[en] Interactions of cosmic rays with the interstellar medium in the disk of the Milky Way provide the majority of the diffuse gamma-ray emission observed by the Fermi Gamma-ray Space Telescope. In addition to the gas which is densely concentrated along the galactic plane, hydrodynamical simulations and observational evidence favor the presence of a halo of hot (T ∼ 10⁶ K) ionized hydrogen (H II), extending with non-negligible densities out to the virial radius of the Milky Way. We show that cosmic-ray collisions with this circum-galactic gas should be expected to provide on the order of 3%-10% of the observed isotopic gamma-ray background at energies above 1 GeV. In addition, gamma rays originating from the extended H II halos of other galaxies along a given line of sight should contribute at a similar level.

[en] We present measurements of two types of cluster galaxy alignments based on a volume limited and highly pure (≥90%) sample of clusters from the GMBCG catalog derived from Data Release 7 of the Sloan Digital Sky Survey (SDSS DR7). We detect a clear brightest cluster galaxy (BCG) alignment (the alignment of major axis of the BCG toward the distribution of cluster satellite galaxies). We find that the BCG alignment signal becomes stronger as the redshift and BCG absolute magnitude decrease and becomes weaker as BCG stellar mass decreases. No dependence of the BCG alignment on cluster richness is found. We can detect a statistically significant (≥3σ) satellite alignment (the alignment of the major axes of the cluster satellite galaxies toward the BCG) only when we use the isophotal fit position angles (P.A.s), and the satellite alignment depends on the apparent magnitudes rather than the absolute magnitudes of the BCGs. This suggests that the detected satellite alignment based on isophotal P.A.s from the SDSS pipeline is possibly due to the contamination from the diffuse light of nearby BCGs. We caution that this should not be simply interpreted as non-existence of the satellite alignment, but rather that we cannot detect them with our current photometric SDSS data. We perform our measurements on both SDSS r-band and i-band data, but do not observe a passband dependence of the alignments.

[en] Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X_CO, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The X_CO factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X_CO on gas properties and averaging scale using a model that is based on a combination of results of sub-parsec scale magnetohydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts X_CO ≈ (2-4) × 10²⁰ K^–1 cm^–2 km^–1 s, consistent with the Galactic value, for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X_CO varies by only a factor of two for gas surface densities in the range Σ_H2∼50-500 M_sunpc^-2. However, the model also predicts that more generally on the scale of GMCs, X_CO is a strong function of metallicity and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H₂ column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on ∼kiloparsec scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X_CO∝Z^–γ with γ ≈ 0.5-0.8.

[en] Molecular hydrogen (H₂) is the primary component of the reservoirs of cold, dense gas that fuel star formation in our Galaxy. While the H₂ abundance is ultimately regulated by physical processes operating on small scales in the interstellar medium (ISM), observations have revealed a tight correlation between the ratio of molecular to atomic hydrogen in nearby spiral galaxies and the pressure in the mid-plane of their disks. This empirical relation has been used to predict H₂ abundances in galaxies with potentially very different ISM conditions, such as metal-deficient galaxies at high redshifts. Here, we test the validity of this approach by studying the dependence of the pressure-H₂ relation on environmental parameters of the ISM. To this end, we follow the formation and destruction of H₂ explicitly in a suite of hydrodynamical simulations of galaxies with different ISM parameters. We find that a pressure-H₂ relation arises naturally in our simulations for a variety of dust-to-gas ratios or strengths of the interstellar radiation field in the ISM. Fixing the dust-to-gas ratio and the UV radiation field to values measured in the solar neighborhood results in fair agreement with the relation observed in nearby galaxies with roughly solar metallicity. However, the parameters (slope and normalization) of the pressure-H₂ relation vary in a systematical way with ISM properties. A particularly strong trend is the decrease of the normalization of the relation with a lowering of the dust-to-gas ratio of the ISM. We show how this trend and other properties of the pressure-H₂ relation arise from the atomic-to-molecular phase transition in the ISM caused by a combination of H₂ formation, destruction, and shielding mechanisms.

[en] There is ample observational evidence that the star formation rate (SFR) surface density, Σ_SFR, is closely correlated with the surface density of molecular hydrogen, Σ_H2. This empirical relation holds both for galaxy-wide averages and for individual ∼>kpc sized patches of the interstellar medium, but appears to degrade substantially at a sub-kpc scale. Identifying the physical mechanisms that determine the scale-dependent properties of the observed Σ_H2-Σ_SFR relation using a set of cosmological, galaxy formation simulations with a peak resolution of ∼100 pc. These simulations include a chemical network for molecular hydrogen, a model for the CO emission, and a simple, stochastic prescription for star formation that operates on ∼100 pc scales. Specifically, star formation is modeled as a Poisson process in which the average SFR is directly proportional to the present mass of H₂. The predictions of our numerical model are in good agreement with the observed Kennicutt-Schmidt and Σ_H2-Σ_SFR relations. We show that observations based on CO emission are ill suited to reliably measure the slope of the latter relation at low (∼< 20 M _☉ pc^–2) H₂ surface densities on sub-kpc scales. Our models also predict that the inferred Σ_H2-Σ_SFR relation steepens at high H₂ surface densities as a result of the surface density dependence of the CO/H₂ conversion factor. Finally, we show that on sub-kpc scales most of the scatter of the relation is a consequence of discreteness effects of the star formation process. In contrast, variations of the CO/H₂ conversion factor are responsible for most of the scatter measured on super-kpc scales.

[en] Accurate measurements of galaxy masses and sizes are key to tracing galaxy evolution over time. Cosmological zoom-in simulations provide an ideal test bed for assessing the recovery of galaxy properties from observations. Here, we utilize galaxies with $M_{\ast <!--\; ???\; -->}\sim <!--\; ???\; -->{10}^{10}\text{--}{10}^{11.5}{M}_{\odot <!--\; ???\; -->}$ at z ∼ 1.7–2 from the MassiveFIRE cosmological simulation suite, part of the Feedback in Realistic Environments (FIRE) project. Using mock multi-band images, we compare intrinsic galaxy masses and sizes to observational estimates. We find that observations accurately recover stellar masses, with a slight average underestimate of $\sim <!--\; ???\; -->0.06\mathrm{d}\mathrm{e}\mathrm{x}$ and $\mathrm{a}\sim <!--\; ???\; -->0.15\mathrm{d}\mathrm{e}\mathrm{x}$ scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of $\sim <!--\; ???\; -->0.1\mathrm{d}\mathrm{e}\mathrm{x}$ (with the half-light radii being larger) and a scatter of $\sim <!--\; ???\; -->0.2\mathrm{d}\mathrm{e}\mathrm{x}$. When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by $\sim <!--\; ???\; -->0.1\mathrm{d}\mathrm{e}\mathrm{x}$. However, if not properly accounted for, aperture effects can bias size estimates by $\sim <!--\; ???\; -->0.1\mathrm{d}\mathrm{e}\mathrm{x}$. No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for ∼25% of the scatter in the recovered masses and sizes. Our results thus suggest that the intrinsic scatter in the mass–size relation may have previously been overestimated by ∼25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by $\sim <!--\; ???\; -->0.5\mathrm{d}\mathrm{e}\mathrm{x}$ at $M_{\ast <!--\; ???\; -->}\sim <!--\; ???\; -->{10}^{11.5}{M}_{\odot <!--\; ???\; -->}$.

[en] The physical mechanisms that quench star formation, turning blue star-forming galaxies into red quiescent galaxies, remain unclear. In this Letter, we investigate the role of gas supply in suppressing star formation by studying the molecular gas content of post-starburst galaxies. Leveraging the wide area of the Sloan Digital Sky Survey, we identify a sample of massive intermediate-redshift galaxies that have just ended their primary epoch of star formation. We present Atacama Large Millimeter/submillimeter Array CO(2-1) observations of two of these post-starburst galaxies at z ∼ 0.7 with $M_{\ast <!--\; ???\; -->}\sim <!--\; ???\; -->2\times <!--\; ??\; -->{10}^{11}{M}_{\odot <!--\; ???\; -->}$. Their molecular gas reservoirs of $(6.4\pm <!--\; ??\; -->0.8)\times <!--\; ??\; -->{10}^9{M}_{\odot <!--\; ???\; -->}$ and $(34.0\pm <!--\; ??\; -->1.6)\times <!--\; ??\; -->{10}^9{M}_{\odot <!--\; ???\; -->}$ are an order of magnitude larger than comparable-mass galaxies in the local universe. Our observations suggest that quenching does not require the total removal or depletion of molecular gas, as many quenching models suggest. However, further observations are required both to determine if these apparently quiescent objects host highly obscured star formation and to investigate the intrinsic variation in the molecular gas properties of post-starburst galaxies.

[en] We study the incidence of nuclear activity in a large sample of massive post-starburst (PSB) galaxies at $z\sim <!--\; ???\; -->0.7$ selected from the Sloan Digital Sky Survey, and identify active galactic nuclei based on radio continuum and optical emission lines. Over our mass range of ${10}^{10.6}\text{--}{10}^{11.5}$ $M_{\odot <!--\; ???\; -->}$, the incidence of radio activity is weakly dependent on stellar mass and independent of stellar age, while radio luminosity depends strongly on stellar mass. Optical nuclear activity incidence depends most strongly on the D_n4000 line index, a proxy for stellar age, with an active fraction that is ∼10 times higher in the youngest versus oldest PSB galaxies. Since a similar trend is seen between age and molecular gas fractions, we argue that, like in local galaxies, the age trend reflects a peak in available fueling rather than feedback from the central black hole on the surrounding galaxy.

[en] We present Gemini Multi-Object Spectrograph (GMOS) integral field unit (IFU) observations of six massive (M _⋆ ≥ 10¹¹ M _⊙) A-star dominated post-starburst galaxies at z ∼ 0.6. These galaxies are a subsample of the $\mathrm{S}\mathrm{Q}\mathrm{u}\mathrm{I}\mathrm{G}\mathrm{G}\stackrel{\to <!--\; ???\; -->}{L}\mathrm{E}$ Survey, which selects intermediate-redshift post-starbursts from the Sloan Digital Sky Survey spectroscopic sample (DR14) with spectral shapes that indicate they have recently shut off their primary epoch of star formation. Using Hδ _A absorption as a proxy for stellar age, we constrain five of the galaxies to have young (∼600 Myr) light-weighted ages at all radii and find that the sample on average has flat age gradients. We examine the spatial distribution of mass-weighted properties by fitting our profiles with a toy model including a young, centrally concentrated burst superimposed on an older, extended population. We find that galaxies with flat Hδ _A profiles are inconsistent with formation via a central secondary starburst. This implies that the mechanism responsible for shutting off this dominant episode of star formation must have done so uniformly throughout the galaxy.

[en] We study the interstellar medium in a sample of 27 high-redshift quasar host galaxies at z ≳ 6, using the [C ii] 158 μm emission line and the underlying dust continuum observed at ∼1 kpc resolution with Atacama Large Millimeter Array. By performing uv-plane spectral stacking of both the high and low spatial resolution data, we investigate the spatial and velocity extent of gas and the size of the dust-emitting regions. We find that the average surface brightness profile of both the [C ii] and the dust continuum emission can be described by a steep component within a radius of 2 kpc and a shallower component with a scale length of 2 kpc, detected up to ∼10 kpc. The surface brightness of the extended emission drops below ∼1% of the peak at radius of ∼5 kpc, beyond which it constitutes 10%–20% of the total measured flux density. Although the central component of the dust continuum emission is more compact than that of the [C ii] emission, the extended components have equivalent profiles. The observed extended components are consistent with those predicted by hydrodynamical simulations of galaxies with similar infrared luminosities, where the dust emission is powered by star formation. The [C ii] spectrum measured in the mean uv-plane stacked data can be described by a single Gaussian, with no observable [C ii] broad-line emission (velocities in excess of ≳500 km s⁻¹), which would be indicative of outflows. Our findings suggest that we are probing the interstellar medium and associated star formation in the quasar host galaxies up to radii of 10 kpc, whereas we find no evidence for halos or outflows.