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Register a new userQuantifying inhomogeneities in the HI distributions of simulated galaxies
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The NIHAO cosmological simulations form a collection of a hundred high-resolution galaxies. We used these simulations to test the impact of stellar feedback on the morphology of the HI distribution in galaxies. We ran a subsample of twenty of the galaxies with different parameterizations of stellar feedback, looking for differences in the HI spatial distribution and morphology. We found that different feedback models do leave a signature in HI, and can potentially be compared with current and future observations. These findings can help inform future modeling efforts in the parameterization of stellar feedback in cosmological simulations of galaxy formation and evolution.
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We use EAGLE to quantify the effect galaxy mergers have on the stellar specific angular momentum of galaxies, $j_{rm stars}$. We split mergers into: dry (gas-poor)/wet (gas-rich), major/minor, and different spin alignments and orbital parameters. Wet (dry) mergers have an average neutral gas-to-stellar mass ratio of $1.1$ ($0.02$), while major (minor) mergers are those with stellar mass ratios $ge 0.3$ ($0.1-0.3$). We correlate the positions of galaxies in the $j_{rm stars}$-stellar mass plane at $z=0$ with their merger history, and find that galaxies of low spins suffered dry mergers, while galaxies of normal/high spins suffered predominantly wet mergers, if any. The radial $j_{rm stars}$ profiles of galaxies that went through dry mergers are deficient by $approx 0.3$~dex at $rlesssim 10,r_{50}$ (with $r_{50}$ being the half-stellar mass radius), compared to galaxies that went through wet mergers. Studying the merger remnants reveals that dry mergers reduce $j_{rm stars}$ by $approx 30$%, while wet mergers increase it by $approx 10$%, on average. The latter is connected to the build-up of the bulge by newly formed stars of high rotational speed. Moving from minor to major mergers accentuates these effects. When the spin vectors of the galaxies prior to the dry merger are misaligned, $j_{rm stars}$ decreases to a greater magnitude, while in wet mergers co-rotation and high orbital angular momentum efficiently spun-up galaxies. We predict what would be the observational signatures in the $j_{rm stars}$ profiles driven by dry mergers: (i) shallow radial profiles and (ii) profiles that rise beyond $approx 10,r_{50}$, both of which are significantly different from spiral galaxies.
We highlight two research strands related to our ongoing chemodynamical Galactic Archaeology efforts: (i) the spatio-temporal infall rate of gas onto the disk, drawing analogies with the infall behaviour imposed by classical galactic chemical evolution models of inside-out disk growth; (ii) the radial age gradient predicted by spectrophometric models of disk galaxies. In relation to (i), at low-redshift, we find that half of the infall onto the disk is gas associated with the corona, while half can be associated with cooler gas streams; we also find that gas enters the disk preferentially orthogonal to the system, rather than in-plane. In relation to (ii), we recover age gradient troughs/inflections consistent with those observed in nature, without recourse to radial migrations.
We present a new method to identify and characterize the structure of the intracluster medium (ICM) in simulated galaxy clusters. The method uses the median of gas properties, such as density and pressure, which we show to be very robust to the presence of gas inhomogeneities. In particular, we show that the radial profiles of median gas properties are smooth and do not exhibit fluctuations at locations of massive clumps in contrast to mean and mode properties. It is shown that distribution of gas properties in a given radial shell can be well described by a log-normal PDF and a tail. The former corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the volume, while the tail corresponds to high density inhomogeneities. We show that this results in a simple and robust separation of the diffuse and clumpy components of the ICM. The FWHM of the density distribution grows with radius and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed clusters. The small scatter in the width between relaxed clusters suggests that the degree of inhomogeneity is a robust characteristic of the ICM. It broadly agrees with the amplitude of density perturbations in the Coma cluster. We discuss the origin of ICM density variations in spherical shells and show that less than 20% of the width can be attributed to the triaxiality of the cluster gravitational potential. As a link to X-ray observations of real clusters we evaluated the ICM clumping factor with and without high density inhomogeneities. We argue that these two cases represent upper and lower limits on the departure of the observed X-ray emissivity from the median value. We find that the typical value of the clumping factor in the bulk component of relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in broad agreement with recent observations.
The shape of a galaxys spatially unresolved, globally integrated 21-cm emission line depends on its internal gas kinematics: galaxies with rotation-supported gas disks produce double-horned profiles with steep wings, while galaxies with dispersion-supported gas produce Gaussian-like profiles with sloped wings. Using mock observations of simulated galaxies from the FIRE project, we show that one can therefore constrain a galaxys gas kinematics from its unresolved 21-cm line profile. In particular, we find that the kurtosis of the 21-cm line increases with decreasing $V/sigma$, and that this trend is robust across a wide range of masses, signal-to-noise ratios, and inclinations. We then quantify the shapes of 21-cm line profiles from a morphologically unbiased sample of $sim$2000 low-redshift, HI-detected galaxies with $M_{rm star} = 10^{7-11} M_{odot}$ and compare to the simulated galaxies. At $M_{rm star} gtrsim 10^{10} M_{odot}$, both the observed and simulated galaxies produce double-horned profiles with low kurtosis and steep wings, consistent with rotation-supported disks. Both the observed and simulated line profiles become more Gaussian-like (higher kurtosis and less-steep wings) at lower masses, indicating increased dispersion support. However, the simulated galaxies transition from rotation to dispersion support more strongly: at $M_{rm star} = 10^{8-10}M_{odot}$, most of the simulations produce more Gaussian-like profiles than typical observed galaxies with similar mass, indicating that gas in the low-mass simulated galaxies is, on average, overly dispersion-supported. Most of the lower-mass simulated galaxies also have somewhat lower gas fractions than the median of the observed population. The simulations nevertheless reproduce the observed line-width baryonic Tully-Fisher relation, which is insensitive to rotation vs. dispersion support.
Simulations have indicated that most of the escaped Lyman continuum photons escape through a minority of solid angles with near complete transparency, with the remaining majority of the solid angles largely opaque, resulting in a very broad and skewed probability distribution function (PDF) of the escape fraction when viewed at different angles. Thus, the escape fraction of Lyman continuum photons of a galaxy observed along a line of sight merely represents the properties of the interstellar medium along that line of sight, which may be an ill-representation of true escape fraction of the galaxy averaged over its full sky. Here we study how Lyman continuum photons escape from galaxies at $z=4-6$, utilizing high-resolution large-scale cosmological radiation-hydrodynamic simulations. We compute the PDF of the mean escape fraction ($left<f_{rm esc,1D}right>$) averaged over mock observational samples, as a function of the sample size, compared to the true mean (had you an infinite sample size). We find that, when the sample size is small, the apparent mean skews to the low end. For example, for a true mean of 6.7%, an observational sample of (2,10,50) galaxies at $z=4$ would have have 2.5% probability of obtaining the sample mean lower than $left<f_{rm esc,1D}right>=$(0.007%, 1.8%, 4.1%) and 2.5% probability of obtaining the sample mean being greater than (43%, 18%, 11%). Our simulations suggest that at least $sim$ 100 galaxies should be stacked in order to constrain the true escape fraction within 20% uncertainty.
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