Kinematics of the Circumgalactic Gas

Co-leading (with Prof. Crystal Martin at UCSB) the Quasars Probing Galaxies Program, we observed pairs of z  ≈ 0.2, star-forming galaxies and background quasars, measured the low-ionization-state gas (in absorption) in the quasar sightlines, and then characterized the gas kinematics in the reference frame of the galactic disk.

Here are two main results:

  1. the cool CGM (~10^4 K, traced by the low-ionization-state ions) corotates with the galactic disk, ruling out the schema that the CGM lacks angular momentum (Ho et al. 2017; Martin et al. 2019), and

  2. galactic outflows kinematically disturb the CGM, producing excess absorption in minor-axis sightlines (Martin et al. 2019).

Hence, our results suggest that​ both angular momentum and galactic outflows should be included in modeling low-redshift CGM.

See the press release in June 2019: [Keck] [UCSB] 

We developed geometrical models to explain the measured gas kinematics.  These models are incorporated with the 3D galactic disk orientation* to constrain the inflow and outflow parameters (Martin et al. 2019; Ho & Martin 2020).

3D disk orientation:

  • Describing the galactic disk orientation on the sky with only the position angle and axis ratio leaves an ambiguity about which side of the minor axis is tipped towards the observer.

  • Which way the disk is tilted alters the predictions (e.g., Doppler shift, LOS velocity range, inflow speed, etc.) from inflow and outflow modeling.

  • How to break the degeneracy: Because disks rotate differentially, and only trailing spirals are long-lived, spiral arms generally trail the direction of rotation.  By resolving the galaxy spiral arm structures and determining their wrapping direction, together with the galaxy rotation curves measured, we can determine which way the disk tilts on the sky.


We also presented one of the first observations with low-ionization-state absorption detected at both the transverse quasar sightline near the galaxy minor axis and "down-the-barrel" measured in the galaxy spectrum (Kacprzak et al. 2014).

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

  • Ho, S. H. & Martin, C. L., Resolving 3D Disk Orientation Using High-Resolution Images: New Constraints on Circumgalactic Gas Inflows.  2020, ApJ, 888, 14 [abstract/selected figures] [ADS]

  • Martin, C. L., Ho, S. H., Kacprzak, G. G. & Churchill, C. W., Kinematics of Circumgalactic Gas: Feeding and Feedback.  2019, ApJ, 878, 84 [abstract/selected figures] [ADS]

  • Ho, S. H., Martin, C. L., Kacprzak, G. G. & Churchill, C. W., Quasar Probing Galaxies: Signatures of Gas Accretion at Redshift z ≈ 0.2.  2017, ApJ, 835, 267 [abstract/selected figures] [ADS]

  • Kacprzak, G. G., Martin, C. L., Bouché, N., Churchill, C. W., Cooke, J., LeReun, A., Schroetter, I., Ho, S. H. & Klimek, E.  2014, New Perspective on Galaxy Outflows from the First Detection of Both Intrinsic and Traverse Metal-line Absorption.  2014, ApJL, 792, L12 [abstract/selected figures] [ADS]


We are expanding our study from the cool phase of the CGM to the warm-hot phase.  See our HST/COS program here.

Resolving 3D Disk Orientation Using High-Resolution Images:
New Constraints on Circumgalactic Gas Inflows

Stephanie H. Ho and Crystal L. Martin
​Published 2019 Dec 30; The Astrophysical Journal, Volume 888, Issue 1, article id. 14, 11 pp.



We constrain gas inflow speeds in star-forming galaxies with color gradients consistent with inside-out disk growth.  Our method combines new measurements of disk orientation with previously described circumgalactic absorption in background quasar spectra.  Two quantities, a position angle and an axis ratio, describe the projected shape of each galactic disk on the sky, leaving an ambiguity about which side of the minor axis is tipped toward the observer.  This degeneracy regarding the 3D orientation of disks has compromised previous efforts to measure gas inflow speeds.  We present Hubble Space Telescope and Keck/LGSAO imaging that resolves the spiral structure in five galaxies at redshift z ≈ 0.2.  We determine the sign of the disk inclination for four galaxies, under the assumption that spiral arms trail the rotation.  We project models for both radial infall in the disk plane and circular orbits onto each quasar sightline.  We compare the resulting line-of-sight velocities to the observed velocity range of Mg II absorption in spectra of background quasars, which intersect the disk plane at radii between 69 and 115 kpc.  For two sightlines, we constrain the maximum radial inflow speeds as 30-40 km/s.   We also rule out a velocity component from radial inflow in one sightline, suggesting that the structures feeding gas to these growing disks do not have unity covering factor.  We recommend appropriate selection criteria for building larger samples of galaxy-quasar pairs that produce orientations sensitive to constraining inflow properties.

Kinematics of Circumgalactic Gas: Feeding Galaxies and Feedback

Crystal L. Martin, Stephanie H. Ho, Glenn G. Kacprzak, and Christopher W. Churchill

Published 2019 Jun 18; The Astrophysical Journal, Volume 878, Issue 2, article id. 84, 28 pp.



We present observations of 50 pairs of redshift z ≈ 0.2 star-forming galaxies and background quasars.  These sightlines probe the circumgalactic medium (CGM) out to half the virial radius, and we describe the circumgalactic gas kinematics relative to the reference frame defined by the galactic disks.  We detect halo gas in Mg II absorption, measure the equivalent-width-weighted Doppler shifts relative to each galaxy, and find that the CGM has a component of angular momentum that is aligned with the galactic disk.  No net counter-rotation of the CGM is detected within 45º of the major axis at any impact parameter.  The velocity offset of the circumgalactic gas correlates with the projected rotation speed in the disk plane out to disk radii of roughly 70 kpc.  We confirm previous claims that the Mg II absorption becomes stronger near the galactic minor axis, and we show that the equivalent width correlates with the velocity range of the absorption.  We cannot directly measure the location of any absorber along the sightline, but we explore the hypothesis that individual velocity components can be associated with gas orbiting in the disk plane or flowing radially outward in a conical outflow.  We conclude that centrifugal forces partially support the low-ionization gas and galactic outflows kinematically disturb the CGM producing excess absorption.  Our results firmly rule out schema for the inner CGM that lack rotation and suggest that angular momentum as well as galactic winds should be included in any viable model for the low-redshift CGM.

Quasar Probing Galaxies: I. Signatures of Gas Accretion at Redshift z ≈ 0.2

Stephanie H. Ho, Crystal L. Martin, Glenn G. Kacprzak, and Christopher W. Churchill
Published 2017 Feb 1; The Astrophysical Journal, Volume 835, Issue 2, article id. 267, 22 pp.



We describe the kinematics of circumgalactic gas near the galactic plane, combining new measurements of galaxy rotation curves and spectroscopy of background quasars.  The sightlines pass within 19-93 kpc of the target galaxy and generally detect Mg II absorption.  The Mg II Doppler shifts have the same sign as the galactic rotation, so the cold gas co-rotates with the galaxy.  Because the absorption spans a broader velocity range than disk rotation can explain, we explore simple models for the circumgalactic kinematics.  Gas spiraling inwards (near the disk plane) offers a successful description of the observations.  An Appendix describes the addition of tangential and radial gas flows and illustrates how the sign of the disk inclination produces testable differences in the projected line-of-sight velocity range.  This inflow interpretation implies that cold flow disks remain common down to redshift z ≈ 0.2 and prolong star formation by supplying gas to the disk.

New Perspective on Galaxy Outflows from the First Detection of Both Intrinsic and Traverse Metal-line Absorption

Glenn G. Kacprzak, Crystal L. Martin, Nicolas Bouché, Christopher W. Churchill, Jeff Cooke, Audrey LeReun, Ilane Schroetter, Stephanie H. Ho, and Elizabeth Klimek

Published 2014 August 18; The Astrophysical Journal Letters, Volume 792, Issue 1, article id. L12, 6 pp.



We present the first observation of a galaxy (z = 0.2) that exhibits metal-line absorption back-illuminated by the galaxy (down-the-barrel) and transversely by a background quasar at a projected distance of 58 kpc. Both absorption systems, traced by Mg II, are blueshifted relative to the galaxy systemic velocity.  The quasar sight line, which resides almost directly along the projected minor axis of the galaxy, probes Mg I and Mg II absorption obtained from the Keck/Low Resolution Imaging Spectrometer as well as Lyα, Si II, and Si III absorption obtained from the Hubble Space Telescope/Cosmic Origins Spectrograph.  For the first time, we combine two independent models used to quantify the outflow properties for down-the-barrel and transverse absorption.  We find that the modeled down-the-barrel deprojected outflow velocities range between V_dtb = 45-255 km/s. The transverse bi-conical outflow model, assuming constant-velocity flows perpendicular to the disk, requires wind velocities V outflow = 40-80 km/s to reproduce the transverse Mg II absorption kinematics, which is consistent with the range of V_dtb. The galaxy has a metallicity, derived from Hα and N II, of [O/H] = -0.21 ± 0.08, whereas the transverse absorption has [X/H] = -1.12 ± 0.02. The galaxy star formation rate is constrained between 4.6-15 M_sun/yr while the estimated outflow rate ranges between 1.6-4.2 M_sun/yr and yields a wind loading factor ranging between 0.1-0.9.  The galaxy and gas metallicities, the galaxy-quasar sight-line geometry, and the down-the-barrel and transverse modeled outflow velocities collectively suggest that the transverse gas originates from ongoing outflowing material from the galaxy.  The ~1 dex decrease in metallicity from the base of the outflow to the outer halo suggests metal dilution of the gas by the time it reached 58 kpc.


© 2019 by Stephanie Ho. (Last update: March 9, 2020)