Simulation Analysis using EAGLE

While I mainly conduct observational studies, analyzing cosmological simulations provide a different perspective of understanding the CGM.  In particular, there exists very few direct observations of gas accretion onto galaxies.  Mapping the CGM using quasar sightline observations is also challenging, because most observations only have one sightline per galaxy.

I analyzed the EAGLE cosmological simulations* to study galaxy gas accretion and the observational signatures of the gas kinematics (Ho, Martin & Turner 2019).  I also analyzed the circumgalactic Mg II gas around low-redshift galaxies in EAGLE; I focused on the Mg II morphological and rotation structures and their dependence on galaxy properties (Ho, Martin & Schaye 2020).

Recently, I focus on the circumgalactic O VI gas around low-redshift star-forming galaxies.  I showed that observational bias could lead observers to overestimate the O VI column density and covering fraction (Ho, Martin & Schaye 2021).  I am also analyzing the O VI kinematics in EAGLE and comparing that to observational analyses (Ho et al. ,in prep.).


* EAGLE (Evolution and Assembly of GaLaxies and their Environments):

  • is a suite of of cosmological, hydrodynamical simulations of a standard lambda cold dark matter universe,

  • was run using the smoothed particle hydrodynamics (SPH) code GADGET-3.

  • links: @ Leiden, @ Durham

Related publications in reverse chronological order (also see the abstracts and selected figures below):

  • Ho, S. H., Martin, C. L. & Schaye, J., How Identifying Circumgalactic Gas by Line-of-sight Velocity instead of the Location in 3D Space Affects O VI Measurements.  2021, ApJ, in press [arXiv:2110.01633] [abstract/selected figures][ADS]

  • Ho, S. H., Martin, C. L. & Schaye, J., Morphological and Rotation Structures of Circumgalactic Mg II Gas in the EAGLE Simulation and the Dependence on Galaxy Properties.  2020, ApJ, 904, 76. [abstract/selected figures] [ADS]
    [4-min result video]

  • Ho, S. H., Martin, C. L. & Turner, M. L., How Gas Accretion Feeds Galactic Disks.  2019, ApJ, 875, 54 [abstract/selected figures] [ADS]

 

How Identifying Circumgalactic Gas by Line-of-sight Velocity instead of the Location in 3D Space Affects O VI Measurements

Stephanie H. Ho, Crystal L. Martin and Joop Schaye
Published 2021 Dec 16; The Astrophysical Journal, Volume 923, Issue 2, article id. 137, 10 pp.

[ADS]

Abstract:

The high incidence rate of the O VI 1032,1038 absorption around low-redshift, ~L* star-forming galaxies has generated interest in studies of the circumgalactic medium.  We use the high-resolution EAGLE cosmological simulation to analyze the circumgalactic O VI gas around z = 0.3 star-forming galaxies.  Motivated by the limitation that observations do not reveal where the gas lies along the line-of-sight, we compare the O VI measurements produced by gas within fixed distances around galaxies and by gas selected using line-of-sight velocity cuts commonly adopted by observers.  We show that gas selected by a velocity cut of +/-300 km/s or +/-500 km/s produces a higher O VI column density, a flatter column density profile, and a higher covering fraction compared to gas within one, two, or three times the virial radius (r_vir) of galaxies.  The discrepancy increases with impact parameter and worsens for lower mass galaxies.  For example, compared to the gas within 2 r_vir, identifying the gas using velocity cuts of 200-500 km/s increases the O VI column density by 0.2 dex (0.1 dex) at 1 r_vir to over 0.75 dex (0.7 dex) at ~2 r_vir for galaxies with stellar masses of 10^9 - 10^9.5 M_sun (10^10 - 10^10.5 M_sun).  We furthermore estimate that excluding O VI outside r_vir decreases the circumgalactic oxygen mass measured by Tumlinson et al. (2011) by over 50%.  Our results demonstrate that gas at large line-of-sight separations but selected by conventional velocity windows has significant effects on the O VI measurements and may not be observationally distinguishable from gas near the galaxies.

Selecting gas using a LOS velocity window overestimates O VI column density
Selecting gas using a LOS velocity window overestimates O VI column density

O VI column density as a function of impact parameter. Different colors show the results from the gas within 1, 2, or 3 r_vir of individual galaxies or within a LOS velocity window of +/- 300 or 500 km/s from the galaxy systemic velocity.

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O VI column density profiles for galaxies in different stellar mass bins
O VI column density profiles for galaxies in different stellar mass bins

In all mass bins, gas selected by the velocity window of +/-500 km/s (dashed) produces a higher O VI column density than that from gas within 2 r_vir (solid); the difference increases with impact parameter.

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What is the spatial extent of the O VI gas around galaxies?
What is the spatial extent of the O VI gas around galaxies?

Gas within 30 r_vir (light green solid) of individual galaxies has to be included to produce a median O VI column density profile comparable to those measured from gas selected by the LOS velocity window of +/- 200, 300, or 500 km/s.

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Selecting gas using a LOS velocity window overestimates O VI column density
Selecting gas using a LOS velocity window overestimates O VI column density

O VI column density as a function of impact parameter. Different colors show the results from the gas within 1, 2, or 3 r_vir of individual galaxies or within a LOS velocity window of +/- 300 or 500 km/s from the galaxy systemic velocity.

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Morphological and Rotation Structures of Circumgalactic Mg II Gas in the EAGLE Simulation and the Dependence on Galaxy Properties

Stephanie H. Ho, Crystal L. Martin and Joop Schaye
Published 2020 Nov 23; The Astrophysical Journal, Volume 904, Issue 1, article id. 76, 20 pp.

[ADS]

Abstract:

Low-ionization-state Mg II gas has been extensively studied in quasar sightline observations to understand the cool, ~10^4 K gas in the circumgalactic medium.  Motivated by recent observations showing that the Mg II gas around low-redshift galaxies has significant angular momentum, we use the high-resolution EAGLE cosmological simulation to analyze the morphological and rotation structures of the z  0.3 circumgalactic Mg II gas and examine how they change with the host galaxy properties.  Around star-forming galaxies, we find that the Mg II gas has an axisymmetric instead of a spherical distribution, and the axis of symmetry aligns with that of the Mg II gas rotation.  A similar rotating structure is less commonly found in the small sample of simulated quiescent galaxies.  We also examine how often Mg II gas around galaxies selected using a line-of-sight velocity cut includes gas physically outside of the virial radius (r_vir).  For example, we show that at an impact parameter of 100 pkpc, a +/- 500$ km/s velocity cut around galaxies with stellar masses of 10^9 - 10^9.5 M_sun (10^10 - 10^10.5 M_sun) selects Mg II gas beyond the virial radius 80% (6%) of the time.  Because observers typically select Mg II gas around target galaxies using such a velocity cut, we discuss how this issue affects the study of circumgalactic Mg II gas properties, including the detection of corotation.  While the corotating Mg II gas generally extends beyond 0.5 r_vir, the Mg II gas outside of the virial radius contaminates the corotation signal and makes observers less likely to conclude that gas at large impact parameters (e.g., >~ 0.25 r_vir) is corotating.

Mg II detection fraction of star-forming galaxies: Mg II gas has an axisymmetric distribution
Mg II detection fraction of star-forming galaxies: Mg II gas has an axisymmetric distribution

For each pixel, the Mg II detection fraction is the number of galaxies with Mg II gas ''detected'' (i.e., N(Mg II) >= 10^11.5/cm^2) divided by the total number of galaxies within the stack. Each red dashed circle shows the median 0.5 r_vir of the stacked galaxies. The Mg II gas is not spherically distributed and generally extends to larger radii for more massive galaxies.

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Fraction of Mg II that is corotating & detectable for star-forming galaxies
Fraction of Mg II that is corotating & detectable for star-forming galaxies

This fraction is calculated by dividing the number of galaxies with Mg II gas ''detected'' (i.e., N(Mg II) >= 10^11.5/cm^2) and corotating by the total number of galaxies in the stack. The shape of the contours changes with galaxy inclination angle. A higher fraction of Mg II is corotating and detectable near the galaxy major axes.

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Observers are less likely to conclude that gas at large impact parameters is corotating
Observers are less likely to conclude that gas at large impact parameters is corotating

Top: fraction of Mg II that is corotating & detectable; Bottom: fraction of detectable Mg II that is corotating. Darker (lighter) lines represent regions closer to the galaxy major (minor) axes. The solid and dashed curves represent the fractions obtained from gas within r_vir or a LOS velocity window around the galaxy systemic velocity. The difference between selecting Mg II gas by the two methods becomes prominent at >0.25 r_vir; the Mg II outside r_vir contaminates the corotation signal.

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Mg II detection fraction of star-forming galaxies: Mg II gas has an axisymmetric distribution
Mg II detection fraction of star-forming galaxies: Mg II gas has an axisymmetric distribution

For each pixel, the Mg II detection fraction is the number of galaxies with Mg II gas ''detected'' (i.e., N(Mg II) >= 10^11.5/cm^2) divided by the total number of galaxies within the stack. Each red dashed circle shows the median 0.5 r_vir of the stacked galaxies. The Mg II gas is not spherically distributed and generally extends to larger radii for more massive galaxies.

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How Gas Accretion Feeds Galactic Disks

Stephanie H. Ho, Crystal L. Martin and Monica L. Turner
Published 2019 April 16; The Astrophysical Journal, Volume 875, Issue 1, article id. 54, 19 pp.

[ADS]

Abstract:

Numerous observations indicate that galaxies need a continuous gas supply to fuel star formation and explain the star formation history.  However, direct observational evidence of gas accretion remains rare.  Using the EAGLE cosmological hydrodynamic simulation suite, we study cold gas accretion onto galaxies and the observational signatures of the cold gas kinematics.  For EAGLE galaxies at z 0.27, we find that cold gas accretes onto galaxies anisotropically with typical inflow speeds between 20 and 60 km/s.  Most of these galaxies have comparable mass inflow rates and star formation rates, implying that the cold inflowing gas plausibly accounts for sustaining the star-forming activities of the galaxies.  As motivation for future work to compare the cold gas kinematics with measurements from quasar sightline observations, we select an EAGLE galaxy with an extended cold gas disk, and we probe the cold gas using mock quasar sightlines.  We demonstrate that by viewing the disk edge on, sightlines at azimuthal angles below 10\deg and impact parameters out to 60 pkpc can detect cold gas that corotates with the galaxy disk.  This example suggests that cold gas disks extending beyond the optical disks possibly explain the sightline observations that detect corotating cold gas near galaxy major axes.

Angular Momentum of Cold Gas
Angular Momentum of Cold Gas

LOS velocity (top), infall prediction from the ballistic assumption (middle), and angular momentum distribution (bottom) of an EAGLE galaxy. Most infalling gas particles lack sufficient angular momentum to maintain circular orbits.

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Inflowing Gas Sustaining Galaxy SF
Inflowing Gas Sustaining Galaxy SF

Both particle tracking and ballistic assumption show that most galaxies have comparable mass inflow rates and star formation rates. This implies that the cold inflowing gas plausibly accounts for sustaining the galaxy star-forming activities.

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Angular Momentum of Cold Gas
Angular Momentum of Cold Gas

LOS velocity (top), infall prediction from the ballistic assumption (middle), and angular momentum distribution (bottom) of an EAGLE galaxy. Most infalling gas particles lack sufficient angular momentum to maintain circular orbits.

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