16 research outputs found
Recommended from our members
Observationally Constrained Metal Signatures of Galaxy Evolution in the Stars and Gas of Cosmological Simulations
The halos of galaxies - consisting of gas, stars, and satellite galaxies - are formed and shaped by the most fundamental processes: hierarchical merging and the flow of gas into and out of galaxies. While these processes are hard to disentangle, metals are tied to the gas that fuels star formation and entrained in the wind that the deaths of these stars generate. As such, they can act as important indicators of the star formation, the chemical enrichment, and the outflow histories of galaxies. Thus, this thesis aims to take advantage of such metal signatures in the stars and gas to place observational constraints on current theories of galaxy evolution as implemented in cosmological simulations.
The first two chapters consider the metallicities of stars in the stellar halo of the Milky Way and its surviving satellite dwarf galaxies. Chapter 2 pairs an N-body simulation with a semi-analytic model for supernova-driven winds to examine the early environment of a Milky Way-like galaxy. At z=10, progenitors of surviving z=0 satellite galaxies are found to sit preferentially on the outskirts of progenitor halos of the eventual main halo. The consequence of these positions is that main halo progenitors are found to more effectively cross-pollute each other than satellite progenitors. Thus, inhomogeneous cross-pollution as a result of different high-z spatial locations of different progenitors can help to explain observed differences in abundance patterns measured today. Chapter 3 expands this work into the analysis of a cosmological, hydrodynamical simulation of dwarf galaxies in the early universe. We find that simple assumptions for modeling the extent of supernova-driven winds used in Chapter 2 agree well with the simulation whereas the presence of inhomogeneous mixing in the simulation has a large effect on the stellar metallicities. Furthermore, the star-forming halos show both bursty and continuous SFHs, two scenarios proposed by stellar metallicity data. However, the metallicity distribution functions of the simulated halos are both too metal rich and too peaked when compared to the data. This comparison reveals that a complex SFH and a broad metallicity distribution can develop rapidly in the early Universe.
The third chapter moves to the present day with a consideration of the circumgalactic medium (CGM) around nearby Milky Way-like galaxies. We compare a cosmological simulation of a Milky Way-like galaxy to recent absorption line data and find that a reduced extragalactic ultraviolet background brings the column density predictions into better agreement with the data. Similarly, when the observationally derived physical properties of the gas are compared to the simulation, we find that the simulation gas is always at temperatures approximately 0.5 dex higher. Thus, similar column densities can be produced from fundamentally different gas. Metal-line emission is then considered as a complementary approach to studying the CGM. From the simulations, we find that the brightest emission is less sensitive to the extragalactic background and that it closely follows the fundamental filamentary structure of the halo. This becomes increasingly true as the galaxy evolves from z = 1 to z = 0 and the majority of the gas transitions to a hotter, more diffuse phase. Finally, resolution is a limiting factor for the conclusions we can draw from emission observations but with moderate resolution and reasonable detection limits, upcoming instrumentation should place constraints on the physical properties of the CGM.
Future work advancing the techniques in this thesis remain promising for putting new observational constraints on our theories of galaxy evolution
Figuring Out Gas & Galaxies in Enzo (FOGGIE). II. Emission from the z=3 Circumgalactic Medium
Observing the circumgalactic medium (CGM) in emission provides 3D maps of the
spatial and kinematic extent of the gas that fuels galaxies and receives their
feedback. We present mock emission-line maps of highly resolved CGM gas from
the FOGGIE project (Figuring Out Gas & Galaxies in Enzo) and link these maps
back to physical and spatial properties of the gas. By increasing the spatial
resolution alone, the total luminosity of the line emission increases by an
order of magnitude. This increase arises in the abundance of dense small-scale
structure resolved when the CGM gas is simulated to < 100 pc scales. Current
integral field unit instruments like KCWI and MUSE should be able to detect the
brightest knots and filaments of such emission, and from this to infer the bulk
kinematics of the CGM gas with respect to the galaxy. We conclude that
accounting for small-scale structure well below the level of instrument spatial
resolution is necessary to properly interpret such observations in terms of the
underlying gas structure driving observable emission.Comment: 18 pages, 10 figures. Submitted to ApJ. Comments welcom
Figuring Out Gas & Galaxies in Enzo (FOGGIE). III. The Mocky Way:Investigating Biases in Observing the Milky Way's Circumgalactic Medium
The circumgalactic medium (CGM) of the Milky Way is mostly obscured by nearby
gas in position-velocity space because we reside inside the Galaxy. Substantial
biases exist in most studies on the Milky Way's CGM that focus on
easier-to-detect high-velocity gas. With mock observations on a Milky-Way
analog from the FOGGIE simulation, we investigate four observational biases
related to the Milky Way's CGM. First, QSO absorption-line studies probe a
limited amount of the CGM mass: only 35% of the mass is at high Galactic
latitudes degrees, of which only half is moving at km s. Second, the inflow rate () of the cold
gas observable in HI 21cm is reduced by a factor of as we switch from
the local standard of rest to the galaxy's rest frame; meanwhile of
the cool and warm gas does not change significantly. Third, OVI and NV are
promising ions to probe the Milky Way's outer CGM (15 kpc), but CIV
may be less sensitive. Lastly, the scatter in ion column density is a factor of
2 higher if the CGM is observed from inside-out than from external views
because of the gas radial density profile. Our work highlights that
observations of the Milky Way's CGM, especially those using HI 21cm and QSO
absorption lines, are highly biased. We demonstrate that these biases can be
quantified and calibrated through synthetic observations with simulated
Milky-Way analogs.Comment: ApJ in pres
Understanding the circumgalactic medium is critical for understanding galaxy evolution
The circumgalactic medium is a major reservoir of baryons and metals, playing a key role in the long cycles of accretion, feedback, and recycling of gas driving galaxy evolution. Fundamental progress on major issues in galaxy evolution depends critically on improved empirical characterization and theoretical understanding of circumgalactic gas
Figuring Out Gas & Galaxies In Enzo (FOGGIE). IV. The Stochasticity of Ram Pressure Stripping in Galactic Halos
We study ram pressure stripping in simulated Milky Way-like halos at z>=2
from the Figuring Out Gas & Galaxies In Enzo (FOGGIE) project. These
simulations reach exquisite resolution in their circumgalactic medium (CGM) gas
owing to FOGGIE's novel refinement scheme. The CGM of each halo spans a wide
dynamic range in density and velocity over its volume---roughly 6 dex and 1000
km/s, respectively---translating into a 5 dex range in ram pressure imparted to
interacting satellites. The ram pressure profiles of the simulated CGM are
highly stochastic, owing to kpc-scale variations of the density and velocity
fields of the CGM gas. As a result, the efficacy of ram pressure stripping
depends strongly on the specific path a satellite takes through the CGM. The
ram-pressure history of a single satellite is generally unpredictable and not
well correlated with its approach vector with respect to the host galaxy. The
cumulative impact of ram pressure on the simulated satellites is dominated by
only a few short strong impulses---on average, 90% of the total surface
momentum gained through ram pressure is imparted in 20% or less of the total
orbital time. These results reveal an erratic mode of ram pressure stripping in
Milky-Way like halos at high redshift---one that is not captured by a smooth
spherically-averaged model of the circumgalactic medium.Comment: 18 pages, 10 figures. Submitted to Ap
Figuring Out Gas & Galaxies in Enzo (FOGGIE). I. Resolving Simulated Circumgalactic Absorption at 2 ≤ z ≤ 2.5
We present simulations from the new "Figuring Out Gas & Galaxies in Enzo" (FOGGIE) project. In contrast to most extant simulations of galaxy formation, which concentrate computational resources on galactic disks and spheroids with fluid and particle elements of fixed mass, the FOGGIE simulations focus on extreme spatial and mass resolution in the circumgalactic medium (CGM) surrounding galaxies. Using the Enzo code and a new refinement scheme, FOGGIE reaches spatial resolutions of 381 comoving h^(−1) pc and resolves extremely low masses (≾1–100 M⊙) out to 100 comoving h^(−1) kpc from the central halo. At these resolutions, cloud and filament-like structures giving rise to simulated absorption are smaller, and better resolved, than the same structures simulated with standard density-dependent refinement. Most of the simulated absorption arises in identifiable and well-resolved structures with masses ≾10^4 M⊙, well below the mass resolution of typical zoom simulations. However, integrated quantities such as mass surface density and ionic covering fractions change at only the ≾30% level as resolution is varied. These relatively small changes in projected quantities—even when the sizes and distribution of absorbing clouds change dramatically—indicate that commonly used observables provide only weak constraints on the physical structure of the underlying gas. Comparing the simulated absorption features to the KODIAQ (Keck Observatory Database of Ionized Absorption toward Quasars) survey of z ~ 2–3.5 Lyman limit systems, we show that high-resolution FOGGIE runs better resolve the internal kinematic structure of detected absorption and better match the observed distribution of absorber properties. These results indicate that circumgalactic medium resolution is key in properly testing simulations of galaxy evolution with circumgalactic observations
Figuring Out Gas & Galaxies in Enzo (FOGGIE). I. Resolving Simulated Circumgalactic Absorption at 2 ≤ <i>z ≤ </i>2.5
We present simulations from the new "Figuring Out Gas & Galaxies in Enzo"
(FOGGIE) project. In contrast to most extant simulations of galaxy formation,
which concentrate computational resources on galactic disks and spheroids with
fluid and particle elements of fixed mass, the FOGGIE simulations focus on
extreme spatial and mass resolution in the circumgalactic medium (CGM)
surrounding galaxies. Using the Enzo code and a new refinement scheme, FOGGIE
reaches spatial resolutions of 381 comoving pc and resolves extremely
low masses (-- Msun out to 100 comoving kpc from the
central halo. At these resolutions, cloud and filament-like structures giving
rise to simulated absorption are smaller, and better resolved, than the same
structures simulated with standard density-dependent refinement. Most of the
simulated absorption arises in identifiable and well-resolved structures with
masses Msun, well below the mass resolution of typical zoom
simulations. However, integrated quantities such as mass surface density and
ionic covering fractions change at only the % level as resolution
is varied. This relatively small changes in projected quantities---even when
the sizes and distribution of absorbing clouds change dramatically---indicate
that commonly used observables provide only weak constraints on the physical
structure of the underlying gas. Comparing the simulated absorption features to
the KODIAQ (Keck Observatory Database of Ionized Absorption toward Quasars)
survey of -- Lyman limit systems, we show that high-resolution
FOGGIE runs better resolve the internal kinematic structure of detected
absorption, and better match the observed distribution of absorber properties.
These results indicate that CGM resolution is key in properly testing
simulations of galaxy evolution with circumgalactic observations.Comment: ApJ, in press. New appendix with results from simulation with 78pc
resolution CGM at z=2.5 now included. Appendix C with additional velocity
phase diagrams has been omitted to fit within the arXiv space restraint
Understanding the circumgalactic medium is critical for understanding galaxy evolution
Galaxies evolve under the influence of gas flows between their interstellar
medium and their surrounding gaseous halos known as the circumgalactic medium
(CGM). The CGM is a major reservoir of galactic baryons and metals, and plays a
key role in the long cycles of accretion, feedback, and recycling of gas that
drive star formation. In order to fully understand the physical processes at
work within galaxies, it is therefore essential to have a firm understanding of
the composition, structure, kinematics, thermodynamics, and evolution of the
CGM. In this white paper we outline connections between the CGM and galactic
star formation histories, internal kinematics, chemical evolution, quenching,
satellite evolution, dark matter halo occupation, and the reionization of the
larger-scale intergalactic medium in light of the advances that will be made on
these topics in the 2020s. We argue that, in the next decade, fundamental
progress on all of these major issues depends critically on improved empirical
characterization and theoretical understanding of the CGM. In particular, we
discuss how future advances in spatially-resolved CGM observations at high
spectral resolution, broader characterization of the CGM across galaxy mass and
redshift, and expected breakthroughs in cosmological hydrodynamic simulations
will help resolve these major problems in galaxy evolution.Comment: Astro2020 Decadal Science White Pape