The Pico dos Dias Survey Starburst Galaxies

Abstract

We discuss the nature of the galaxies found in the Pico dos Dias Survey (PDS) for young stellar objects. The PDS galaxies were selected from the IRAS Point Source catalog. They have flux density of moderate or high quality at 12, 25, and 60 μm and spectral indices in the ranges -3.00 ≤ α(25, 12) ≤ + 0.35 and -2.50 ≤ α(60, 25) ≤ +0.85. These criteria allowed the detection of 382 galaxies, which are a mixture of starburst and Seyfert galaxies. Most of the PDS Seyfert galaxies are included in the catalog of warm IRAS sources by de Grijp et al. The remaining galaxies constitute a homogeneous sample of luminous [log F (LB/L☉) = 9.9 ± 0.4] starburst galaxies, 67% of which were not recognized as such before. The starburst nature of the PDS galaxies is established by comparing their LIR/LB ratios and IRAS colors with a sample of emission-line galaxies from the literature already classified as starburst galaxies. The starburst galaxies show an excess of FIR luminosity, and their IRAS colors are significantly different from those of Seyfert galaxies-99% of the starburst galaxies in our sample have a spectral index α(60, 25) -2.5. This color cutoff also marks a change in the dominant morphologies of the galaxies: the normal IRAS galaxies are preferentially late-type spirals (Sb and later), while the starbursts are more numerous among early-type spirals (earlier than Sbc). This preference of starbursts for early-type spirals is not new, but a trait of the massive starburst nucleus galaxies (Coziol et al.). As in other starburst nucleus galaxy samples, the PDS starbursts show no preference for barred galaxies. No difference is found between the starbursts detected in the FIR and those detected on the basis of UV excess. The PDS starburst galaxies represent the FIR luminous branch of the UV-bright starburst nucleus galaxies, with mean FIR luminosity log (LIR/L☉) = 10.3 ± 0.5 and redshifts smaller than 0.1. They form a complete sample limited in flux in the FIR at 2 × 10-10 ergs cm-2 s-1.