NGC 2770: high supernova rate due to interaction
  Galaxies which hosted many core-collapse supernovae (SN) explosions can be
used to study the conditions necessary for the formation of massive stars. NGC
2770 was dubbed a SN factory, because it hosted four core-collapse SNe in 20
years (three type Ib and one type IIn). Its star formation rate (SFR) was
reported not to be enhanced and therefore not compatible with such a high SN
rate. We aim at explaining the high SN rate of NGC 2770. We used archival HI
line data for NGC 2770 and reinterpret the Halpha and optical continuum data.
Even though the continuum-based SFR indicators do not yield high values, the
dust-corrected Halpha luminosity implies a high SFR, consistent with the high
SN rate. Such disparity between the SFR estimators is an indication of recently
enhanced star formation activity, because the continuum indicators trace long
timescale of the order of 100 Myr, unlike the line indicators, which trace
timescales of the order of 10 Myr. Hence, the unique feature of NGC 2770
compared to other galaxies is the fact that it is observed very recently after
the enhancement of the SFR. It also has high dust extinction, E(B-V) above 1
mag. We provide support for the hypothesis that the increased SFR in NGC 2770
is due to the interaction with its companion galaxies. We report an HI bridge
between NGC 2770 and its closest companion and the existence of a total of four
companions within 100 kpc (one identified for the first time). There are no
clear HI concentrations close to the positions of SNe in NGC 2770 such as those
detected for hosts of gamma-ray bursts (GRBs) and broad-lined SNe type Ic
(IcBL). This suggests that the progenitors of type Ib SNe are not born out of
recently accreted atomic gas, as was suggested for GRB and IcBL SN progenitors.
