A STUDY OF HEATING AND COOLING OF THE ISM IN NGC 1097 WITHHERSCHEL-PACS ANDSPITZER-IRS

Abstract

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [C ii]158 μm and [O i]63 μm lines and mid-infrared H 2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([C ii]158 μm+[O i]63 μm)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 μm PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [C ii]158 μm/PAH(5.5-14 μm) is found. PAHs in the ring are responsible for a factor of two more [C ii]158 μm and [O i]63 μm emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G 0 ∼ 10 2.3 and n H ∼ 10 3.5 cm −3 in the ring. For these values of G 0 and n H , PDR models cannot reproduce the observed H 2 emission. Much of the H 2 emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks.

The Origins of [C ii] Emission in Local Star-forming Galaxies

“…Given the sensitivity of Herschel, one possible method to isolate the contribution of [C II] emission from ionized gas is to use the [N II] 122 μm line, which is emitted purely from the ionized phase (Beirão et al 2012;Croxall et al 2012). The ratio of [C II] 158 μm to [N II] 122 μm is, however, density dependent.…”

Submillimeter-HCN Diagram for Energy Diagnostics in the Centers of Galaxies

“…Indeed, the line luminosity of HCN(4-3) in NGC 1097 is totally comparable to that of the individual molecular clump seen at the center of NGC 253 (∼20 pc in size; Sakamoto et al 2011), suggesting the similarly small size of the line-emitting region in NGC 1097. Radiative feedback from SB activity would be discarded considering the high gas temperature (100 K) suggested for the molecular phase (I13; Beirão et al 2012). On the other hand, mechanical heating can be another choice, which is supported by the detection of a compact radio jet (largest beam-convolved and projected size is ∼90 pc; Thean et al 2000).…”

SHOCK-ENHANCED C+EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USINGHERSCHEL

“…Here we define PAH tot as the sum of the PAH feature fluxes from 6 to 17 μm. For diffuse regions in the Galaxy (e.g., Habart et al 2003), this is typically few percent, and has been directly measured recently in a variety of extragalactic environments (Beirão et al 2012;Croxall et al 2012) ranging from 2% to 5%. Figure 8 (see also Table 1, Column 7) shows the [C ii]/PAH ratio for the five PACS regions plotted against far-IR color temperature.…”