Variability-selected intermediate mass black hole candidates in dwarf galaxies from ZTF and WISE

Abstract

While it is difficult to observe the first black hole seeds in the early Universe, we can study intermediate mass black holes (IMBHs) in local dwarf galaxies for clues about their origins. In this paper we present a sample of variability-selected AGN in a parent sample of dwarf galaxies using optical photometry from the Zwicky Transient Facility (ZTF) and forward-modeled mid-IR photometry of time-resolved Wide-field Infrared Survey Explorer (WISE) coadded images. We found that 44 out of 25,714 dwarf galaxies had optically variable AGN candidates, and 158 out of 79,879 dwarf galaxies had mid-IR variable AGN candidates, corresponding to active fractions of 0.17 ± 0.03% and 0.20 ± 0.02% respectively. Only two objects, NSA164884 and NSA451469, broad line AGN with virial masses M BH = 10 6.9 M and M BH = 10 6.3 M respectively, were found in both optical and mid-IR searches. We find that spectroscopic approaches to AGN identification would have missed 81% of our ZTF IMBH candidates and 69% of our WISE IMBH candidates. Only 9 candidates have been detected previously in radio, X-ray, and variability searches for dwarf galaxy AGN. The IMBHs with broad Balmer lines have virial masses of 10 5 M < M BH < 10 7 M but for the rest of the sample, BH masses predicted from host galaxy mass range between 10 4.8 M < M BH < 10 7 M . We found that only 5 of 152 previously reported variability-selected AGN candidates from the Palomar Transient Factory in common with our parent sample were variable in ZTF. We determined that the characteristic variability timescales of the IMBH candidates were significantly shorter than a control sample of AGN in higher mass galaxies, as expected if they scale with the black hole mass. We also determined a nuclear supernova fraction of 0.05 ± 0.01% year −1 for dwarf galaxies in ZTF. Our ZTF and WISE IMBH candidates show the promise of variability searches for discovery of otherwise hidden low mass AGN in preparation for the Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory.

Dwarf AGNs from Optical Variability for the Origins of Seeds (DAVOS): insights from the dark energy survey deep fields

“…The typical (mean) uncertainties in stellar mass and redshift are shown at the bottom of the plot. For comparison, we show the variability-selected dwarf AGNs from SDSS/PTF (blue dots; Baldassare et al 2018Baldassare et al , 2020 and ZTF imaging (orange dots; Ward et al 2021a ). Our deep-field variability-selected AGNs are at relatively lower mass at a given redshift than previous optically selected AGNs.…”

“…The selection rates are expected to depend on the sensitivity/photometric precision of the surv e y and the exact selection criteria used (Burke et al 2022 ). Ho we ver, using v ariability as a complementary AGN selection technique to identify dwarf AGNs is a relatively new technique (Baldassare, Geha & Greene 2018 ;Baldassare, Geha & Greene 2020 ;Guo et al 2020 ;Mart ínez-Palomera et al 2020 ;Ward et al 2021a ) In this work, we perform a systematic search for variable AGN using Dark Energy Surv e y (DES; Dark Energy Surv e y Collaboration 2016 ) deep field imaging (Hartley et al 2022 ). We choose the DES deep fields because of the exceptional depth ( g ∼ 24.6), ∼7 d cadence with a total baseline of ∼6 yr, and availability of multiwavelength imaging and spectroscopy.…”

Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material

“…With the time-domain surveys made accessible by the WISE/NEOWISE mission and groundbased JHK surveys, more authors have explored use of IR variability to find AGNs [e.g., 17,317]. Recently, Elmer et al [65] explored high-z AGN selection with long-term near-IR variability using archival UKIDSS data.…”

Detecting intermediate mass black holes in midiquasars with current and future surveys

“…This allows us to exclude already known sources such as low luminosity stars, high-z quasars, and galaxies. Dwarf galaxies with AGN activity have been found to show atypical spectral features than higher mass AGN (Baldassare et al 2020;Ward et al 2021). Therefore, our criteria allow us to produce a sample 789,372 "unidentified/unclassified" sources.…”