Adrienne M. Stilp

Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation

Motivated by the observed connection between molecular hydrogen (H2) and star formation, we present a method for tracking the non-equilibrium abundance and cooling processes of H2 and H2-based star formation in smoothed particle hydrodynamic simulations. The local abundances of H2 are calculated by integrating over the hydrogen chemical network. This calculation includes the gas phase and dust grain formation of H2, shielding of H2 and photodissociation of H2 by Lyman–Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low-metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H2 abundance. This link limits star formation to ‘star-forming regions’, represented by areas with abundant H2. We use simulations of isolated disc galaxies to verify that the transition from atomic to molecular hydrogen occurs at realistic densities and surface densities. Using these same isolated galaxies, we establish that gas particles of 104 M⊙ or less are necessary to follow the molecular gas in this implementation. With this implementation, we determine the effect of H2 on star formation in a cosmological simulation of a dwarf galaxy. This simulation is the first cosmological simulation with non-equilibrium H2 abundances to be integrated to a redshift of zero or to include efficient supernova feedback. We analyse the amount and distribution of star formation in the galaxy using simulated observations of the H i gas and in various optical bands. From these simulated observations, we find that our simulations are consistent with the observed Tully–Fisher, global Kennicutt–Schmidt and resolved Kennicutt–Schmidt relations. We find that the inclusion of shielding of both the atomic and molecular hydrogen and, to a lesser extent, the additional cooling from H2 at temperatures between 200 and 5000 K increases the amount of cold gas in the galaxies. The changes to the interstellar medium (ISM) result in an increased amount of cold, dense gas in the disc of the galaxy and the formation of a clumpier ISM. The explicit link between star formation and H2 and the clumpier ISM results in a bluer galaxy with a greater spatial distribution of star formation at a redshift of zero.

Control for Population Structure and Relatedness for Binary Traits in Genetic Association Studies via Logistic Mixed Models

Linear mixed models (LMMs) are widely used in genome-wide association studies (GWASs) to account for population structure and relatedness, for both continuous and binary traits. Motivated by the failure of LMMs to control type I errors in a GWAS of asthma, a binary trait, we show that LMMs are generally inappropriate for analyzing binary traits when population stratification leads to violation of the LMMs constant-residual variance assumption. To overcome this problem, we develop a computationally efficient logistic mixed model approach for genome-wide analysis of binary traits, the generalized linear mixed model association test (GMMAT). This approach fits a logistic mixed model once per GWAS and performs score tests under the null hypothesis of no association between a binary trait and individual genetic variants. We show in simulation studies and real data analysis that GMMAT effectively controls for population structure and relatedness when analyzing binary traits in a wide variety of study designs.

The Formation of Polar Disk Galaxies

Polar ring galaxies, such as NGC 4650A, are a class of galaxies that have two kinematically distinct components that are inclined by almost 90° to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain and is the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that polar ring galaxies are simply an extreme example of the misalignment of angular momentum that occurs during the hierarchical structure formation characteristic of a cold dark matter cosmology. In our model, polar ring galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.

The Formation of Kiloparsec-scale H I Holes in Dwarf Galaxies

The origin of kpc-scale holes in the atomic hydrogen (H i) distributions of some nearby dwarf irregular galaxies presents an intriguing problem. Star formation histories (SFHs) derived from resolved stars give us the unique opportunity to study past star-forming events that may have helped shape the currently visible Hi distribution. Our sample of five nearby dwarf irregular galaxies spans over an order of magnitude in both total Hi mass and absolute B-band magnitude and is at the low-mass end of previously studied systems. We use Very Large Array Hi line data to estimate the energy required to create the centrally dominant hole in each galaxy. We compare this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope. The inferred integrated stellar energy released within the characteristic ages exceeds our energy estimates for creating the holes in all cases, assuming expected efficiencies. Therefore, it appears that stellar feedback provides sufficient energy to produce the observed holes. However, we find no obvious signature of single star-forming events responsible for the observed structures when comparing the global SFHs of each galaxy in our sample to each other or to those of dwarf irregular galaxies reported in the literature. We also fail to find evidence of a central star cluster in FUV or Hα imaging. We conclude that large Hi holes are likely formed from multiple generations of star formation and only under suitable interstellar medium conditions.

PRESSURE SUPPORT IN GALAXY DISKS: IMPACT ON ROTATION CURVES AND DARK MATTER DENSITY PROFILES

Rotation curves constrain a galaxys underlying mass density profile, under the assumption that the observed rotation produces a centripetal force that exactly balances the inward force of gravity. However, most rotation curves are measured using emission lines from gas, which can experience additional forces due to pressure. In realistic galaxy disks, the gas pressure declines with radius, providing additional radial support to the disk. The measured tangential rotation speed will therefore tend to lag the true circular velocity of a test particle. The gas pressure is dominated by turbulence, and we evaluate its likely amplitude from recent estimates of the gas velocity dispersion and surface density. We show that where the amplitude of the rotation curve is comparable to the characteristic velocities of the interstellar turbulence, pressure support may lead to underestimates of the mass density of the underlying dark matter halo and the inner slope of its density profile. These effects may be significant for galaxies with rotation speeds 75 km s-1 but are unlikely to be significant in higher-mass galaxies. We find that pressure support can be sustained over long timescales, because any reduction in support due to the conversion of gas into stars is compensated for by an inward flow of gas. However, we point to many uncertainties in assessing the importance of pressure support in real or simulated galaxies. Thus, while pressure support may help to alleviate possible tensions between rotation curve observations and ΛCDM on kiloparsec scales, it should not be viewed as a definitive solution at this time.

THE ADVANCED CAMERA FOR SURVEYS NEARBY GALAXY SURVEY TREASURY. IV. THE STAR FORMATION HISTORY OF NGC 2976

We present resolved stellar photometry of NGC 2976 obtained with the Advanced Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury (ANGST) program. The data cover the radial extent of the major axis of the disk out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of M F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of M F606W ~ ?1. Through detailed analysis and modeling of the resulting color-magnitude diagrams, we have reconstructed the star formation history (SFH) of the stellar populations currently residing in these portions of the galaxy, finding similar ancient populations at all radii but significantly different young populations at increasing radii. In particular, outside of the well-measured break in the disk surface brightness profile, the age of the youngest population increases with distance from the galaxy center, suggesting that star formation is shutting down from the outside-in. We use our measured SFH, along with H I surface density measurements, to reconstruct the surface density profile of the disk during previous epochs. Comparisons between the recovered star formation rates and reconstructed gas densities at previous epochs are consistent with star formation following the Schmidt law during the past 0.5 Gyr, but with a drop in star formation efficiency at low gas densities, as seen in local galaxies at the present day. The current rate and gas density suggest that rapid star formation in NGC 2976 is currently in the process of ceasing from the outside-in due to gas depletion. This process of outer disk gas depletion and inner disk star formation was likely triggered by an interaction with the core of the M81 group 1 Gyr ago that stripped the gas from the galaxy halo and/or triggered gas inflow from the outer disk toward the galaxy center.

GLOBAL H I KINEMATICS IN DWARF GALAXIES

H I line widths are typically interpreted as a measure of interstellar medium turbulence, which is potentially driven by star formation (SF). In an effort to better understand the possible connections between line widths and SF, we have characterized H I kinematics in a sample of nearby dwarf galaxies by co-adding line-of-sight spectra after removing the rotational velocity to produce average global H I line profiles. These superprofiles are composed of a central narrow peak (~6-10 km s–1) with higher-velocity wings to either side that contain ~10%-15% of the total flux. The superprofiles are all very similar, indicating a universal global H I profile for dwarf galaxies. We compare characteristics of the superprofiles to various galaxy properties, such as mass and measures of SF, with the assumption that the superprofile represents a turbulent peak with energetic wings to either side. We use these quantities to derive average scale heights for the sample galaxies. When comparing to physical properties, we find that the velocity dispersion of the central peak is correlated with ΣH I . The fraction of mass and characteristic velocity of the high-velocity wings are correlated with measures of SF, consistent with the picture that SF drives surrounding H I to higher velocities. While gravitational instabilities provide too little energy, the SF in the sample galaxies does provide enough energy through supernovae, with realistic estimates of the coupling efficiency, to produce the observed superprofiles.

Local Ancestry Inference in a Large US-Based Hispanic/Latino Study: Hispanic Community Health Study/Study of Latinos (HCHS/SOL)

We estimated local ancestry on the autosomes and X chromosome in a large US-based study of 12,793 Hispanic/Latino individuals using the RFMix method, and we compared different reference panels and approaches to local ancestry estimation on the X chromosome by means of Mendelian inconsistency rates as a proxy for accuracy. We developed a novel and straightforward approach to performing ancestry-specific PCA after finding artifactual behavior in the results from an existing approach. Using the ancestry-specific PCA, we found significant population structure within African, European, and Amerindian ancestries in the Hispanic/Latino individuals in our study. In the African ancestral component of the admixed individuals, individuals whose grandparents were from Central America clustered separately from individuals whose grandparents were from the Caribbean, and also from reference Yoruba and Mandenka West African individuals. In the European component, individuals whose grandparents were from Puerto Rico diverged partially from other background groups. In the Amerindian ancestral component, individuals clustered into multiple different groups depending on the grandparental country of origin. Therefore, local ancestry estimation provides further insight into the complex genetic structure of US Hispanic/Latino populations, which must be properly accounted for in genotype-phenotype association studies. It also provides a basis for admixture mapping and ancestry-specific allele frequency estimation, which are useful in the identification of risk factors for disease.

Trans-ethnic Fine Mapping Highlights Kidney-Function Genes Linked to Salt Sensitivity

We analyzed genome-wide association studies (GWASs), including data from 71,638 individuals from four ancestries, for estimated glomerular filtration rate (eGFR), a measure of kidney function used to define chronic kidney disease (CKD). We identified 20 loci attaining genome-wide-significant evidence of association (p < 5 × 10−8) with kidney function and highlighted that allelic effects on eGFR at lead SNPs are homogeneous across ancestries. We leveraged differences in the pattern of linkage disequilibrium between diverse populations to fine-map the 20 loci through construction of “credible sets” of variants driving eGFR association signals. Credible variants at the 20 eGFR loci were enriched for DNase I hypersensitivity sites (DHSs) in human kidney cells. DHS credible variants were expression quantitative trait loci for NFATC1 and RGS14 (at the SLC34A1 locus) in multiple tissues. Loss-of-function mutations in ancestral orthologs of both genes in Drosophila melanogaster were associated with altered sensitivity to salt stress. Renal mRNA expression of Nfatc1 and Rgs14 in a salt-sensitive mouse model was also reduced after exposure to a high-salt diet or induced CKD. Our study (1) demonstrates the utility of trans-ethnic fine mapping through integration of GWASs involving diverse populations with genomic annotation from relevant tissues to define molecular mechanisms by which association signals exert their effect and (2) suggests that salt sensitivity might be an important marker for biological processes that affect kidney function and CKD in humans.

Constraining the age of the NGC 4565 H I disk WARP: Determining the origin of gas WARPS

We have mapped the distribution of young and old stars in the gaseous H I warp of NGC 4565. We find a clear correlation of young stars ( 1 Gyr), which places an upper limit on the age of the structure. The formation rate of the young stars, which increased ∼300 Myr ago relative to the surrounding regions, is (6.3{sub −1.5}{sup +2.5})×10{sup −5} M {sub ☉} yr{sup –1} kpc{sup –2}. This implies a ∼60 ± 20 Gyr depletion time of the H I warp, similar to the timescales calculated for the outer H I disks of nearby spiral galaxies. While some stars associated with the warp fall into the asymptotic giant branch (AGB) region of the color-magnitude diagram, where stars could be as old as 1 Gyr, further investigation suggests that they may be interlopers rather than real AGB stars. We discuss the implications of these age constraints for the formation of H I warps and the gas fueling of disk galaxies.