpc and ~6 kpc x 20 kpc, respectively. The observed Ha equivalent widths range from 4 A to 7 A. In order to explain the presence of the two post-starburst tails, we suggest a possible multiple-merger scenario for Arp 220 in which two post-starburst disk-like structures merged into one, and then caused the two tails. This favors that Arp 220 is a multiple merging system composed of four or more galaxies, arising from a compact group of galaxies. Taking our new results into account, we discuss a star formation history in the last 1 Gyr in Arp 220.
We present high angular resolution (0.7) observations made with the Very Large Array (VLA) of the radio recombination line (RRL) H53alpha and radio continuum emission at 43 GHz from the ultraluminous infrared galaxy (ULIRG) Arp 220. The 43 GHz continuum emission shows a compact structure (~2) with two peaks separated by ~1, the East (E) and West (W) components, that correspond to each galactic nucleus of the merger. The spectral indices for both the E and W components, using radio continuum images at 8.3 and 43 GHz are typical of synchrotron emission (alpha ~ -1.0). Our 43 GHz continuum and H53alpha line observations confirm the flux densities predicted by the models proposed by Anantharamaiah et al. This agreement with the models implies the presence of high-density (~ 100,000 cm^-3) compact HII regions (~ 0.1 pc) in Arp 220. The integrated H53alpha line emission is stronger toward the non-thermal radio continuum peaks, which are also coincident with the peaks of molecular emission of the H2CO. The coincidence between the integrated H53alpha and the H2CO maser line emission suggests that the recent star forming regions, traced by the high density gas, are located mainly in regions that are close to the two radio continuum peaks. A velocity gradient of ~ 0.30 km/s/pc in the H53alpha RRL is observed toward the E component and a second velocity gradient of ~ 0.15 km/s/pc is detected toward the W component. The orientations of these velocity gradients are in agreement with previous CO, HI and OH observations. The kinematics of the high-density ionized gas traced by the H53alpha line are consistent with two counter rotating disks as suggested by the CO and HI observations.
We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp220. the correct spatial configuration of the various componnents of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern nuclear region in MERLIN scales; this gradient is three times smaller than seen in OH and implies that the OH gas lies inside the HI. A re-analysis of previously presented global VLBI data (Lonsdale et al. 1998) reveals a very high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the site of a heavily obscured AGN.
The cores of Arp 220, the closest ultra-luminous infrared starburst galaxy, provide an opportunity to study interactions of cosmic rays under extreme conditions. In this paper, we model the populations of cosmic rays produced by supernovae in the central molecular zones of both starburst nuclei. We find that ~65 - 100% of cosmic rays are absorbed in these regions due to their huge molecular gas contents, and thus, the nuclei of Arp 220 nearly complete proton calorimeters. As the cosmic ray protons collide with the interstellar medium, they produce secondary electrons that are also contained within the system and radiate synchrotron emission. Using results from chi-squared tests between the model and the observed radio spectral energy distribution, we predict the emergent gamma-ray and high-energy neutrino spectra and find the magnetic field to be at milligauss levels. Because of the extremely intense far-infrared radiation fields, the gamma-ray spectrum steepens significantly at TeV energies due to gamma-gamma absorption.
Recent analyses of the gamma-ray spectrum from the ultra-luminous infrared galaxy Arp 220 have revealed a discrepancy in the cosmic ray energy injection rates derived from the gamma-rays versus the radio emission. While the observed radio emission is consistent with the star formation rate inferred from infrared observations, a significantly higher cosmic ray population is necessary to accurately model the measured gamma-ray flux. To resolve this discrepancy between the radio and gamma-ray observations, we find that we must increase the cosmic ray energy injection rate and account for an infrared optical depth greater than unity. Raising the energy injection rate naturally raises the total gamma-ray flux but also raises the radio flux unless there is also an increase in the energy loss rate for cosmic ray leptons. A optically thick medium results in an increase in energy losses via inverse Compton for cosmic ray leptons and preserves agreement with submillimeter, millimeter, and infrared wavelength observations.
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