Dr. Nikole M. Nielsen

My research focuses on studying the baryon cycle, or the flow of gas into and out of galaxies. Cosmological simulations have shown it is vital for regulating galaxy evolution and building up the mass in galaxies. To do this, I examine the properties and motions of gas in the circumgalactic medium (CGM), a massive and diffuse reservoir of gas surrounding galaxies.

By comparing the CGM to the host galaxy properties (color, redshift, orientation, etc.), I determine whether the gas is likely accreting from the intergalactic medium or the CGM (recycled accretion), outflowing from star formation in the galaxy, or being tidally stripped from merging satellite galaxies. This constrains the role that the baryon cycle plays in galaxy evolution and to test predictions from the simulations.

My research generally 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 ISM to the CGM for a comprehensive view of how galaxies obtain and process their gas.

I study the CGM via two methods:
– Direct emission mapping takes an image of the CGM in various emission lines around a single galaxy. 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.
– Quasar absorption lines 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 absorption signatures 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 my primary method for studying the CGM.

I am a leading member of DUVET, whose aim is to test star formation theories, map multiphase outflows, and trace the transfer of gas between the interstellar medium (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 will be key to understanding the outflows we measure in the CGM at Cosmic Noon.

So far we have measured outflows across the entire disk of the pilot DUVET galaxy, similar to the artist's illustration above left. We also traced the full baryon cycle in a single 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.

I am leading the more extended CGM "bonus science" for DUVET, where we have imaged the gas around our pilot DUVET target out to 30 kpc with sub-kiloparsec spatial resolution. We found emission everywhere we observed in three emission lines: [OII], Hβ, and [OIII]. Most excitingly, we revealed the transition between the gas in the disk and the gas in the CGM in both the surface brightness and ionization conditions. Future observations will determine if this transition is a common feature of galaxies or a special case for our first DUVET galaxy!

An illustration of several components of the circumgalactic medium of IRAS08: accreting gas (blue), outflowing gas (white), and widespread diffuse gas (purple).

I compiled the MgII Absorber–Galaxy Catalog, or MAGIICAT, which contains both MgII doublet absorption and galaxy data for 182 isolated absorber–galaxy pairs and 29 group environments. We have a series of papers which describe MAGIICAT and look at basic properties of the cool CGM.

I am building a new sample at Cosmic Noon (z = 2–3) to study gas flows when galaxies are most actively building their mass. We are using the integral field spectrograph KCWI to identify galaxies at this epoch and HIRES/UVES to analyze their CGM properties in as much detail as at lower redshifts. We have published 2 papers with this program so far, where 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 highlighted 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 intergalactic medium (IGM).

I am also a leading member of the Multiphase Galaxy Halos Survey, which studies low redshift (z < 1) absorber–galaxy pairs with OVI absorption. These data come from a Large program on the Hubble Space Telescope with the Cosmic Origins Spectrograph. Most recently, we examined how the CGM metallicity varies with location about 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 the picture is much more complicated.

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