Cool outflows in galaxies and their implications

Neutral-atomic and molecular outflows are a common occurrence in galaxies, near and far. They operate over the full extent of their galaxy hosts, from the innermost regions of galactic nuclei to the outermost reaches of galaxy halos. They carry a substantial amount of material that would otherwise have been used to form new stars. These cool outflows may have a profound impact on the evolution of their host galaxies and environments. This article provides an overview of the basic physics of cool outflows, a comprehensive assessment of the observational techniques and diagnostic tools used to characterize them, a detailed description of the best-studied cases, and a more general discussion of the statistical properties of these outflows in the local and distant universe. The remaining outstanding issues that have not yet been resolved are summarized at the end of the review to inspire new research directions.STFC and ER

Comparing [C II], H I, and CO dynamics of nearby galaxies

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M-dyn 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 mu m line of atomic carbon ([CII]) may be the only way to derive M-dyn. As the distribution and kinematics of the ISM tracer affects the determination of M-dyn, it is important to quantify the relative distributions of HI, CO, and [CII]. HI and CO are well-characterized observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS, and KINGFISH surveys. We find that within R-25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. 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 [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M-dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M-dyn based on [CII] data, is that of the well-characterized CO distribution. This length scale is similar to that of the optical disk

The Morpho-kinematic architecture of super star clusters in the center of NGC 253

The center of the nearby galaxy NGC 253 hosts a population of more than a dozen super star clusters (SSCs) that are still in the process of forming. The majority of the star formation of the burst is concentrated in these SSCs, and the starburst is powering a multiphase outflow from the galaxy. In this work, we measure the 350 GHz dust continuum emission toward the center of NGC 253 at 47 mas (0.8 pc) resolution using data from the Atacama Large Millimeter/submillimeter Array. We report the detection of 350 GHz (dust) continuum emission in the outflow for the first time, associated with the prominent South-West streamer. In this feature, the dust emission has a width of ≈8 pc, is located at the outer edge of the CO emission, and corresponds to a molecular gas mass of ∼(8–17)×106 M⊙. In the starburst nucleus, we measure the resolved radial profiles, sizes, and molecular gas masses of the SSCs. Compared to previous work at the somewhat lower spatial resolution, the SSCs here break apart into smaller substructures with radii 0.4–0.7 pc. In projection, the SSCs, dust, and dense molecular gas appear to be arranged as a thin, almost linear, structure roughly 155 pc in length. The morphology and kinematics of this structure can be well explained as gas following x2 orbits at the center of a barred potential. We constrain the morpho-kinematic arrangement of the SSCs themselves, finding that an elliptical, angular-momentum-conserving ring is a good description of both the morphology and kinematics of the SSCs

COMPARING [C ∥], H i, and CO DYNAMICS of NEARBY GALAXIES

The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M_dyn 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 $\mu$m line of atomic carbon [CII] may be the only way to derive M_dyn. As the distribution and kinematics of the ISM tracer affects the determination of M_dyn, it is important to quantify the relative distributions of HI, CO and [CII]. HI and CO are well-characterised observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS and KINGFISH surveys. We find that within R_25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. 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 [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M_dyn based on [CII] data, is that of the well-characterised CO distribution. This length scale is similar to that of the optical disk.This is the accepted manuscript. It is currently embargoed pending publication

Resolving the far-IR line deficit : photoelectric heating and far-IR line cooling in NGC 1097 and NGC 4559

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 mu m, [O I] 63 mu m, and [N II] 122 mu m in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & 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, nu f(nu)(70 mu m) nu f(nu)(100 mu m), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant similar to 6% over a wide range of far-IR color, 0.5 , 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 mu m PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments

First Results from the Herschel and ALMA Spectroscopic Surveys of the SMC: The Relationship between [C ii ]-bright Gas and CO-bright Gas at Low Metallicity

The Small Magellanic Cloud (SMC) provides the only laboratory to study the structure of molecular gas at high resolution and low metallicity. We present results from the Herschel Spectroscopic Survey of the SMC (HS3), which mapped the key far-IR cooling lines [C ii], [O i], [N ii], and [O iii] in five star-forming regions, and new ALMA 7 m array maps of ${}^{12}\mathrm{CO}$ and ${}^{13}\mathrm{CO}$ $(2-1)$ with coverage overlapping four of the five HS3 regions. We detect [C ii] and [O i] throughout all of the regions mapped. The data allow us to compare the structure of the molecular clouds and surrounding photodissociation regions using ${}^{13}\mathrm{CO}$, ${}^{12}\mathrm{CO}$, [C ii], and [O i] emission at $\lesssim 10^{\prime\prime}$ ($\lt 3$ pc) scales. We estimate ${A}_{V}$ using far-IR thermal continuum emission from dust and find that the CO/[C ii] ratios reach the Milky Way value at high ${A}_{V}$ in the centers of the clouds and fall to \sim 1/5\mbox{--}1/10\times the Milky Way value in the outskirts, indicating the presence of translucent molecular gas not traced by bright ${}^{12}\mathrm{CO}$ emission. We estimate the amount of molecular gas traced by bright [C ii] emission at low ${A}_{V}$ and bright ${}^{12}\mathrm{CO}$ emission at high ${A}_{V}$. We find that most of the molecular gas is at low ${A}_{V}$ and traced by bright [C ii] emission, but that faint ${}^{12}\mathrm{CO}$ emission appears to extend to where we estimate that the ${{\rm{H}}}_{2}$-to-H i transition occurs. By converting our ${{\rm{H}}}_{2}$ gas estimates to a CO-to-${{\rm{H}}}_{2}$ conversion factor (X CO), we show that X CO is primarily a function of ${A}_{V}$, consistent with simulations and models of low-metallicity molecular clouds

The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

The CO-to-H-2 conversion factor (alpha CO) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower alpha CO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner similar to 2 kpc of NGC 3627 and NGC 4321 tracing (CO)-C-12, (CO)-C-13, and (CO)-O-18 lines on similar to 100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of aCO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature Tk greater than or similar to 100 K and density n(H2) > 10(3) cm(-3). Assuming a CO-to-H-2 abundance of 3 x 10(-4), we derive 4-15 times lower alpha(CO) than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of alpha(CO) with low-J (CO)-C-12 optical depths (tau(CO)), as well as an anticorrelation with Tk. The tCO correlation explains most of the aCO variation in the three galaxy centers, whereas changes in T-k influence alpha(CO) to second order. Overall, the observed line width and (CO)-C-12/(CO)-C-13 2-1 line ratio correlate with tCO variation in these centers, and thus they are useful observational indicators for alpha(CO) variation. We also test current simulation-based alpha(CO) prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations

PHANGS-JWST First Results: Mapping the 3.3 μm Polycyclic Aromatic Hydrocarbon Vibrational Band in Nearby Galaxies with NIRCam Medium Bands

We present maps of the 3.3 mu m polycyclic aromatic hydrocarbon (PAH) emission feature in NGC 628, NGC 1365, and NGC 7496 as observed with the Near-Infrared Camera imager on JWST from the PHANGS-JWST Cycle 1 Treasury project. We create maps that isolate the 3.3 mu m PAH feature in the F335M filter (F335M(PAH)) using combinations of the F300M and F360M filters for removal of starlight continuum. This continuum removal is complicated by contamination of the F360M by PAH emission and variations in the stellar spectral energy distribution slopes between 3.0 and 3.6 mu m. We modify the empirical prescription from Lai et al. to remove the starlight continuum in our highly resolved galaxies, which have a range of starlight- and PAH-dominated lines of sight. Analyzing radially binned profiles of the F335M(PAH) emission, we find that between 5% and 65% of the F335M intensity comes from the 3.3 mu m feature within the inner 0.5 r (25) of our targets. This percentage systematically varies from galaxy to galaxy and shows radial trends within the galaxies related to each galaxy's distribution of stellar mass, interstellar medium, and star formation. The 3.3 mu m emission is well correlated with the 11.3 mu m PAH feature traced with the MIRI F1130W filter, as is expected, since both features arise from C-H vibrational modes. The average F335M(PAH)/F1130W ratio agrees with the predictions of recent models by Draine et al. for PAHs with size and charge distributions shifted toward larger grains with normal or higher ionization

PHANGS-JWST first results: stellar-feedback-driven excitation and dissociation of molecular gas in the Starburst Ring of NGC 1365?

We compare embedded young massive star clusters (YMCs) to (sub-)millimeter line observations tracing the excitation and dissociation of molecular gas in the starburst ring of NGC 1365. This galaxy hosts one of the strongest nuclear starbursts and richest populations of YMCs within 20 Mpc. Here we combine near-/mid-IR PHANGS–JWST imaging with new Atacama Large Millimeter/submillimeter Array multi-J CO (1–0, 2–1 and 4–3) and [C i] (1–0) mapping, which we use to trace CO excitation via R42 = ICO(4−3)/ICO(2−1) and R21 = ICO(2−1)/ICO(1−0) and dissociation via RCICO = I[CI](1−0)/ICO(2−1) at 330 pc resolution. We find that the gas flowing into the starburst ring from northeast to southwest appears strongly affected by stellar feedback, showing decreased excitation (lower R42) and increased signatures of dissociation (higher RCICO) in the downstream regions. There, radiative-transfer modeling suggests that the molecular gas density decreases and temperature and [CI/CO] abundance ratio increase. We compare R42 and RCICO with local conditions across the regions and find that both correlate with near-IR 2 μm emission tracing the YMCs and with both polycyclic aromatic hydrocarbon (11.3 μm) and dust continuum (21 μm) emission. In general, RCICO exhibits ∼0.1 dex tighter correlations than R42, suggesting C i to be a more sensitive tracer of changing physical conditions in the NGC 1365 starburst than CO (4–3). Our results are consistent with a scenario where gas flows into the two arm regions along the bar, becomes condensed/shocked, forms YMCs, and then these YMCs heat and dissociate the gas

PHANGS-JWST First Results: A Global and Moderately Resolved View of Mid-infrared and CO Line Emission from Galaxies at the Start of the JWST Era

We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately (∼1 kpc) spatially resolved measurements of CO (1-0) and CO (2-1) intensity, I CO, and mid-IR intensity, I MIR, at 8, 12, 22, and 24 μm. The I CO versus I MIR relationship is reasonably described by a power law with slopes 0.7-1.2 and normalization I CO ∼ 1 K km s−1 at I MIR ∼ 1 MJy sr−1. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO (1-0) and CO (2-1) allow us to infer that R 21 ∝ I MIR 0.2 , in good agreement with other work. The 8 μm and 12 μm bands, with strong polycyclic aromatic hydrocarbon (PAH) features, show steeper CO versus mid-IR slopes than the 22 and 24 μm, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass (M ⋆) and anticorrelates with star formation rate/M ⋆. At ∼1 kpc resolution, the first four PHANGS-JWST targets show CO-to-mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities