VCC 144: a star-bursting dwarf galaxy in the Virgo cluster

We describe results of a multi-spectral study of a blue compact dwarf galaxy in the Virgo Cluster. The object was observed with broad-band and Hα imaging, ultraviolet observations, and radio synthesis. Our data were combined with previously published optical observations, with HI single-beam observation and with far-infrared data, and were compared to results of evolutionary synthesis programs. The radio synthesis observations revealed a compact concentration of HI coincident with the optical galaxy, embedded in a diffuse, asymmetric HI cloud which has no trace of optical emission. While the overall velocity dispersion and size of the HI structure suggests that the total mass in stars and gas is not sufficient to gravitationally bind the system as a whole, the HI clump coincident with the optical galaxy requires little or no dark matter to be self-gravitating. The diffuse cloud has more complex velocity structure and is extended in a direction approximately perpendicular to the optical major axis. The optical-UV data can be explained by a single population of stars formed in a recent burst, indicating that this is a genuine young galaxy. The efficiency of star formation is similar to that in large disk galaxies. The IR emission indicates the presence of dust; this must have been formed very recently, or was already present within the original HI cloud from which the galaxy was formed. The round and smooth isophotes, the correspondence of the optical and HI redshifts, and the lack of any suitable nearby galaxy, indicate that the starburst was probably not triggered by an external interaction with a visible galaxy. The distribution of HI fits better a blow-out scenario than an accretion or collision with a companion. We point out features in common with other actively star-forming dwarf galaxies and conclude that, at least in the southern outskirts of the Virgo cluster, intensive star formation, perhaps for the first time in some objects, takes place at present. Based on observations by the International Ultraviolet Explorer (IUE) collected at the Villafranca Satellite Tracking Station of the European Space Agency