Alatalo, K.13; Appleton, P. N.13; Lisenfeld, U.4; Bitsakis, T.5; Guillard, P.6; Charmandaris, V.14; Cluver, M.15; Dopita, M. A.16; Freeland, E.17; Jarrett, T.15; Kewley, L. J.16; Ogle, P. M.13; Rasmussen, Jesper1; Rich, J. A.13; Verdes-Montenegro, L.11; Xu, C. K.13; Yun, M.18
1 Department of Physics, Technical University of Denmark2 Plasma Physics and Fusion Energy, Department of Physics, Technical University of Denmark3 California Institute of Technology4 Universidad De Granada5 Universidad Nacional Autónoma de México6 Université de Paris-Sud7 University of Crete8 University of Cape Town9 Australian National University10 Stockholm University11 Instituto de Astrofísica de Andalucía12 University of Massachusetts13 California Institute of Technology14 University of Crete15 University of Cape Town16 Australian National University17 Stockholm University18 University of Massachusetts
We present [C II] and [O I] observations from Herschel and CO(1-0) maps from the Combined Array for Research in Millimeter Astronomy (CARMA) of the Hickson compact group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG 57a has been previously shown to contain enhanced quantities of warm molecular hydrogen consistent with shock or turbulent heating. Our observations show that HCG 57d has strong [C II] emission compared to L-FIR and weak CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies at the upper end of the normal distribution of the [C II]/CO and [C II]/FIR ratios, and its far-infrared (FIR) cooling supports a low-density, warm, diffuse gas that falls close to the boundary of acceptable models of a photon-dominated region. However, the power radiated in the [C II] and warm H2 emissions have similar magnitudes, as seen in other shock-dominated systems and predicted by recent models. We suggest that shock heating of the [C II] is a viable alternative to photoelectric heating in violently disturbed, diffuse gas. The existence of shocks is also consistent with the peculiar CO kinematics in the galaxy, indicating that highly noncircular motions are present. These kinematically disturbed CO regions also show evidence of suppressed star formation, falling a factor of 10-30 below normal galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar properties of both galaxies are consistent with a highly dissipative, off-center collisional encounter between HCG 57d and 57a, creating ring-like morphologies in both systems. Highly dissipative gas-on-gas collisions may be more common in dense groups because of the likelihood of repeated multiple encounters. The possibility of shock-induced star-formation suppression may explain why a subset of these HCG galaxies has been found previously to fall in the mid-infrared green valley.
Astrophysical Journal, 2014, Vol 795, Issue 2, p. 1-16
galaxies: evolution; galaxies: individual (NGC 3753, NGC 3754); galaxies: kinematics and dynamics; galaxies: star formation