Powerful H₂ Line Cooling in Stephan’s Quintet. II. Group-wide Gas and Shock Modeling of the Warm H₂ and a Comparison with [C ii] 157.7 μm Emission and Kinematics

Abstract

We map for the first time the two-dimensional H 2 excitation of warm intergalactic gas in Stephanʼs Quintet on group-wide (50×35 kpc 2 ) scales to quantify the temperature, mass, and warm H 2 mass fraction as a function of position using Spitzer. Molecular gas temperatures are seen to rise (to T > 700 K) and the slope of the power-law density-temperature relation flattens along the main ridge of the filament, defining the region of maximum heating. We also performed MHD modeling of the excitation properties of the warm gas, to map the velocity structure and energy deposition rate of slow and fast molecular shocks. Slow magnetic shocks were required to explain the power radiated from the lowest-lying rotational states of H 2 , and strongly support the idea that energy cascades down to small scales and low velocities from the fast collision of NGC 7318b with group-wide gas. The highest levels of heating of the warm H 2 are strongly correlated with the large-scale stirring of the medium as measured by [C II] spectroscopy with Herschel. H 2 is also seen associated with a separate bridge that extends toward the Seyfert nucleus in NGC 7319, from both Spitzer and CARMA CO observations. This opens up the possibility that both galaxy collisions and outflows from active galactic nuclei can turbulently heat gas on large scales in compact groups. The observations provide a laboratory for studying the effects of turbulent energy dissipation on groupwide scales, which may provide clues about the heating and cooling of gas at high z in early galaxy and protogalaxy formation.

Global radiation signature from early structure formation

“…In the Z sfdbk simulation, the fluxes for the pure rotational lines 0-0 S(1), 0-0 S(3) and 0-0 S(5) are much lower (by at least a factor of 10) than the overall flux, and remain two orders of magnitude below the detection limit at z = 7.2, indicating that their detection at such high redshifts will be challenging, requiring some unusual event, such as a starburst in a major merger. For Z Nsfdbk, on the other hand, the flux of the strongest line 0-0 S(1) reaches FH 2 [0-0 S(1)] ∼ 10 −23 W m −2 at z = 7.2, which is possible to detect with a factor of 10 enhancement from (strong) gravitational lensing (Appleton et al 2010) or shocks, as observed in Stephan's Quintet (Appleton et al 2017). Note that the simulated haloes with M ∼ 10 10 M are not the most massive ones at z ∼ 7.…”

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…More recently Spitzer IRS observations have shown that our census of the warm H 2 gas in galaxies may be severely incomplete, revealing a new class of galaxies (including ellipti-cal galaxies, AGN, galaxy groups, and galaxy clusters) with strongly enhanced H 2 rotational emission lines, while classical star formation indicators (far-infrared continuum emission, ionized gas lines, polycyclic aromatic hydrocarbons, PAHs) are strongly suppressed (Appleton et al, 2006;Ogle et al, 2007;Guillard et al, 2009;Cluver et al, 2010;Ogle et al, 2010;Guillard et al, 2012b;Ogle et al, 2012;Peterson et al, 2012;Guillard et al, 2015a). Among the sample of H 2 -luminous objects, the Stephan's Quintet is certainly the object where the astrophysical context is clear enough to identify the dominant source of energy that powers the H 2 emission and to associate it with the mechanical energy released in a galactic collision (Guillard et al, 2009(Guillard et al, , 2012aAppleton et al, 2013Appleton et al, , 2017. In these sources, the luminosity of the H 2 lines cannot be accounted for by UV or X-ray excitation, and their properties suggest that the dissipation of turbulence is the main heating mechanism for the warm H 2 gas.…”

Revisiting Stephan's Quintet with deep optical images

“…This system exhibits a remarkable variety of physical processes at all scales, pertaining to cooling and heating of the interstellar medium (e.g. Appleton et al 2017), star formation in shock regions (e.g. Guillard et al 2012), star cluster formation (e.g.…”