Markarian 52 : a gas-rich starburst galaxy.

Abstract

We have observed the "starburst" galaxy Markarian 52(= NGC 4385) both through broad and narrow band imaging, and spectroscopically in the UV and optical ranges. The narrow band Hα+[N II] continuum-free image shows that the ionized gas extends over at least 19" (4.1 kpc). However, our long slit spectroscopic data reveals a much larger extent of the ionized nebulosity: the [O II] λ 3727 line reaches a 66" (14 kpc) diameter along a slit position angle of 277^deg^. The rotation curve of the ionized gas derived from the [O II] λ 3727 line is smooth, with a solid-body rotation in the central region of the galaxy. The overall far-IR to far-UV continuum of Mkn 52 shows the existence of several components. At least two dust components are present in the far IR, with temperatures of 35 and 150 K, while the optical emission is mostly stellar, and a strong excess over the extrapolation of the optical spectrum is observed in the UV range. This UV excess is interpreted as due to a large number of OB stars, which account for the ionization of the gas. On the other hand, the far-IR luminosity is higher than the UV plus optical one by an order of magnitude, therefore raising the problem of heating the dust. We have analyzed the nuclear emission line spectrum of Mkn 52 in the UV and optical ranges and find evidence for the presence of Wolf-Rayet stars, of dominant subtypes WN7 and WC8. The total mass of ionized gas is 1.0 10^6^ M_sun_ and its filling factor is 0.02-0.04. Line intensities are also given in several other regions of the extended ionized nebulosity, and a more detailed analysis of a bright extranuclear H II region is included. We have constructed chemical and photometric stellar population synthesis models and compared them to the observed metallicity and UBV colours of Mkn 52. The best fitting model is a composite one with an old disk stellar population and a starburst characterized by an Initial Mass Fuction of slope x=0.95, a burst strength b=0.0005 and an age of 1 10^7^ yr. Stars formed in this burst during 4 10^6^ yr. The youngest OB stars are therefore mainly responsible for the ionization of the gas, since the WR contribution to the ionization is only 5%.