AGN Pairs

The majority of my work focuses on finding pairs of supermassive black holes, and learning more about their preferential environments and evolution through mergers. I connect the X-ray activity of merging supermassive black holes with the optical and IR properties of their host galaxies. Below I highlight some recent results.

Investigating the Accretion Nature of Binary Supermassive Black Hole Candidate SDSS J025214.67-002813.7

SDSS J025214.67−002813.7, is a system that has been previously classified as a binary active galactic nucleus (AGN) candidate based on periodic signals detected in the optical light curves. Using available radio−X-ray observations of the system, we investigated whether it was likely a merging SMBH system. Analyzing new observations from XMM-Newton and NuSTAR, and compiling a full multiwavelength spectral energy distribution SED, we also search for signs of circumbinary accretion, such as a “notch” in the continuum due to the presence of minidisks. We found that the radio–X-ray emission agrees with the SED of a standard, radio-quiet, AGN. However, future studies of the expected hard X-ray emission associated with binary AGNs (especially in the unequal-mass regime) will allow for more rigorous analyses of the binary AGN hypothesis.

Read more here: Foord et al. 2022

AGN Triality in Triple Mergers: Faint X-ray Sources and Multi-wavelength Classifications

The overarching goal on this analysis was to identify new triple AGN, which have been theorized to play an important role in the timescales for merging supermassive black holes. However, we hoped to find a range of AGN activity (from 0 AGN, up to 3), in order to measure differences, and possible links, between SMBH triggering and environmental parameters. We found that 1 triple merger has a single AGN; we discovered, for the first time, 4 likely dual AGN; and we confirmed one triple AGN system. Combining the archival Chandra observations with existing SDSS, WISE, and VLA observations we found a trend of increasing levels of gas as a function of increasing dust, reflecting that the motions of gas and dust are coupled in merging environments, where large amounts of both can be funneled into the active central region. Additionally, the one triple AGN system in our sample has the highest levels of gas and dust, while the dual AGN candidates all have lower levels; these results are consistent with theoretical merger simulations that suggest higher levels of nuclear gas are more likely to active AGN in mergers.

Read more here: Foord et al. 2021a and here: Foord et al. 2021b

Four new strong dual AGN candidates discovered in Foord et al. 2021a. I plot the unbinned Chandra data, where counts associated with the primary AGN (yellow circles), secondary AGN (purple squares), and background (triangles). The optical contours of the host galaxy are shown in pink contours.

Using avaible mid-IR observations from WISE, Foord et al. 2021b found a trend of increasing levels of dust (y-axis) as a function of increasing gas levels (as determined by fitting the X-ray spectrum via the available Chandra data, x-axis). Next to each marker I denote how many X-ray AGN were detected in each system. Interestingly, the one triple X-ray AGN system had higher levels of gas and dust than the dual AGN.

Analyzing X-ray Observations of 12 Optically Selected Dual AGN Candidates

BAYMAX was used to analyze Chandra observations of 12 candidate dual AGNs, which were originally identified via double-peaked [O III] λ5007 emission lines in their SDSS spectra. Because the X-ray data are low-count (<100 counts) with small separations (<1"), BAYMAX is necessary to determine whether the X-ray emission from each system is more likely a single or dual point source. We found that 4 of the 12 sources are likely dual X-ray point source systems, and by analyzing each point source's spectra via a Monte Carlo method that probabilistically identifies the likely origin of each photon, we found that one system (SDSS J1126+2944) is a dual AGN. Additionally, using complementary HST observations, we found that dual AGNs may preferentially reside in mergers with small separations, consistent with both simulations and observations. Read more here: Foord et al. 2020a.

Left: Unbinned Chandra data of confirmed dual AGN, SDSS J1126+2944. Filled circles and open-faced squares denote counts associated with the primary and secondary AGN. HST contours overplotted. Right: Joint posterior distribution for separation, r, and count ratio, f, between the primary and secondary AGN.

Levels of gas in the vicinity of the X-ray point sources in each merging system (NH), versus physical separation between each galactic nucleus. Those mergers which BAYMAX found two (one) X-ray point sources are plotted in squares (diamonds). Values of NH using different spectral models are shown red and blue. For one spectral model (red), we find evidence that dual AGN may prefer gas rich and closely separated mergers -- in agreement with theoretical predictions.

SDSS J0914+0853: A Low-mass Dual AGN Candidate

SDSS J0914+0853 is the lowest-mass dual AGN candidate to date, where archival Chandra data shows a possible secondary AGN ~0.3" from the primary. Analyzing a new 50 ks Chandra observation, results from BAYMAX shows that SDSS J0914+0853 is most likely a single AGN with a Bayes factor of 13.5 in favor of a single point source model. Further, posterior distributions from the dual point source model are consistent with emission from a single AGN. Statistical analyses with BAYMAX will be important for robust analyses on all dual AGN candidates! Read more here: Foord et al. 2019

BAYMAX favors the single point source model, and best-fit values for a possible separation and flux ratio are both consistent with zero.

Investigating the Accretion Nature of Binary AGN Candidate PSO J334

PSO J334.2028+01.4075 (PSO J334) has been targeted as a binary AGN candidate based on periodic variations of the optical flux. I carried out a full multi-wavelength analyses of the candidate binary AGN, including an analysis of the first Chandra observation, in order to characterize the mode of accretion. On the multi-wavelength fronts investigated, we found no evidence supporting PSO J334 as a binary-AGN system. I plan to get follow-up spectroscopic data to better understand the AGN's rest-frame optical emission. Read more here: Foord et al. 2017b.

The multi-wavelength SED of PSO J334.