Research

My primary areas of research are focused on aspects of active galactic nuclei (AGN) and galaxy evolution. Below I describe some of my current projects in more detail:

Heavily Obscured AGN:
The growth rates of supermassive black holes (SMBHs) are known to correlate with many properties of their host galaxies, including star formation rates. However, much of this growth may occur in heavily obscured phases due to the high densities of gas and dust that surround AGN. I am leading a project to identify these AGN on a large scale using the Wide-field Infrared Survey Explorer (WISE). This project has produced the WISE AGN Host Galaxy Catalog, a catalog of photometric redshifts, AGN properties, and host galaxy properties for ~695,000 WISE-selected AGN throughout the sky (Barrows et al. 2021).

Sky map of the WISE AGN Catalog from Assef et al. 2018 (top) and the catalog of host galaxy properties for a subset of ~695,000 of them.

Spatially Offset AGN:
AGN are powered by accretion of interstellar medium (ISM) material onto massive black holes, but this process requires the material to lose angular momentum. One process by which this loss can occur is through mergers of galaxies, though it can also occur efficiently in non-interacting galaxies. The dominant process(es) of AGN triggering are unclear and may depend on properties of the host galaxy and AGN itself. To constrain the conditions under which AGN triggering is likely to occur, we have compiled a sample of galaxy mergers identified from the X-ray signature of an AGN that is spatially offset from a galaxy nucleus (Barrows et al. 2016).

Examples from the offset AGN sample of (Barrows et al. 2016). In all panels, North is up and East is to the left. For reference to the galactic environment, the far left panels show a 40x40 kpc field of the SDSS g+r+i composite image centered on the SDSS AGN coordinates. The red box is 8x8" on a side, also centered on the SDSS AGN coordinates, and denotes the field-of-view of the three right-most panels.

Identifying Dual AGN Spectroscopically:
The region of the ISM close enough to be ionized by the AGN accretion disk radiation, but not within the SMBH's sphere of influence, moves more slowly and produces much narrower emission lines; it is called the narrow line region (NLR). The NLR traces AGN-ionized gas on a much larger scale (hundreds to thousands of parsecs) and therefore be can used to identify AGN even for unfavorable accretion disk orientations. I have used measurements of these lines to look for large scale orbital motion on the scale of the NLR, e.g. the earlier stages of a galaxy merger, often called the 'Dual AGN' stage (Comerford et al. 2012; Barrows et al. 2012; Barrows et al. 2013).

NLR spectra of a candidate dual AGN from Barrows et al. (2012). Left: Keck/LRIS spectrum of a double-peaked narrow line AGN at z=1.175 showing various emission lines with double components. Right: Spatially-integrated spectrum of (from left to right) the [NeIII], [OII] and [NeV] emission lines as would be seen through a 1D fiber spectrum.

NLR spectra of a candidate dual AGN from Barrows et al. (2012). Left: Keck/LRIS spectrum of a double-peaked narrow line AGN at z=1.175 showing various emission lines with double components. Right: Spatially-integrated spectrum of (from left to right) the [NeIII], [OII] and [NeV] emission lines as would be seen through a 1D fiber spectrum.

These systems can actually be spatially resolved by current instruments and have the potential to be an excellent observational constraint on the merger rate of galaxies and a predictor of the number of close, SMBH binaries expected to exist. Since dual AGN can be spatially resolved, they can also be discovered through imaging surveys. I am currently involved in several multi-wavelength studies designed to locate these systems (Comerford et al. 2015; Mueller-Sanchez et al. 2015).

Binary Supermassive Black Holes:
One method of estimating SMBH masses is if they are oriented in such a way that we see down to the very central engine. In this case, very (Doppler-)broadened emission lines are evident in their spectra (Assef et al 2011). I am interested in these lines because they can be used to study the region of broad line emitting gas, called the broad line region (BLR) and sometimes even the accretion disk itself. Deviations from single-peaked, symmetric line profiles can provide clues about the distribution of gas near the SMBH, including any disturbances which may exist. I have been involved in investigations of what physical mechanisms may be causing such disturbances: tidal disruptions of stars, hot-spots or spiral arms in the accretion disk, or the presence of a secondary, orbiting SMBH as the result of a merger with another galaxy (Barrows et al. 2011 ;Tsai et al. 2013). These binaries will have separations of less than 1 pc, comparable to the size of the BLR itself.

Hbeta (left) and Halpha (right) emission profiles for a candidate binary SMBH from Tsai et al. (2013).

Spiral Galaxies:
Late-type, disk-dominated galaxies very often exhibit distinct spiral structure which extends all the way to the outer edges of the disk. While such galaxy-wide scales are far beyond the central SMBH's gravitational sphere of influence, features of disk galaxies are well-correlated with the central SMBH mass. This has been, in a general way, inferred from the Hubble Sequence, and I have been involved in studies to quantify this by using a measure of the 'tightness' of spiral arms (Berrier et al. 2012).

Ultimately , I am interested in how these very different phenomena, on physical scales separated by orders of magnitude, are linked - either directly or indirectly - to the mass and growth rate of the central SMBH. Understanding the reasons for these observed connections will shed light on the ways that black holes and galaxies co-evolve.