Insights into gas heating and cooling in the disc of NGC 891 fromHerschelfar-infrared spectroscopy

Abstract

We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in the nearby, edge-on spiral galaxy, NGC 891: [C] with increasing FIR colour suggests that polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in the central regions. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a PDR model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet (FUV) radiation field, G 0 . Resolving details on physical scales of ∼0.6 kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC 891's disc have hydrogen densities of 1 < log(n/cm −3 ) < 3.5 experiencing an incident FUV radiation field with strengths of 1.7 < log G 0 < 3. Although these values we derive for most of the disc are consistent with the gas properties found in PDRs in the spiral arms and inter-arm regions of M 51, observed radial trends in n and G 0 are shown to be sensitive to varying optical thickness in the lines, demonstrating the importance of accurately accounting for optical depth effects when interpreting observations of high inclination systems. Increasing the coverage of our analysis by using an empirical relationship between the MIPS 24 µm and [N] 205 µm emission, we estimate an enhancement of the FUV radiation field strength in the far north-eastern side of the disc relative to the rest of the disc that coincides with the above-average star formation rate surface densities and gas-to-dust ratios. However, an accurate interpretation remains difficult due to optical depth effects, confusion along the line-of-sight and observational uncertainties.

The spatially resolved correlation between [NII] 205μm line emission and the 24μm continuum in nearby galaxies

“…In this paper, we thus attempt to assess whether the [N] 205−24 µm correlation has some physical origin or is merely an artefact of lineof-sight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of face-on and interacting galaxies, and compare our findings to the NGC 891 correlation of Hughes et al (2015). We show that not only is this empirical relationship also observed in face-on systems, but also that this correlation appears to be driven by the star formation rate.…”

“…Firstly, as mentioned in Hughes et al (2015), despite the high quality of observations of the far-infrared, fine-structure lines made possible by the Herschel Space Observatory, which enable us to resolve features on sub-kiloparsec scales, one of the main restrictions was the relatively sparse coverage and low spatial resolution of the [N] 205 µm line. There is also uncertainty in the SFR calibration.…”

“…1, we quantify the correlation between [N] 205 µm line emission and MIPS 24 µm emission for each galaxy. Figure 1e presents the original correlation in NGC 891 reported by Hughes et al (2015). To briefly recap their results, the Spearman (ρ S ) and Pearson (ρ P ) coefficients of rank correlation are 0.94 and 0.88, respectively, where a value of 1 represents a perfect correlation.…”

“…The SPIRE FTS instrument (Griffin et al 2010) Hughes et al (2015), Parkin et al (2013), Wu et al (2015) and Schirm et al (2014), and we adopt the same distances of 9.6, 8.4, 3.6, and 20.0 Mpc, respectively. Here, we briefly summarize the observations and data reduction for each galaxy, whilst full details may be found in these corresponding papers.…”

“…In a recent study of the nearby edge-on spiral galaxy NGC 891, Hughes et al (2015) exploited such an empirical relationship to predict the [N] 205 µm line emission from a higher resolution 24 µm image. Although this appears to be a powerful and promising technique for estimating the [N] 205 µm line emission from more widely available Spitzer-MIPS data, the underlying nature of the correlation is still unknown in part because of uncertainties associated with observations of high inclination systems.…”