A physical model for the [CII]-FIR deficit in luminous galaxies

Observations of ionised carbon at 158 micron ([CII]) from luminous star-forming galaxies at z~0 show that their ratios of [CII] to far infrared (FIR) luminosity are systematically lower than those of more modestly star-forming galaxies. In this paper, we provide a theory for the origin of this so called "[CII] deficit" in galaxies. Our model treats the interstellar medium as a collection of clouds with radially-stratified chemical and thermal properties, which are dictated by the clouds' volume and surface densities, as well as the interstellar radiation and cosmic ray fields to which they are exposed. [CII] emission arises from the outer, HI dominated layers of clouds, and from regions where the hydrogen is H2 but the carbon is predominantly C+. In contrast, the most shielded regions of clouds are dominated by CO and produce little [CII] emission. This provides a natural mechanism to explain the observed [CII]-star formation relation: galaxies' star formation rates are largely driven by the surface densities of their clouds. As this rises, so does the fraction of gas in the CO-dominated phase that produces little [CII] emission. Our model further suggests that the apparent offset in the [CII]-FIR relation for high-z sources compared to those at present epoch may arise from systematically larger gas masses at early times: a galaxy with a large gas mass can sustain a high star formation rate even with relatively modest surface density, allowing copious [CII] emission to coexist with rapid star formation.Comment: Accepted by MNRAS; minor revisions that include additional comparisons to observation

Why is the Milky Way X-factor Constant?

The CO-H2 conversion factor (Xco; otherwise known as the X-factor) is observed to be remarkably constant in the Milky Way and in the Local Group (aside from the SMC). To date, our understanding of why Xco should be so constant remains poor. Using a combination of extremely high resolution (~ 1 pc) galaxy evolution simulations and molecular line radiative transfer calculations, we suggest that Xco displays a narrow range of values in the Galaxy due to the fact that molecular clouds share very similar physical properties. In our models, this is itself a consequence of stellar feedback competing against gravitational collapse. GMCs whose lifetimes are regulated by radiative feedback show a narrow range of surface densities, temperatures and velocity dispersions with values comparable to those seen in the Milky Way. As a result, the X-factors from these clouds show reasonable correspondence with observed data from the Local Group, and a relatively narrow range. On the other hand, feedback-free clouds collapse to surface densities that are larger than those seen in the Galaxy, and hence result in X-factors that are systematically too large compared to the Milky Way's. We conclude that radiative feedback within GMCs can generate cloud properties similar to those observed in the Galaxy, and hence a roughly constant Milky Way X-factor in normal, quiescent clouds.Comment: MNRAS Accepte

Galaxy Gas Fractions at High-Redshift: The Tension between Observations and Cosmological Simulations

CO measurements of z ∼ 1–4 galaxies have found that their baryonic gas fractions are significantly higher than those for galaxies at z = 0, with values ranging from 20 to 80 per cent. Here, we suggest that the gas fractions inferred from observations of star-forming galaxies at high-z are overestimated, owing to the adoption of locally calibrated CO–H2conversion factors (αCO). Evidence from both observations and numerical models suggests that αCO varies smoothly with the physical properties of galaxies, and that αCO can be parametrized simply as a function of both gas-phase metallicity and observed CO surface brightness. When applying this functional form, we find fgas ≈ 10–40 per cent in galaxies with M* = 1010–1012  M⊙. Moreover, the scatter in the observed fgas–M* relation is lowered by a factor of 2. The lower inferred gas fractions arise physically because the interstellar media of high-z galaxies have higher velocity dispersions and gas temperatures than their local counterparts, which results in an αCO that is lower than the z = 0 value for both quiescent discs and starbursts. We further compare these gas fractions to those predicted by cosmological galaxy formation models. We show that while the canonically inferred gas fractions from observations are a factor of 2–3 larger at a given stellar mass than predicted by models, our rederived αCOvalues for z = 1–4 galaxies result in revised gas fractions that agree significantly better with the simulations

Variability Tests for Intrinsic Absorption Lines in Quasar Spectra

Quasar spectra have a variety of absorption lines whose origins range from energetic winds expelled from the central engines to unrelated, intergalactic clouds. We present multiepoch, medium-resolution spectra of eight quasars at z ~ 2 that have narrow associated absorption lines (AALs, within ±5000 km s-1 of the emission redshift). Two of these quasars were also known previously to have high-velocity mini-broad absorption lines (mini-BALs). We use these data, spanning ~17 yr in the observed frame with 2-4 observations per object, to search for line-strength variations as an identifier of absorption that occurs physically near ( intrinsic to) the central active galactic nucleus. Our main results are the following: Two out of the eight quasars with narrow AALs exhibit variable AAL strengths. Two out of two quasars with high-velocity mini-BALs exhibit variable mini-BAL strengths. We also marginally detect variability in a high-velocity narrow absorption line system, blueshifted ~32,900 km s-1 with respect to the emission lines. No other absorption lines in these quasars appeared to vary. The outflow velocities of the variable AALs are 3140 and 1490 km s-1. The two mini-BALs identify much higher velocity outflows of ~28,400 and ~52,000 km s-1. Our temporal sampling yields upper limits on the variation timescales from 0.28 to 6.1 yr in the quasar rest frames. The corresponding minimum electron densities in the variable absorbers, based on the recombination timescale, are ~40,000 to ~1900 cm-3. The maximum distances of the absorbers from the continuum source, assuming photoionization with no spectral shielding, range from ~1.8 to ~7 kpc

The Kennicutt-Schmidt Star Formation Relation at z~2

Recent observations of excited CO emission lines from z∼ 2 disc galaxies have shed light on the SFR ∝ρN relation at high z via observed ΣSFR–ΣαCOJ=2−1 and ΣSFR–ΣαCOJ=3−2 relations. Here, we describe a novel methodology for utilizing these observations of high-excitation CO to derive the underlying Schmidt (SFR ∝ρN) relationship. To do this requires an understanding of the potential effects of differential CO excitation with SFR. If the most heavily star-forming galaxies have a larger fraction of their gas in highly excited CO states than the lower SFR galaxies, then the observed molecular Kennicutt–Schmidt index, α, will be less than the underlying SFR ∝ρN index, N. Utilizing a combination of SPH models of galaxy evolution and molecular line radiative transfer, we present the first calculations of CO excitation in z∼ 2 disc galaxies with the aim of developing a mapping between various observed ΣSFR–ΣαCO relationships and the underlying SFR ∝ρN relation. We find that even in relatively luminous z∼ 2 discs, differential excitation does indeed exist, resulting in α \u3c N for highly excited CO lines. This means that an observed (e.g.) ΣSFR–ΣαCOJ=3−2 relation does not map linearly to a ΣSFR–ΣαH2 relation. We utilize our model results to provide a mapping from α to N for the range of Schmidt indices N= 1–2. By comparing to recent observational surveys, we find that the observed and ΣSFR–ΣαCOJ=3−2 relations suggest that an underlying SFR ∝ρ1.5 relation describes z∼ 2 disc galaxies

The Nature of CO Emission from z~6 Quasars

We investigate the nature of molecular gas emission from z ~ 6 quasars via the commonly observed tracer of H2, carbon monoxide (CO). We achieve this by combining non-LTE radiative transfer calculations with merger-driven models of z ~ 6 quasar formation that arise naturally in Λ cold dark matter structure formation simulations. Motivated by observational constraints, we consider four representative z ~ 6 quasars formed in the halo mass range ~1012-1013 M☉ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase, and the CO flux density peaks between J = 5 and 8. The CO morphology of z ~ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ~ 6, in quasar J1148+5251. Quasars which form at z ~ 6 display a large range of sight line-dependent line widths. The sight line dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the LB is largest) and broadest when the quasar is seen edge-on (and the LB is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ~ 6 having line widths narrower than the median. The mean sight line-averaged line width is found to be reflective of the circular velocity of the host halo and thus scales with halo mass. For example, the mean line width for the ~1012 M☉ halo is σ ~ 300 km s−1, while the median for the ~1013 M☉ quasar host is σ ~ 650 km s−1. Depending on the host halo mass, approximately 2%-10% of sight lines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10%-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ~ 2. Finally, we find that the dynamical mass derived from the mean sight line-averaged line widths provide a good estimate of the total mass and allow for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local MBH-Mbul relation

Simulated Molecular Outflows in Galaxy Mergers with Embedded AGN

We study the effects of feedback from active galactic nuclei (AGNs) on emission from molecular gas in galaxy mergers by combining hydrodynamic simulations that include black holes with a three-dimensional, non-local thermodynamic equilibrium (LTE) radiative transfer code. We find that molecular clouds entrained in AGN winds produce an extended CO morphology with significant off-nuclear emission, which may be detectable via contour mapping. Furthermore, kinematic signatures of these molecular outflows are visible in emission-line profiles when the outflow has a large line-of-sight velocity. Our results can help interpret current and upcoming observations of luminous infrared galaxies, as well as provide a detailed test of subresolution prescriptions for supermassive black hole growth in galaxy-scale hydrodynamic simulations