Optical spectroscopy of Arp220: the star formation history of the closest ULIRG

Abstract

We present optical spectra of the merging system Arp 220,taken with the William Herschel Telescope(WHT) on La Palma. These data were taken with the aim of investigating the evolution and star formation history of this object. Spectral synthesis modelling has been used to estimate the ages of the stellar populations found in the diffuse light sampled by the spectra. The data show a remarkable uniformity in the stellar populations across the full 65 arcsec covered by our slit positions, sampling the measurable extent of the galaxy. The results are consistent with a dominant intermediate-age stellar population (ISP) with age 0.5 < t_{ISP} $\leq$ 0.9 Gyr that is present at all locations, with varying contributions from a young ($\leq$ 0.1 Gyr) stellar population (YSP) component. However, it is notable that while the flux contribution of the YSP component in the extended regions is relatively small ($\leq$ 40%), adequate fits in the nuclear region are only found for combinations with a significant contribution of a YSP component (22 - 63%). Moreover, while a low intrinsic reddening (E(B - V)$\lsim$ 0.3) is found for the ISPs in the extended regions, intrinsic reddening values as high as E(B - V) ~ 1.0 are required in the galactic center. This clearly reflects the presence of a reddening gradient, with higher concentrations of gas and dust towards the nuclear regions, coinciding with dust lanes in the HST images. Overall, our results are consistent with models that predict an epoch of enhanced star formation coinciding with the first pass of the merging nuclei (represented by the ISP), with a further episode of star formation occurring as the nuclei finally merge together (represented by the YSP and ULIRG).