Dynamic and thermal processes regulate the structure of the multi-phase 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 (\(P_{\rm th}\)) of the diffuse, neutral gas. By combining C II 158 \(\mu\)m, HI, 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 \(\sim\)1 kiloparsec. We find a distribution of thermal pressures in the \(P_{\rm th}/k\sim10^3-10^5\) 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 \(P_{\rm th}/k\approx3600\) K cm\(^{-3}\). These results are consistent with thermal pressure measurements using other observational methods. We find that \(P_{\rm th}\) 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 medium could exist in pressure equilibrium. Finally, we find that the midplane thermal pressure of the diffuse gas is about \(\sim30\)% of the vertical weight of the overlying ISM, consistent with results from hydrodynamical simulations of self-regulated star formation in galactic disks.
To compute the SFR of galaxies from the rest-frame UV it is essential to take into account the obscuration by dust. To do so, one of the most popular methods consists in combining the UV with the ...emission from the dust itself in the IR. Yet, different studies have derived different estimators, showing that no such hybrid estimator is truly universal. In this paper we aim at understanding and quantifying what physical processes drive the variations between different hybrid estimators. Doing so, we aim at deriving new universal UV+IR hybrid estimators to correct the UV for dust attenuation, taking into account the intrinsic physical properties of galaxies. We use the CIGALE code to model the spatially-resolved FUV to FIR SED of eight nearby star-forming galaxies drawn from the KINGFISH sample. This allows us to determine their local physical properties, and in particular their UV attenuation, average SFR, average specific SFR (sSFR), and their stellar mass. We then examine how hybrid estimators depend on said properties. We find that hybrid UV+IR estimators strongly depend on the stellar mass surface density (in particular at 70 and 100 micron) and on the sSFR (in particular at 24 micron and the TIR). Consequently, the IR scaling coefficients for UV obscuration can vary by almost an order of magnitude. This result contrasts with other groups who found relatively constant coefficients with small deviations. We exploit these variations to construct a new class of hybrid estimators based on observed UV to near-IR colours and near-IR luminosity densities per unit area. We find that they can reliably be extended to entire galaxies. The new estimators provide better estimates of attenuation-corrected UV emission than classical hybrid estimators. Naturally taking into account the variable impact of dust heated by old stellar populations, they constitute a step towards universal estimators.
The NII 122 and 205 \mu m transitions are powerful tracers of the ionized gas in the interstellar medium. By combining data from 21 galaxies selected from the Herschel KINGFISH and Beyond the Peak ...surveys, we have compiled 141 spatially resolved regions with a typical size of ~1 kiloparsec, with observations of both NII far-infrared lines. We measure NII 122/205 line ratios in the ~0.6-6 range, which corresponds to electron gas densities \(n_e\)~1-300 cm\(^{-3}\), with a median value of \(n_e\)=30 cm\(^{-3}\). Variations in the electron density within individual galaxies can be as a high as a factor of ~50, frequently with strong radial gradients. We find that \(n_e\) increases as a function of infrared color, dust-weighted mean starlight intensity, and star formation rate surface density (\(\Sigma_{SFR}\)). As the intensity of the NII transitions is related to the ionizing photon flux, we investigate their reliability as tracers of the star formation rate (SFR). We derive relations between the NII emission and SFR in the low-density limit and in the case of a log-normal distribution of densities. The scatter in the correlation between NII surface brightness and \(\Sigma_{SFR}\) can be understood as a property of the \(n_e\) distribution. For regions with \(n_e\) close to or higher than the NII line critical densities, the low-density limit NII-based SFR calibration systematically underestimates the SFR since NII emission is collisionally quenched. Finally, we investigate the relation between NII emission, SFR, and \(n_e\) by comparing our observations to predictions from the MAPPINGS-III code.
We have mapped the superwind/halo region of the nearby starburst galaxy M82 in the mid-infrared with \(Spitzer-IRS\). The spectral regions covered include the H\(_2 S(1)-S(3)\), NeII, NeIII emission ...lines and PAH features. We estimate the total warm H\(_2\) mass and the kinetic energy of the outflowing warm molecular gas to be between \(M_{warm}\sim5-17\times10^6\) M\(_{\odot}\) and \(E_{K}\sim6-20\times10^{53}\) erg. Using the ratios of the 6.2, 7.7 and 11.3 micron PAH features in the IRS spectra, we are able to estimate the average size and ionization state of the small grains in the superwind. There are large variations in the PAH flux ratios throughout the outflow. The 11.3/7.7 and the 6.2/7.7 PAH ratios both vary by more than a factor of five across the wind region. The Northern part of the wind has a significant population of PAH's with smaller 6.2/7.7 ratios than either the starburst disk or the Southern wind, indicating that on average, PAH emitters are larger and more ionized. The warm molecular gas to PAH flux ratios (H\(_2/PAH\)) are enhanced in the outflow by factors of 10-100 as compared to the starburst disk. This enhancement in the H\(_2/PAH\) ratio does not seem to follow the ionization of the atomic gas (as measured with the NeIII/NeII line flux ratio) in the outflow. This suggests that much of the warm H\(_2\) in the outflow is excited by shocks. The observed H\(_2\) line intensities can be reproduced with low velocity shocks (\(v < 40\) km s\(^{-1}\)) driven into moderately dense molecular gas (\(10^2 <n_H < 10^4\) cm\(^{-3}\)) entrained in the outflow.
'Normal' galaxies observed at z>6, when the Universe was <1 billion years old, thus far show no evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas ...mass ratio is 1%. A prototypical example is 'Himiko' (z=6.6), which a mere 840 Myr after the Big Bang is forming stars at a rate of 30-100 Msun/yr, yielding a mass assembly time M^{star}/SFR 150x10^6 yr. Himiko is estimated to have a low fraction (2-3% of the Solar value) of elements heavier than helium (metallicity), and although its gas mass cannot be asserted at this time its dust-to-stellar mass ratio is constrained to be <0.05%. The local galaxy I Zw 18, with a metallicity 4% solar and forming stars less rapidly than Himiko but still vigorously for its mass (M^{star}/SFR 1.6x10^9 yr), is also very dust deficient and perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18 from which we determine its dust mass to be 450-1800 Msun, yielding a dust-to-stellar mass ratio \approx 10^{-6}-10^{-5} and a dust-to-gas mass ratio 3.2-13x10^{-6}. If I Zw 18 is a reasonable analog of Himiko, then Himiko's dust mass is \approx 50,000 Msun, a factor of 100 below the current upper limit. These numbers are considerably uncertain, but if most high-z galaxies are more like Himiko than like the quasar host SDSS J114816.64+525150.3, then the prospects for detecting the gas and dust in them are much poorer than hitherto anticipated.
The CII 157.74 $\mu$m transition is the dominant coolant of the neutral
interstellar gas, and has great potential as a star formation rate (SFR)
tracer. Using the Herschel KINGFISH sample of 46 ...nearby galaxies, we
investigate the relation of CII surface brightness and luminosity with SFR.
We conclude that CII can be used for measurements of SFR on both global and
kiloparsec scales in normal star-forming galaxies in the absence of strong
active galactic nuclei (AGN). The uncertainty of the $\Sigma_{\rm
CII}-\Sigma_{\rm SFR}$ calibration is $\pm$0.21 dex. The main source of
scatter in the correlation is associated with regions that exhibit warm IR
colors, and we provide an adjustment based on IR color that reduces the
scatter. We show that the color-adjusted $\Sigma_{\rmCII}-\Sigma_{\rm SFR}$
correlation is valid over almost 5 orders of magnitude in $\Sigma_{\rm SFR}$,
holding for both normal star-forming galaxies and non-AGN luminous infrared
galaxies. Using CII luminosity instead of surface brightness to estimate SFR
suffers from worse systematics, frequently underpredicting SFR in luminous
infrared galaxies even after IR color adjustment (although this depends on the
SFR measure employed). We suspect that surface brightness relations are better
behaved than the luminosity relations because the former are more closely
related to the local far-UV field strength, most likely the main parameter
controlling the efficiency of the conversion of far-UV radiation into gas
heating. A simple model based on Starburst99 population-synthesis code to
connect SFR to CII finds that heating efficiencies are $1\%-3\%$ in normal
galaxies.
Since the classical work by Purcell (1979) it has been generally accepted
that most interstellar grains rotate suprathermally. Suprathermally rotating
grains would be nearly perfectly aligned with ...the magnetic field by
paramagnetic dissipation if not for ``crossovers'', intervals of low angular
velocity resulting from reversals of the torques responsible for suprathermal
rotation; during crossovers grains are susceptible to disalignment by random
impulses.
Lazarian and Draine (1997) identified thermal fluctuations within grain
material as an important component of crossover dynamics. For grains of size
less than 0.1 micron, these fluctuations ensure good correlation of angular
momentum before and after crossover resulting in good alignment, in accord with
observations of starlight polarization. In the present paper we discuss two new
processes which are important for the dynamics of grains with a<0.1 micron. The
first -- ``thermal flipping'' -- offers a way for small grains to bypass the
period of greatly reduced angular momentum which would otherwise take place
during a crossover, thereby enhancing the alignment of small grains. The second
effect -- ``thermal trapping'' -- arises when thermal flipping becomes rapid
enough to prevent the systematic torques from driving the grain to suprathermal
rotation. This effect acts to reduce the alignment of small grains.
The observed variation of grain alignment with grain size would then result
from a combination of the thermal flipping process -- which suppresses
suprathermal rotation of small grains -- and due to molecular hydrogen
formation and starlight -- which drive large grains to suprathermal rotation
rates.
We model the infrared to submillimeter spectral energy distribution of 11 nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and compare model extrapolations at 870um (using ...different fitting techniques) with LABOCA 870um observations. We investigate how the differences between predictions and observations vary with model assumptions or environment. At global scales, we find that modified blackbody models using realistic cold emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed emission within the uncertainties for most of the sample. Low values (beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local scales, we observe a systematic 870um excess when using beta_=2.0. The beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with observations in part of the disks. Some of the remaining excesses occur towards the centres and can be partly or fully accounted for by non-dust contributions such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In three non-barred galaxies, the remaining excesses rather occur in the disk outskirts. This could be a sign of a flattening of the submm slope (and decrease of the effective emissivity index) with radius in these objects.
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 160um. 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+/-0.3K) is cooler than that of the inner galaxy (T=25.7+/-0.7K). 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.
