arXiv subjects

John-Michael Eberhard

Publications and source records attributed to John-Michael Eberhard.

3 recordsLinked to original sources

arXiv2026

Dwarf Galaxies Hosting Extreme Star-Forming Regions and (Variable) AGNs at Radio Wavelengths

We present a detailed study of radio-detected dwarf galaxies (with stellar masses less than 3 billion solar masses) to characterize extreme star formation and search for (variable) radio AGNs. Our sample comes from Reines et al. (2020) (arXiv:1909.04670), who used the Karl G. Jansky Very Large Array (VLA) with 0.25 arcsecond resolution to observe 111 dwarf galaxies with lower-resolution (5 arcsecond) detections in the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey. While that work identified and focused on 13 compact radio AGN candidates in dwarf galaxies, here we focus on 16 compact radio sources consistent with star formation in dwarf galaxies. We find that these objects are dominated by thermal HII regions with ages less than 10 Myr, and the most extreme sources have ionizing luminosities requiring the equivalent of around 10,000 to 100,000 O-type stars. We also investigate the dwarf galaxies detected in FIRST but not detected in the high-resolution follow-up observations. Using the infrared-radio correlation parameter, we identify eight sources consistent with radio-excess AGNs. Five of these objects plus another 15 dwarf galaxies have no corresponding detections in the VLA Sky Survey (VLASS) indicating variability between the FIRST and VLASS observations. The FIRST radio luminosities of these sources are significantly higher than expected for supernova-related emission, suggesting the radio variability is likely associated with AGNs. Together, these results provide new context for the presence of compact star formation and massive black holes in dwarf galaxies, and highlight the utility of radio variability and multi-resolution data for identifying the dominant power sources in low-mass systems.

astro-ph.GA↗

arXiv2025

A New Sample of Dwarf Galaxies with X-ray-Selected AGN Candidates from the eROSITA All-Sky Survey

To investigate the population of massive black holes in dwarf galaxies, we conduct a systematic search for active galactic nuclei (AGNs) using data from the first data release of the eROSITA All-Sky Survey (eRASS1). We crossmatch dwarf galaxy positions in the NASA-Sloan Atlas with X-ray sources from eRASS and apply rigorous criteria to remove contaminants such as background sources, galaxies with dubious stellar masses, and X-ray binaries. Potential contamination from ultraluminous X-ray sources is also assessed. We ultimately assemble a sample of 27 X-ray-selected AGN candidates in dwarf galaxies with stellar masses $M_{*} = 10^{7.5-9.5} M_{\odot}$ and redshifts $z<0.15$. An analysis of the host galaxy properties reveals that the AGN candidates are predominantly situated in systems with $g-r$ colors, star formation rates, and concentrations typical of the broader dwarf galaxy population. Detailed examination of the X-ray sources indicates that most AGN candidates in our sample are consistent with being located in the nuclear regions of their host galaxies and exhibit Eddington ratios ranging between $λ_{Edd} \sim 10^{-3}-10^{-1}$, with a few radiating at/above their Eddington limit. Notably, our methodology identifies 15 previously unreported AGNs candidates, highlighting the complementary nature of our approach to the existing literature.

astro-ph.GA↗

arXiv2024

Dwarf Galaxies with Radio-excess AGNs in the VLA Sky Survey

We present a systematic search for radio active galactic nuclei (AGNs) in dwarf galaxies using recent observations taken by the Very Large Array Sky Survey (VLASS). To select these objects, we first establish a criterion to identify radio-excess AGNs using the infrared-radio correlation (IRRC) parameter, $q$, that describes the tight relation between radio and IR emission in star forming (SF) galaxies. We find a $2σ$ threshold of $q < 1.94$ to select radio-excess AGNs, which is derived from a sample of $\sim 7,000$ galaxies across the full mass range in the NASA-Sloan Atlas (NSA) that have radio and IR detections from VLASS and the Wide-Field Infrared Survey Explorer, respectively. We create catalogs of radio-excess AGNs and SF galaxies and make these available to the community. Applying our criterion to dwarf galaxies with stellar masses $M_\star \lesssim 3 \times 10^9 M_\odot$ and redshifts $z \le 0.15$, and carefully removing interlopers, we find 10 radio-excess AGNs with radio-optical positional offsets between $\sim$ 0 and 2.3 arcseconds (0 - 2.7 kpc). Based on statistical arguments and emission line diagnostics, we expect the majority of these radio-excess AGNs to be associated with the dwarf host galaxies rather than background AGNs. Five of the objects have evidence for hosting AGNs at other wavelengths, and 5 objects are identified as AGNs in dwarf galaxies for the first time. We also identify 8 variable radio sources in dwarf galaxies by comparing the VLASS epoch 1 and epoch 2 observations to FIRST detections presented in arXiv:1909.04670.

astro-ph.GA↗