You Are What You Eat: The Circumgalactic Medium Around BreakBRD Galaxies has Low Mass and Angular Momentum

Observed breakBRD ("break bulges in red disks") galaxies are a nearby sample of face-on disk galaxies with particularly centrally-concentrated star formation: they have red disks but recent star formation in their centers as measured by the D$_n$4000 spectral index. In Kopenhafer et al. (2020), a comparable population of breakBRD analogues was identified in the TNG simulation, in which the central concentration of star formation was found to reflect a central concentration of dense, starforming gas caused by a lack of dense gas in the galaxy outskirts. In this paper we examine the circumgalactic medium of the central breakBRD analogues to determine if the extended halo gas also shows differences from that around comparison galaxies with comparable stellar mass. We examine the circumgalactic medium gas mass, specific angular momentum, and metallicity in these galaxy populations. We find less gas in the circumgalactic medium of breakBRD galaxies, and that the breakBRD circumgalactic medium is slightly more concentrated than that of comparable stellar mass galaxies. In addition, we find that the angular momentum in the circumgalactic medium of breakBRD galaxies tends to be low for their stellar mass, and show more misalignment to the angular momentum vector of the stellar disk. Finally, we find that the circumgalactic medium metallicity of breakBRD galaxies tends to be high for their stellar mass. Together with their low SFR, we argue that these CGM properties indicate a small amount of disk feeding concentrated in the central regions, and a lack of low-metallicity gas accretion from the intergalactic medium.Comment: Published in The Astrophysical Journal, July 202

The Angular Momentum of the Circumgalactic Medium and its Connection to Galaxies in the Illustris and TNG Simulations

A galaxy's angular momentum is known to be correlated with its morphology: at a given mass, spiral galaxies have higher angular momenta than elliptical galaxies. A galaxy's angular momentum is also largely set by its formation history: in particular, how much gas and the kinematic state of the gas that both accretes onto it and is expelled in galactic outflows from AGN and supernovae. All gas inflowing to and outflowing from the galaxy interacts with gas in the region surrounding the galaxy called the circumgalactic medium (CGM), which means at a fundamental level, the CGM controls the angular momentum of the galaxy. Therefore, to really understand the origins of galactic angular momentum, it is necessary to understand the angular momentum of the CGM itself. In this dissertation, I present a series of projects aimed at studying angular momentum in the CGM using the Illustris and IllustrisTNG cosmological hydrodynamical simulations suites. In an appendix, I also present a project on searching a survey of neutral hydrogen for previously undetected ultra-faint dwarf galaxies in and around the Milky Way's CGM. First, to understand how present-day galaxies acquire their observed angular momentum, I analyze the evolution of the angular momentum of Lagrangian gas mass elements as they accrete onto dark matter halos, condense into Milky Way-scale galaxies, and join the z=0 stellar phase of those galaxies. I find that physical feedback from the galaxy is essential in order to produce reasonable values of galactic angular momentum, and that most of the effects of this feedback occur in the CGM, necessitating studying the angular momentum of the CGM itself. Following on from this result, I then characterize the angular momentum distribution and structure within the CGM of simulated galaxies over a much larger range of halo masses and redshifts, with the goal of determining if there are common angular momentum properties in CGM populations. I indeed find that the angular momentum of the CGM is larger and better aligned around disk galaxies that themselves have high angular momentum. I also identify rotating structures of cold gas that are generally present around galactic disks. This clear connection of the CGM to the galaxy motivated a detailed comparison to observations of cold CGM gas. I perform this comparison in the following chapter where I use the highest-resolution simulation from the IllustrisTNG suite of cosmological magneto-hydrodynamical simulations to generate synthetic observations of cold CGM gas around star-forming galaxies in order to study kinematics and compare them to line-of-sight observations of cold gas near comparable galaxies. With this direct comparison to observations of the CGM, I show that IllustrisTNG produces rotating CGM gas consistent with observations to a high degree. In the penultimate chapter I present unpublished work where I begin to examine angular momentum evolution in the CGM on much finer timescales than can be resolved with the cosmological simulations I have used thus far. Preliminary results suggest that gas can experience large changes in angular momentum very quickly, and that these changes may be connected to corresponding changes in the temperature of the gas. Finally, I conclude by summarizing my main results and briefly discussing what questions still remain unanswered and my plans and strategies for pursuing these questions in my future work

The Angular Momentum of the Circumgalactic Medium in the TNG100 Simulation

We present an analysis of the angular momentum content of the circumgalactic medium (CGM) using TNG100, one of the flagship runs of the IllustrisTNG project. We focus on Milky Way-mass halos ($\sim 10^{12} \; M_{\odot}$) at $z=0$ but also analyze other masses and redshifts up to $z=5$. We find that the CGM angular momentum properties are strongly correlated with the stellar angular momentum of the corresponding galaxy: the CGM surrounding high-angular momentum galaxies has a systematically higher angular momentum and is better aligned to the rotational axis of the galaxy itself than the CGM surrounding low-angular momentum galaxies. Both the hot and cold phases of the CGM show this dichotomy, though it is stronger for colder gas. The CGM of high-angular momentum galaxies is characterized by a large wedge of cold gas with rotational velocities at least $\sim1/2$ of the halo's virial velocity, extending out to $\sim 1/2$ of the virial radius, and by biconical polar regions dominated by radial velocities suggestive of galactic fountains; both of these features are absent from the CGM of low-angular momentum galaxies. These conclusions are general to halo masses $\lesssim 10^{12} \; M_{\odot}$ and for $z \lesssim 2$, but they do not apply for more massive halos or at the highest redshift studied. By comparing simulations run with alterations to the fiducial feedback model, we identify the better alignment of the CGM to high-angular momentum galaxies as a feedback-independent effect and the galactic winds as a dominant influence on the CGM's angular momentum.Comment: Accepted to ApJ. 16 pages, 12 figure

Magnetic Inflation and Stellar Mass. I. Revised Parameters for the Component Stars of the Kepler Low-mass Eclipsing Binary T-Cyg1-12664

Several low-mass eclipsing binary stars show larger than expected radii for their measured mass, metallicity, and age. One proposed mechanism for this radius inflation involves inhibited internal convection and starspots caused by strong magnetic fields. One particular eclipsing binary, T-Cyg1-12664, has proven confounding to this scenario. Çakırlı et al. measured a radius for the secondary component that is twice as large as model predictions for stars with the same mass and age, but a primary mass that is consistent with predictions. Iglesias-Marzoa et al. independently measured the radii and masses of the component stars and found that the radius of the secondary is not in fact inflated with respect to models, but that the primary is, which is consistent with the inhibited convection scenario. However, in their mass determinations, Iglesias-Marzoa et al. lacked independent radial velocity measurements for the secondary component due to the star's faintness at optical wavelengths. The secondary component is especially interesting, as its purported mass is near the transition from partially convective to a fully convective interior. In this article, we independently determined the masses and radii of the component stars of T-Cyg1-12664 using archival Kepler data and radial velocity measurements of both component stars obtained with IGRINS on the Discovery Channel Telescope and NIRSPEC and HIRES on the Keck Telescopes. We show that neither of the component stars is inflated with respect to models. Our results are broadly consistent with modern stellar evolutionary models for main-sequence M dwarf stars and do not require inhibited convection by magnetic fields to account for the stellar radii

The effect of cosmic rays on the observational properties of the CGM

International audienceThe circumgalactic medium (CGM) contains information on the cumulative effect of galactic outflows over time, generally thought to be caused by feedback from star formation and active galactic nuclei. Observations of such outflows via absorption in CGM gas of quasar sightlines show a significant amount of cold (⁠|${\lesssim}10^4\,{\rm K}$|⁠) gas, which cosmological simulations struggle to reproduce. Here, we use the adaptive mesh refinement hydrodynamical code Ramses to investigate the effect of cosmic rays (CR) on the cold gas content of the CGM using three zoom realizations of a z = 1 star-forming galaxy with supernova mechanical feedback: one with no CR feedback (referred to as no-CR), one with a medium CR diffusion coefficient |$\kappa = 10^{28} \, \rm {cm^{2}\, s^{-1}}$| (CR−κ_med), and one with a high rate of diffusion of |$\kappa = 3\times 10^{29} \, \rm {cm^{2}\,\, s^{-1}}$| (CR−κ_high). We find that, for CR−κ_med, the effects of CRs are largely confined to the galaxy itself as CRs do not extend far into the CGM. However, for CR−κ_high, the CGM temperature is lowered and the amount of outflowing gas is boosted. Our CR simulations fall short of the observed Mg ii covering fraction, a tracer of gas at temperatures |${\lesssim}10^4\,{\rm K}$|⁠, but the CR−κ_high simulation is more in agreement with covering fractions of C iv and O vi, which trace higher temperature gas

A Comparison of Circumgalactic MgII Absorption between the TNG50 Simulation and the MEGAFLOW Survey

in press to ApJ. 15 pages, 10 figures. The TNG50 data is now publicly available at https://www.tng-project.orgInternational audienceThe circumgalactic medium (CGM) contains information on gas flows around galaxies, such as accretion and supernova-driven winds, which are difficult to constrain from observations alone. Here, we use the high-resolution TNG50 cosmological magneto-hydrodynamical simulation to study the properties and kinematics of the CGM around star-forming galaxies in $10^{11.5}-10^{12}\;M_{\odot}$ halos at $z\simeq1$ using mock MgII absorption lines, which we generate by post-processing halos to account for photoionization in the presence of a UV background. We find that the MgII gas is a very good tracer of the cold CGM, which is accreting inwards at an inflow velocity of $\sim$50 km s$^{-1}$. For sightlines aligned with the galaxy's major axis, we find that MgII absorption lines are kinematically shifted due to the cold CGM's significant corotation at speeds up to 50% of the virial velocity for impact parameters up to 60 kpc. We compare mock MgII spectra to observations from the MusE GAs FLow and Wind (MEGAFLOW) survey of strong MgII absorbers ($EW^{2796\r{A}}_{0}>0.5 \; \r{A}$). After matching the equivalent width (EW) selection, we find that the mock MgII spectra reflect the diversity of observed kinematics and EWs from MEGAFLOW, even though the sightlines probe a very small fraction of the CGM. MgII absorption in higher-mass halos is stronger and broader than in lower-mass halos but has qualitatively similar kinematics. The median specific angular momentum of the MgII CGM gas in TNG50 is very similar to that of the entire CGM and only differs from non-CGM components of the halo by normalization factors of $\lesssim$ 1 dex