We compare the infrared excess (IRX) and Balmer decrement (Hα/Hβ) as dust attenuation indicators in relation to other galaxy parameters using a sample of ∼32 000 local star-forming galaxies (SFGs) carefully selected from SDSS, GALEX and WISE. While at fixed Hα/Hβ, IRX turns out to be independent on galaxy stellar mass, the Balmer decrement does show a strong mass dependence at fixed IRX. We find the discrepancy, parameterized by the color excess ratio R EBV ≡ E(B−V ) IRX /E(B−V ) Hα/Hβ , is not dependent on the gas-phase metallicity and axial ratio but on the specific star formation rate (SSFR) and galaxy size (R e ) following R EBV = 0.79+0.15 log(SSFR/R 2 e ). This finding reveals that the nebular attenuation as probed by the Balmer decrement becomes increasingly larger than the global (stellar) attenuation of SFGs with decreasing SSFR surface density. This can be understood in the context of an enhanced fraction of intermediate-age stellar populations that are less attenuated by dust than the H ii region-traced young population, in conjunction with a decreasing dust opacity of the diffuse ISM when spreading over a larger spatial extent. Once the SSFR surface density of an SFG is known, the conversion between attenuation of nebular and stellar emission can be well estimated using our scaling relation.

We investigate the properties of long tidal tails using the largest to date sample of 461 merging galaxies with $\log (M_\ast /\rm M_\odot )\ge 9.5$ within 0.2 ≤ z ≤ 1 from the COSMOS survey in combination with Hubble Space Telescope imaging data. Long tidal tails can be briefly divided into three shape types: straight (41 per cent), curved (47 per cent) and plume (12 per cent). Their host galaxies are mostly at late stages of merging, although 31 per cent are galaxy pairs with projected separations d > 20 kpc. The high formation rate of straight tidal tails needs to be understood as the projection of curved tidal tails accounts for only a small fraction of the straight tails. We identify 165 tidal dwarf galaxies (TDGs), yielding a TDG production rate of 0.36 per merger. Combined with a galaxy merger fraction and a TDG survival rate from the literature, we estimate that ∼ 5 per cent of local dwarf galaxies are of tidal origin, suggesting the tidal formation is not an important formation channel for the dwarf galaxies. More than half of TDGs are located at the tip of their host tails. These TDGs have stellar masses in the range of $7.5\le \log (M_\ast /\rm M_\odot )\le 9.5$ and appear compact with half-light radii following the M* - Re relation of low-mass elliptical galaxies. However, their surface brightness profiles are generally flatter than those of local disc galaxies. Only 10 out of 165 TDGs have effective radii larger than 1.5 kpc and would qualify as unusually bright ultra-diffuse galaxies.