Dorado and its member galaxies. Ha imaging of the group backbone
  ABRIDGED: Dorado is a nearby, rich and clumpy galaxy group that extends for
several degrees in the Southern Hemisphere. For the first time, we map the Ha
distribution as a possible indicator of star formation (SF) activity of Dorado
members a large fraction of which show interaction and merging signatures. With
the 2.5m du Pont and the 1m Swope telescopes we obtained narrow-band,
calibrated images of 14 galaxies, forming the backbone of the group, mapping
Ha+[N II] down to few 10$^{-17}$ erg~cm$^{-2}$~s$^{-1}$~arcsec$^{-2}$. We
estimated the galaxy star formation rate (SFR) from the Ha fluxes, corrected
for Galaxy foreground extinction and [N II] contamination. Ha+[N II] emission
has been detected in all galaxies. HII regions clearly emerge in late-type
galaxies (LTGs) , while in early-type galaxies (ETGs) the Ha+[N II] emission is
dominated by [N II], especially in the central regions. However, HII complexes
are also revealed in four ETGs. Considering the Dorado group as a whole, we
notice that the Ha+[N II] equivalent width, a measure of the specific SF,
increases with the morphological type, from early to late-type members,
although it remains lower that what observed in similar surveys of spiral
galaxies. The SFR of the spiral members is in the range of what observed in
similar galaxies surveys James+ (2004), although, in three spirals the SFR is
well below the median for their morphological classes. The SFR of some
early-type members tends, at odds, to be higher than the average derived from
Ha+[N II] surveys of this morphological family. We detected in Ha+[N II] all
the ETGs observed and half of them show HII regions. These findings suggest
that ETGs in this group are not dead galaxies: their SF has not shut down yet.
Mechanisms such as gas stripping and gas accretion, through galaxy-galaxy
interaction, seem relevant in modifying the SF in this evolutionary phase of
Dorado.
