Abstract
Abstract Determining the fraction of nearby dwarf galaxies hosting massive black holes (BHs) can inform our understanding of the origin of “seed” BHs at high redshift. Here we search for signatures of accreting massive BHs in a sample of dwarf galaxies (M ⋆ ≤ 3 × 109 M ⊙, z ≤ 0.15) selected from the NASA-Sloan Atlas (NSA) using X-ray observations from the eROSITA Final Equatorial Depth Survey (eFEDS). On average, our search is sensitive to active galactic nuclei (AGNs) in dwarf galaxies that are accreting at ≳1% of their Eddington luminosity. Of the ∼28,000 X-ray sources in eFEDS and the 495 dwarf galaxies in the NSA within the eFEDS footprint, we find six galaxies hosting possible active massive BHs. If the X-ray sources are indeed associated with the dwarf galaxies, the X-ray emission is above that expected from star formation, with X-ray source luminosities of L 0.5–8 keV ∼ 1039–40 erg s−1. Additionally, after accounting for chance alignments of background AGNs with dwarf galaxies, we estimate there are between zero and nine real associations between dwarf galaxies and X-ray sources in the eFEDS field at the 95% confidence level. From this we find an upper limit on the eFEDS-detected dwarf galaxy AGN fraction of ≤1.8%, which is broadly consistent with similar studies at other wavelengths. We extrapolate these findings from the eFEDS sky coverage to the planned eROSITA All-Sky Survey and estimate that upon completion, the all-sky survey could yield as many as ∼1350 AGN candidates in dwarf galaxies at low redshift.
Highlights
Massive black holes (BHs) with MBH ∼ 106−9 M reside at the center of most massive galaxies
Following the same general method as in Kovács et al (2020), we start with our estimation of how many (NSA-detected) dwarf galaxies lie in the eROSITA Final Equatorial Depth Survey (eFEDS) footprint: 495 galaxies
From matching dwarf galaxies in the NASA-Sloan Atlas (NSA) to X-ray sources in the recently released eFEDS catalog, and accounting for chance alignments with background AGNs, we find that there could be between 0-9 real associations between dwarf galaxies and X-ray sources
Summary
Massive black holes (BHs) with MBH ∼ 106−9 M reside at the center of most massive galaxies. Previous X-ray studies have been used to explore the BH occupation fraction in low mass early-type spheroidal galaxies and late-type spirals, as well as to search for AGNs in the general population of dwarf galaxies. Miller et al (2015) used Chandra X-ray data from the AMUSE1 -Virgo (Gallo et al 2008; Gallo et al.2010) and AMUSE-Field (Miller et al 2012) surveys to constrain the BH occupation fraction to be > 20% for early-type galaxies with M? EFEDS has a sky footprint of ∼140 deg (1/300th of the sky), which is broken up into four ∼35 deg rectangular, semi-adjacent sub-fields of 4.2◦ ×7.0◦ (for a visualization, see Figure 1 in Brunner et al 2021) This observational strategy was designed to provide uniform exposure over this field and to be. Compare the eFEDS source fluxes to those of sources in the same footprint appearing in the XMM-ATLAS survey (Ranalli et al 2015), and find no evidence for strong systematic offsets
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