Research

Observations have shown that every massive galaxy (i.e. the size of our Milky Way and larger) contains a super-massive black hole at the center. However, the population of massive black holes at the centers of dwarf galaxies remains relatively elusive. By searching for and studying black holes in the smallest galaxies, I hope to better understand massive black hole formation and growth over cosmic time. Find out more about my projects below. 

 
 

Optical variability as a tool for selecting AGNs in low-mass galaxies

 BPT diagram showing the positions of 121 objects with AGN-like variability. Points are colored by stellar mass, and objects with broad H-alpha emission are circled. The lowest-mass objects in the sample have narrow emission line ratios dominated by star formation. 

BPT diagram showing the positions of 121 objects with AGN-like variability. Points are colored by stellar mass, and objects with broad H-alpha emission are circled. The lowest-mass objects in the sample have narrow emission line ratios dominated by star formation. 

While optical spectroscopy has been very successful at identifying AGNs in low-mass galaxies, we may be missing a significant number of these systems due to star formation dilution and metallicity effects. Motivated by the potential for identifying AGNs in low-mass galaxies missed by other selection techniques, I recently carried out a search for AGNs in low-mass galaxies via low-level optical variability.

Using difference imaging of SDSS g-band images, We constructed light curves of ~28,000 nearby (z < 0.15) galaxies with SDSS spectroscopy in Stripe 82.  We selected variable AGN by assessing whether detected variability is well-described by a damped random walk model. We found 135 galaxies with AGN-like nuclear variability. As shown in the figure to the left, while most of the variability-selected AGNs have narrow emission lines consistent with the presence of an AGN, a small fraction have narrow emission lines dominated by star formation. Those with star formation-dominated narrow lines are also the lowest mass galaxies in the sample, demonstrating that this may be a promising avenue for detecting AGNs in low-mass systems dominated by star formation. 

While the sensitivity of SDSS imaging means that we are only sensitive down to BHs of roughly one million solar masses, the upcoming Large Synoptic Space Telescope should be able to detect subtle variations in light output from lower-mass BHs accreting at lower Eddington fractions.  


Multi-wavelength properties of active galactic nuclei in dwarf galaxies

 RGG 119 has broad H-alpha emission observed in several epochs of spectroscopy, and narrow line ratios consistent with photo-ionization from an AGN. Its black hole is 300,000 times the mass of the Sun. (Figure from Baldassare et al. 2016)

RGG 119 has broad H-alpha emission observed in several epochs of spectroscopy, and narrow line ratios consistent with photo-ionization from an AGN. Its black hole is 300,000 times the mass of the Sun. (Figure from Baldassare et al. 2016)

Active galactic nuclei in dwarf galaxies comprise a new class of AGN that are relatively unexplored. I'm analyzing high resolution optical spectroscopy, X-ray imaging, and UV imaging of dwarf galaxies with candidate broad-line AGN to determine the origin of the broad emission, and investigate the accretion properties of confirmed AGN. Some of my key results are: 

  • Broad H-alpha emission in star forming dwarf galaxies is likely due to transient stellar processes (i.e., supernovae).
  • Broad emission in dwarf galaxies with narrow emission lines supporting the presence of an AGN is likely emitted from dense gas orbiting around the black hole.
  • Dwarf galaxies with optical broad and narrow emission line AGN signatures are all X-ray detected, at luminosities higher than would be expected from X-ray binaries.  
  • Inferred Eddington fractions for broad line AGN in dwarf galaxies range from 0.1-50%. 

A 50,000 solar mass black hole in a dwarf spiral galaxy

 RGG 118 contains the smallest super-massive black hole yet reported.&nbsp;Credit: X-ray: NASA/CXC/Univ of Michigan/V.F.Baldassare, et al; Optical: SDSS; Illustration: NASA/CXC/M.Weiss

RGG 118 contains the smallest super-massive black hole yet reported. Credit: X-ray: NASA/CXC/Univ of Michigan/V.F.Baldassare, et al; Optical: SDSS; Illustration: NASA/CXC/M.Weiss

I led the analysis leading to the discovery of a 50,000 solar mass black hole in the dwarf galaxy RGG 118: the smallest yet reported. This object was first identified as an AGN candidate by Reines et al. (2013) based on the relative strengths of its photo-ionized emission lines. A follow-up spectrum taken with the MagE spectrograph on the Clay Telescope clearly revealed broad hydrogen emission, which we used to measure the mass of the black hole. Chandra X-ray observations showed a nuclear X-ray point source with a luminosity of ~1% the Eddington luminosity for a 50,000 solar mass black hole. Furthermore, this system sits on the relation between black hole mass and host stellar velocity dispersion, extrapolated to low galaxy/black hole masses. 

More recently, I analyzed Hubble Space Telescope WFC3 imaging of RGG 118 (Cycle 23; PI Baldassare). 


Nuclear star clusters in early-type galaxies

 Elliptical galaxy NGC 1331 contains a compact and massive cluster of stars at its center.

Elliptical galaxy NGC 1331 contains a compact and massive cluster of stars at its center.

Dense nuclear star clusters are often found at the centers of low-mass galaxies, but there are many open questions relating to their formation. Using Hubble Space Telescope images, I modeled the two-dimensional light profiles of a sample of 22 early-type Field galaxies to identify and characterize nuclear star clusters. I then compared the fraction of Field early-type galaxies hosting NSCs to that which was found for early-type Virgo Cluster members, and found that there was no statistically significant difference in the fraction of galaxies hosting a NSC between the two environments (after controlling for the mass of the host galaxy). This suggests that the formation of a NSC is not strongly dependent on galaxy environment.