CALIFA Spectroscopy of the Interacting Galaxy NGC 5394 (Arp 84): Starbursts, Enhanced [NII]6584 and Signs of Outflows and Shocks

Abstract

We investigate the spiral galaxy NGC 5394, which is strongly interacting with the larger spiral NGC 5395 (the pair is Arp 84), using optical integral-field spectroscopy from the CALIFA survey. Spatially-resolved equivalent-widths, emission-line ratios and kinematics reveal many features related to the interaction, which has reshaped the galaxy. $\rm Hα$ maps (with other diagnostic emission lines) show a concentrated central ($r<1$ kpc) starburst and three less luminous star-forming regions (one knot far out in the northern arm), and we estimate the dust-corrected total star-formation rate as 3.39 $\rm M_{\odot}yr^{-1}$. However, much of the galaxy, especially the outer tidal arms, has a post-starburst spectrum, evidence of a more extensive episode of star-formation a few $\times 10^8$ yr ago, triggered by the previous perigalacticon. The $\rm [NII]6584/Hα$ ratio is high in the nucleus, reaching 0.63 at the centre, which we interpret as related to high electron density ($n_e\simeq 750$ $\rm cm^{-3}$ from the $\rm [SII]{6717\over 6731}$ ratio). We find a central region of strong and blueshifted NaI(5890,5896) absorption, indicative of a starburst-driven outflow from the nucleus at an estimated velocity $\sim 223$ km $\rm s^{-1}$. The CALIFA data also show an annular region at radii 2.25--4 kpc from the nucleus, with elevated ratios of [NII], [OI]6300 etc. to the Balmer lines -- this is evidence of shock excitation, which might be the result of interaction-triggered gas inflow.