[en] The H i and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass $M_{\mathrm{d}\mathrm{y}\mathrm{n}}$ of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 μm line of atomic carbon ([C ii]) may be the only way to derive $M_{\mathrm{d}\mathrm{y}\mathrm{n}}$. As the distribution and kinematics of the ISM tracer affects the determination of $M_{\mathrm{d}\mathrm{y}\mathrm{n}}$, it is important to quantify the relative distributions of H i, CO, and [C ii]. H i and CO are well-characterized observationally, however, for [C ii] only very few measurements exist. Here we compare observations of CO, H i, and [C ii] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS, and KINGFISH surveys. We find that within R ₂₅, the average [C ii] exponential radial profile is slightly shallower than that of the CO, but much steeper than the H i distribution. This is also reflected in the integrated spectrum (“global profile”), where the [C ii] spectrum looks more like that of the CO than that of the H i. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [C ii] data. Using high-spectral-resolution SOFIA [C ii] data of a number of star forming regions in two nearby galaxies, we find that their [C ii] linewidths agree better with those of the CO than the H i. As the radial extent of a given ISM tracer is a key input in deriving $M_{\mathrm{d}\mathrm{y}\mathrm{n}}$ from spatially unresolved data, we conclude that the relevant length-scale to use in determining $M_{\mathrm{d}\mathrm{y}\mathrm{n}}$ based on [C ii] data, is that of the well-characterized CO distribution. This length scale is similar to that of the optical disk.

[en] We examine Herschel Space Observatory images of one nearby prototypical outer ring galaxy, NGC 1291, and show that the ring becomes more prominent at wavelengths longer than 160 μm. The mass of cool dust in the ring dominates the total dust mass of the galaxy, accounting for at least 70% of it. The temperature of the emitting dust in the ring (T = 19.5 ± 0.3 K) is cooler than that of the inner galaxy (T = 25.7 ± 0.7 K). We discuss several explanations for the difference in dust temperature, including age and density differences in the stellar populations of the ring versus the bulge.

[en] We present the detection of extended (∼30 kpc²) dust emission in the tidal H I arm near NGC 3077 (member of the M 81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is typically detected where the H I column densities are >10²¹ cm^-2. The SPIRE band ratios show that the dust in the tidal arm is significantly colder (∼13 K) than in NGC 3077 itself (∼31 K), consistent with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ∼1.8 x 10⁶ M_sun (assuming β = 2), i.e., substantially larger than the dust mass associated with NGC 3077 (∼2 x 10⁵ M_sun). Where dust is detected, the dust-to-gas ratio is (6 ± 3) x 10^-3, consistent within the uncertainties with what is found in NGC 3077 and nearby spiral galaxies with Galactic metallicities. The faint H II regions in the tidal arm cannot be responsible for the detected enriched material and are not the main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic H I, molecules, and dust) in this tidal feature was pre-enriched and stripped off NGC 3077 during its recent interaction (∼3 x 10⁸ yr ago) with M 82 and M 81. This implies that interaction can efficiently remove heavy elements and enriched material (dust and molecular gas) from galaxies. As interactions were more frequent at large look-back times, it is conceivable that they could substantially contribute (along with galactic outflows) to the enrichment of the intergalactic medium.

[en] 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 H₂ 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 G₀ ∼ 10^2.3 and n_H ∼ 10^3.5 cm^–3 in the ring. For these values of G₀ and n_H, PDR models cannot reproduce the observed H₂ emission. Much of the H₂ emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks.

[en] The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 μm, [O I] 63 μm, and [N II] 122 μm in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera and Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, νf_ν(70 μm) νf_ν(100 μm), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant ∼6% over a wide range of far-IR color, 0.5 < νf_ν(70 μm) νf_ν(100 μm) ∼< 0.95. In the warmest regions, where νf_ν(70 μm) νf_ν(100 μm) ∼> 0.95, the ratio ([C II] + [O I])/PAH drops rapidly to 4%. We derived representative values of the local ultraviolet radiation density, G₀, and the gas density, n_H, by comparing our observations to models of photodissociation regions. The ratio G₀/n_H, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, (U), derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 μm PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments.

[en] Dynamic and thermal processes regulate the structure of the multiphase interstellar medium (ISM), and ultimately establish how galaxies evolve through star formation. Thus, to constrain ISM models and better understand the interplay of these processes, it is of great interest to measure the thermal pressure ($P_{\mathrm{t}\mathrm{h}}$) of the diffuse, neutral gas. By combining [C ii] 158 μm, H i, and CO data from 31 galaxies selected from the Herschel KINGFISH sample, we have measured thermal pressures in 534 predominantly atomic regions with typical sizes of ∼1 kiloparsec. We find a distribution of thermal pressures in the $P_{\mathrm{t}\mathrm{h}}/k\sim <!--\; ???\; -->{10}^3\text{--}{10}^5$ K cm⁻³ range. For a sub-sample of regions with conditions similar to those of the diffuse, neutral gas in the Galactic plane, we find thermal pressures that follow a log-normal distribution with a median value of P_th/k ≈ 3600 K cm⁻³. These results are consistent with thermal pressure measurements using other observational methods. We find that $P_{\mathrm{t}\mathrm{h}}$ increases with radiation field strength and star formation activity, as expected from the close link between the heating of the gas and the star formation rate. Our thermal pressure measurements fall in the regime where a two-phase ISM with cold and warm neutral media could exist in pressure equilibrium. Finally, we find that the midplane thermal pressure of the diffuse gas is about ∼30% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.

[en] We investigate the far infrared (IR) spectrum of NGC 1266, a S0 galaxy that contains a massive reservoir of highly excited molecular gas. Using the Herschel Fourier Transform Spectrometer, we detect the ¹²CO ladder up to J = (13-12), [C I] and [N II] lines, and also strong water lines more characteristic of UltraLuminous IR Galaxies (ULIRGs). The ¹²CO line emission is modeled with a combination of a low-velocity C-shock and a photodissociation region. Shocks are required to produce the H₂O and most of the high-J CO emission. Despite having an IR luminosity 30 times less than a typical ULIRG, the spectral characteristics and physical conditions of the interstellar medium of NGC 1266 closely resemble those of ULIRGs, which often harbor strong shocks and large-scale outflows

[en] We report the discovery of a powerful molecular wind from the nucleus of the non-interacting nearby S0 field galaxy NGC 1266. The single-dish CO profile exhibits emission to ±400 km s^-1 and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally concentrated molecular component with a mass of 1.1 x 10⁹ M_sun and a molecular outflow with a molecular mass of ∼2.4 x 10⁷ M_sun. The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1 x 10⁸ M_sun within a radius of ∼60 pc. This compact molecular nucleus has a surface density of ∼2.7 x 10⁴ M_sun pc^-2, more than two orders of magnitude larger than that of giant molecular clouds in the disk of the Milky Way, and it appears to sit on the Kennicutt-Schmidt relation despite its extreme kinematics and energetic activity. We interpret this nucleus as a disk that confines the outflowing wind. A mass outflow rate of ∼13 M_sun yr^-1 leads to a depletion timescale of ∼<85 Myr. The star formation in NGC 1266 is insufficient to drive the outflow, and thus it is likely driven by the active galactic nucleus. The concentration of the majority of the molecular gas in the central 100 pc requires an extraordinary loss of angular momentum, but no obvious companion or interacting galaxy is present to enable the transfer. NGC 1266 is the first known outflowing molecular system that does not show any evidence of a recent interaction.

[en] New far-infrared and submillimeter photometry from the Herschel Space Observatory is presented for 61 nearby galaxies from the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel (KINGFISH) sample. The spatially integrated fluxes are largely consistent with expectations based on Spitzer far-infrared photometry and extrapolations to longer wavelengths using popular dust emission models. Dwarf irregular galaxies are notable exceptions, as already noted by other authors, as their 500 μm emission shows evidence for a submillimeter excess. In addition, the fraction of dust heating attributed to intense radiation fields associated with photodissociation regions is found to be (21 ± 4)% larger when Herschel data are included in the analysis. Dust masses obtained from the dust emission models of Draine and Li are found to be on average nearly a factor of two higher than those based on single-temperature modified blackbodies, as single blackbody curves do not capture the full range of dust temperatures inherent to any galaxy. The discrepancy is largest for galaxies exhibiting the coolest far-infrared colors.

[en] We characterize the dust in NGC 628 and NGC 6946, two nearby spiral galaxies in the KINGFISH sample. With data from 3.6 μm to 500 μm, dust models are strongly constrained. Using the Draine and Li dust model (amorphous silicate and carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass surface density, (2) dust mass fraction contributed by polycyclic aromatic hydrocarbons, (3) distribution of starlight intensities heating the dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR luminosity originating in regions with high starlight intensity. We obtain maps for the dust properties, which trace the spiral structure of the galaxies. The dust models successfully reproduce the observed global and resolved spectral energy distributions (SEDs). The overall dust/H mass ratio is estimated to be 0.0082 ± 0.0017 for NGC 628, and 0.0063 ± 0.0009 for NGC 6946, consistent with what is expected for galaxies of near-solar metallicity. Our derived dust masses are larger (by up to a factor of three) than estimates based on single-temperature modified blackbody fits. We show that the SED fits are significantly improved if the starlight intensity distribution includes a (single intensity) 'delta function' component. We find no evidence for significant masses of cold dust (T ∼< 12 K). Discrepancies between PACS and MIPS photometry in both low and high surface brightness areas result in large uncertainties when the modeling is done at PACS resolutions, in which case SPIRE, MIPS70, and MIPS160 data cannot be used. We recommend against attempting to model dust at the angular resolution of PACS.