Dorado and its member galaxies. III. Mapping star formation with FUV imaging from UVIT

Context. We are investigating the star formation in galaxies of the actively evolving Dorado group where, for a large fraction of both early- and late-type galaxies, signatures of interactions and merging events are revealed by optical and radio observations. Aims: Our previous Hα+[N II] study, probing ≈10 Myr timescales, suggested that star formation is still ongoing in early-type galaxies. In this work, we use far-UV (FUV) imaging to map recent star formation on longer timescales, on the order of 100 Myr. Methods: We used the Ultraviolet Imaging Telescope UVIT on board Astrosat to image the galaxies of the Dorado backbone previously observed in Hα+[N II], with the far-UV filter FUV.CaF2 (1300-1800 Å). The sample included NGC 1536, NGC 1546, NGC 1549, [CMI2001]4136-01, NGC 1553, IC 2058, PGC 75125, NGC 1566, NGC 1596, and NGC 1602; for the two latter galaxies, the UVIT data provided the first view in far-UV. For the others, previously observed by GALEX, the UVIT data afford a ∼5× improvement in spatial resolution. Results: FUV.CaF2 emission is revealed in all the Dorado galaxies observed, tracing young stellar populations in ring structures and showing tidal distortions. The Sérsic index, derived by fitting the luminosity profiles, is always n < 3, suggesting that the FUV.CaF2 emission originates from a disk also in early-type galaxies. The star formation rate (SFR) ranges from 0.004 ± 0.001 M⊙ yr−1 for [CMI2001]4136-01 to 2.455 ± 0.027 M⊙ yr−1 for NGC 1566. Most of the recent star formation is found at the periphery of the Dorado group, where most of late-type galaxies are located. For these galaxies, the ratio SFRHα/SFRFUV.CaF2 is close to 1, except for the edge-on IC 2058, similar to previously reported relations for local volume samples. For early-type galaxies, however, SFRHα is about 15 times higher than SFRFUV. Dorado's early-type galaxies define a separate locus in SFRFUV, SFRHα space with respect to the late-type galaxies, which is well represented by the relation log (SFRFUV.CaF2) = 0.70 × log (SFRHα)−1.26. Conclusions: The disk structure of the FUV.CaF2 emitting populations discovered in all the early-types galaxies implies dissipative processes and wet merging events. The systematic discrepancy between SFRs derived from Hα and FUV fluxes suggests that rejuvenation episodes in early-type galaxies cannot sustain constant star formation over ∼100 Myr timescales.