X-ray measurements suggest that the abundance of calcium in the intracluster medium is higher than can be explained using favored models for core-collapse and Type Ia supernovae alone. We investigate whether the "calcium conundrum" in the intracluster medium can be alleviated by including a contribution from the recently discovered subclass of supernovae known as calcium-rich gap transients. Although the calcium-rich gap transients make up only a small fraction of all supernovae events, we find that their high calcium yields are sufficient to reproduce the X-ray measurements found for nearby rich clusters. We find the chi(2) goodness-of-fit metric improves from 84 to 2 by including this new class. Moreover, calcium-rich supernovae preferentially occur in the outskirts of galaxies making it easier for the nucleosynthesis products of these events to be incorporated in the intracluster medium via ram-pressure stripping. The discovery of calcium-rich gap transients in clusters and groups far from any individual galaxy suggests that supernovae associated with intracluster stars may play an important role in enriching the intracluster medium. Calcium-rich gap transients may also help explain anomalous calcium abundances in many other astrophysical systems including individual stars in the Milky Way, the halos of nearby galaxies, and the circumgalactic medium. Our work highlights the importance of considering the diversity of supernovae types and corresponding yields when modeling the abundance of the intracluster medium and other gas reservoirs.

We examine the statistics of the low-redshift Ly alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate (Gamma(HI)) required by our simulations to match the observed properties of the low-redshift Ly alpha forest is a factor of five larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch in Gamma(HI) results in the mean flux decrement of the Ly alpha forest being overpredicted by at least a factor of two (a 10 sigma discrepancy with observations) and a column density distribution of Ly alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must contribute considerably more than current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.

We examine how HI and metal absorption lines within low-redshift galaxy haloes trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work, showing that the ionization level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionization metal absorbers (e. g. MgII) tend to arise in gas that will fall on to galaxies within several Gyr, while high-ionization metal absorbers (e.g. OVI) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow; hence, accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower mass haloes since high-mass haloes have more hot gas that is able to support itself against infall. Low-mass haloes also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower mass galaxies. Typical HI absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger HI absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle.

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z <= 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected one-dimensional and two-dimensional distribution of C iv absorption, we find that Civ is detected out to approximate to 100 kpc (corresponding roughly to approximate to 0.5 R-vir) of the host galaxies. The Civ absorption strength falls off radially as a power law, and beyond approximate to 0.5 R-vir, no Civ absorption is detected above our sensitivity limit of approximate to 50-100 m angstrom . We find a tentative correlation between detected C iv absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L similar to L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the circumgalactic medium (CGM) of these galaxies, corresponding to a minimum carbon mass of greater than or similar to 1.2 x 10(6) M circle dot out to similar to 110 kpc. This mass is comparable to the carbon mass in the interstellar medium and exceeds the carbon mass currently in the stars of these galaxies. The Civ absorption seen around these sub-L* galaxies can account for almost two-thirds of all W-r >= 100 m angstrom C iv absorption detected at low z. Comparing the Civ covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

We combine cosmological hydrodynamic simulations with analytic models to evaluate the role of galaxy-scale gravitational torques on the evolution of massive black holes at the centers of star-forming galaxies. We confirm and extend our earlier results to show that torque-limited growth yields black holes and host galaxies evolving on average along the M-BH-M-bulge relation from early times down to z = 0 and that convergence onto the scaling relation occurs independent of the initial conditions and with no need for mass averaging through mergers or additional self-regulation processes. Smooth accretion dominates the long-term evolution, with black hole mergers with mass ratios greater than or similar to 1: 5 representing typically a small fraction of the total growth. Winds from the accretion disk are required to eject significant mass to suppress black hole growth, but there is no need for coupling this wind to galactic-scale gas to regulate black holes in a nonlinear feedback loop. Torque-limited growth yields a close-to-linear <(M) over dot(BH)> proportional to star formation rate (SFR) relation for the black hole accretion rate averaged over galaxy evolution timescales. However, the SFR-AGN connection has significant scatter owing to strong variability of black hole accretion at all resolved timescales. Eddington ratios can be described by a broad lognormal distribution with median value evolving roughly as lambda(MS) (1 + z)(1.9), suggesting a main sequence for black hole growth similar to the cosmic evolution of specific SFRs. Our results offer an attractive scenario consistent with available observations in which cosmological gas infall and transport of angular momentum in the galaxy by gravitational instabilities regulate the long-term co-evolution of black holes and star-forming galaxies.

The instantaneous HI content of galaxies is thought to be governed by recent accretion and environment. We examine these effects within a cosmological hydrodynamic simulation that includes a heuristic galactic outflow model that reproduces basic observed trends of HI in galaxies. We show that this model reproduces the observed HI mass function in bins of stellar mass, as well as the HI richness (M-HI / M-*) versus local galaxy density. For satellite galaxies in massive (greater than or similar to 10(12)M(circle dot)) haloes, the HI richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of HI entering a massive halo is more rapid, in contrast to the specific star formation rate which shows little variation in the attenuation rate versus halo mass. This suggests that, up to the halo mass scales probed here (less than or similar to 10(14)M(circle dot)), star formation is mainly attenuated by starvation, but HI is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the HI richness of satellites is independent of radius, while in very massive haloes they become gas-poor towards the centre, confirming the increasing strength of the stripping with halo mass. Mergers somewhat increase the HI richness and its scatter about the mean relation, tracking the metallicity in a way consistent with it arising from inflow fluctuations, while star formation is significantly boosted relative to HI.

We analyse the low-redshift (z a parts per thousand 0.2) circumgalactic medium (CGM) by comparing absorption-line data from the COS-Halos survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We compare equivalent widths, covering factors, ion ratios, and kinematics. Both wind models show generally a parts per thousand(2) 1 sigma agreement with these observations for H i and certain low-ionization metal lines, but poorer agreement with higher ionization metal lines including Si iii and O vi that are well observed by COS-Halos. This suggests that both models predict too much cool, metal-enriched gas and not enough hot gas, and / or that metals are not sufficiently mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by superheating interstellar medium gas. This produces low-ionization absorption broadly consistent with observations, but substantial O vi absorption inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of a parts per thousand(2) 50 kpc, recycling winds dominate the absorption of low ions and even H i, while O vi generally arises from metals ejected a parts per thousand(3) 1 Gyr ago. The similarity between the wind models is surprising, since they differ substantially in the amount and phase distribution of halo gas. We show that this similarity owes mainly to our comparison at fixed stellar (not halo) mass, suggesting that CGM properties are more closely tied to galaxy's stellar (versus halo) mass.

We explore the implications of the observed lowspin of GW150914 within the context of stellar astrophysics and progenitor models. We conclude that many of the recently proposed scenarios are in marked tension with this observation. We derive a simple model for the observed spin in the case that the progenitor system was a field binary composed of a black hole (BH) and a Wolf-Rayet star and explore the implications of the observed spin for this model. The spin observation allows us to place a lower limit for the delay time between the formation of the BH+BH binary and the actual merger, t(merge). We use typical values for these systems to derive t(merge) greater than or similar to 10(8) yr, which proves to be an important diagnostic for different progenitor models. We anticipate the next series of events, and the associated spin parameters, will ultimately yield critical constraints on formation scenarios and on stellar parameters describing the late-stage evolution of massive stars.

We present an overview of the Carnegie-Chicago Hubble Program, an ongoing program to obtain a 3% measurement of the Hubble constant (H-0) using alternative methods to the traditional Cepheid distance scale. We aim to establish a completely independent route to H-0 using RR Lyrae variables, the tip of the red giant branch (TRGB), and Type Ia supernovae (SNe Ia). This alternative distance ladder can be applied to galaxies of any Hubble type, of any inclination, and, using old stars in low-density environments, is robust to the degenerate effects of metallicity and interstellar extinction. Given the relatively small number of SNe. Ia host galaxies with independently measured distances, these properties provide a great systematic advantage in the measurement of H-0 via the distance ladder. Initially, the accuracy of our value of H-0 will be set by the five Galactic RR Lyrae calibrators with Hubble Space Telescope Fine-Guidance Sensor parallaxes. With Gaia, both the RR Lyrae zero-point and TRGB method will be independently calibrated, the former with at least an order of magnitude more calibrators and the latter directly through parallax measurement of tip red giants. As the first end-to-end "distance ladder" completely independent of both Cepheid variables and the Large Magellanic Cloud, this path to H-0 will allow for the high-precision comparison at each rung of the traditional distance ladder that is necessary to understand tensions between this and other routes to H-0.

Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia's internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error sigma(pi(0)) will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for pi(0). Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.