Research

Exploring how galaxies evolve across cosmic time

Illuminating the Vast Cosmic Ecosystems of Galaxies

The OU CGM group studies the baryon cycle, or the flow of gas into and out of galaxies. Cosmological simulations show this cycling of gas is vital for regulating galaxy evolution and building up the mass in galaxies. We examine the properties and motions of the circumgalactic medium (CGM), a massive and diffuse reservoir of gas surrounding galaxies out to 100 times the size of their stellar disks.

By comparing the CGM to host galaxy properties (star formation activity, redshift, orientation, environment, etc.), we determine whether the extended gas is accreting from the intergalactic medium (IGM) or CGM (via recycled accretion), outflowing from star formation in the galaxy, or being tidally stripped from merging satellite galaxies and galaxy–galaxy interactions. This constrains the role that the baryon cycle plays in galaxy evolution and tests predictions from simulations.

Our research aims to:

  • Determine the spatial, geometric, and kinematic distribution of the CGM.
  • Use gas kinematics, metallicities, and geometry to quantify gas flow rates.
  • Determine how changes in galaxy star formation rate change gas flow rates and the CGM.
  • Connect the interstellar medium (ISM) to the CGM for a comprehensive view of how galaxies obtain and process their gas.
Comparison between the quasar absorption line and direct emission mapping methods

We study the CGM via two methods:

  1. Quasar absorption lines: the "traditional" one pencil-beam sightline per galaxy method.
  2. Direct emission mapping: equivalent to thousands of quasar sightlines through a single galaxy!

Quasar Absorption Lines

Quasar absorption line technique

We use light from a background quasar to pierce the CGM of a foreground galaxy with instruments like HIRES on Keck or UVES on the VLT. The resulting absorption signatures imprinted on the quasar spectra are sensitive to very diffuse gas and provide detailed gas properties such as the ionization balance, chemical content, density, temperature, and kinematics. Combining many galaxy–quasar pairs is required to statistically characterize the CGM. Up until recently, this was the primary method for studying the CGM.

See details below on some of our projects!

A cat wearing a magic hat and holding a magic wand

MAGIICAT: The MgII Absorber–Galaxy Catalog

MAGIICAT contains both MgII doublet absorption and galaxy data for 182 isolated absorber–galaxy pairs and 29 group environments. We have a series of papers that describe the sample and look at basic properties of the cool CGM. The data from all six series papers are also available to download.

The CGM at Cosmic Noon with KCWI

We are building a new sample at Cosmic Noon (z=2–3) to study gas flows when galaxies are most actively building their mass. The galaxies are identified with the Keck Cosmic Web Imager, KCWI (an integral field spectrograph), and we use spectrographs such as HIRES/Keck and UVES/VLT to analyze their CGM properties in as much detail as at lower redshifts.

We have published 2 papers with this program so far. In the first paper we modeled outflows along the minor axis of an isolated galaxy. AAS Nova highlighted this work in ''Seeing Star Formation at Cosmic Noon.''

Our second paper explored a highly complex absorption system associated with a compact group of galaxies. The galaxies give rise to metal-rich high velocity outflows and tidal streams. We also found evidence for a significant amount of metal-poor accretion onto the galaxies from the IGM.

A cat wearing a magic hat and holding a magic wand

Multiphase Galaxy Halos Survey

With this survey, we study low redshift (z < 1) absorber–galaxy pairs in OVI (tracing the warm-hot CGM) and other multiphase absorption. These data were obtained through a Large program on the Hubble Space Telescope with the Cosmic Origins Spectrograph, COS.

We examined how CGM metallicity varies with location around galaxies. We expected to find pristine (low metallicity) gas accreting along galaxy disks and enriched (high metallicity) gas outflowing perpendicular to galaxies. Our work showed that our simple picture of CGM gas flows is much more complicated, where also accounting for the galaxy's ISM metallicity does not yet help disentangle inflows from outflows alone.

Most recently we explored whether and how the multiphase absorption aligns with the rotation of galaxy disks. Neutral hydrogen persistently co-rotates with galaxy disks to large distances within the CGM. As the ionization of the gas increases, gas co-rotates less strongly with the galaxy. Lower ionization, lower temperature, and more dense gas is more clearly an important tracer of gas accreting from the intergalactic medium, adding angular momentum and fresh star-forming material to galaxy disks.

Direct Emission Mapping

Purpe diffuse CGM, blue accretion, and white outflows surrounding a star-forming galaxy

With this method, we take an image of the CGM in various emission lines around a single galaxy, effectively probing the galaxy with thousands of sightlines with a broad spatial distribution. This has only recently become possible with new instruments like the Keck Cosmic Web Imager (KCWI) and MUSE. These images can spatially resolve the gas out to several tens of kiloparsecs from nearby galaxies and obtain information about the distribution and ionization conditions of the more dense gas in the CGM.

Ultimately, we hope to create real images of gas surrounding galaxies like the illustration on the right!

DUVET: We've got you covered!

We are members of DUVET (''Deep near-UV observations of Entrained gas in Turbulent galaxies''), whose aim is to test star formation theories, map multiphase outflows and trace the transfer of gas between the ISM and the CGM with KCWI. We target low redshift galaxies that are analogs of z=1–3 star-forming galaxies and link outflow properties to local star formation rates, star formation efficiencies, gas mass surface densities, and pressures. The outflows we measure in these galaxies are key to understanding the outflows we measure in the CGM at Cosmic Noon.

Looking down onto a bed with colorful gas flows as the blanket
A bed from the side with colorful flows of gas above and below
A bedroom with colorful gas exploding around
An edge-on galaxy with outflowing gas above and below the stellar disk

Gas flows in and very near to face-on and edge-on galaxies in DUVET

So far we have measured outflows across the entire disk of the nearly face-on pilot DUVET galaxy, IRAS08. From the larger DUVET face-on sample, we find that the outflows are common and are driven by energetic supernovae from the starbursts in each galaxy.

We also traced the full baryon cycle in a single very edge-on galaxy, Mrk 1486, using direct metallicity measurements, with metal-poor accretion along the major axis and metal-rich outflows along the minor axis. This was highlighted in the press release "Research reveals how star-making pollutes the cosmos." Most recently we used a new method to characterize the shape and flow rate of the large-scale outflow being driven from this galaxy out to nearly 10 kpc.

These methods for tracing the gas flows around both face-on and edge-on starbursting galaxies are currently being applied to the rest of the DUVET sample (Papers!).

Oxygen emission map surrounding a nearly face-on starbursting galaxy.

Beyond DUVET: Mapping extended gaseous environments

At OU, we are leading the more extended CGM ''bonus science'' for DUVET, where we have directly imaged the gas around IRAS08 out to 30 kpc (10 times the size of the stellar disk!) with sub-kiloparsec spatial resolution. We found emission everywhere we observed in both oxygen (traced by [OII] and [OIII]) and hydrogen (traced by Hβ), which probe low ionization gas that is likely infalling onto the galaxy or outflowing from star formation.

Most excitingly, we revealed the transition between the gas in the galaxy disk and the gas in the CGM in both the surface brightness and ionization conditions. This has important implications for how galaxies interact with their gaseous surroundings. Future observations and analyses will determine if this transition is a common feature of galaxies or a special case for our first DUVET galaxy!

Check out our press releases, recorded interviews, and news articles about this galaxy!