The Intermediate Mass Black Hole in Omega Centauri: Constraints on Accretion from JWST

TL;DR

The paper investigates the presence of an intermediate-mass black hole in Omega Centauri using JWST infrared observations cross-matched to the HST-based oMEGACat catalog. By constructing UV-to-IR SEDs and comparing with the P+21 radiative accretion model, the authors translate non-detections into constraints on the black hole mass $M_{\rm BH}$ and the accretion parameter $\dot{m}_{0}$, under plausible intracluster medium conditions. They find no IR counterpart consistent with an isolated IMBH, and show that JWST limits are particularly constraining for $M_{\rm BH} \lesssim 2\times10^{4}\,M_{\odot}$ given reasonable $\dot{m}_{0}$ and gas densities, while larger masses or different gas properties can weaken the constraints. The results are complementary to dynamical and radio/X-ray limits and emphasize the value of deeper JWST observations and precise center/medium characterization for tightening IMBH constraints in crowded GC centers.

Abstract

We analyze JWST observations of the central region of the globular cluster $ω$ Centauri (NGC 5139, $ω$ Cen hereafter), around the position of the candidate IMBH inferred by Haberle et al. (2024a) from the motion of fast-moving stars in multi-epoch HST observations. We performed PSF-fitting photometry for sources in NIRCam (F200W and F444W) and MIRI (F770W and F1500W) and constructed UV to IR SEDs for sources within the central region of the cluster by using HST photometry from oMEGACat (Haberle et al. 2024b). None of the SEDs of reliably measured sources within this region resembles the SEDs computed from models of Pesce et al. 2021 for IMBHs accreting from intracluster medium at low rates. Our JWST limits place constraints on combinations of IMBH mass and accretion efficiency, either due to the amount of material available to be accreted, or due to the fraction of accreting matter that actually falls into the IMBH. Our non-detection then does not contradict the mass range of the IMBH inferred from the fast moving stars. We discuss these constraints in the context of the model of Pesce et al. 2021. We find that JWST limits are more restrictive than the existing radio limits for IMBH masses $\lesssim 20,000 M_{\odot}$. It is also possible that the faint IMBH emission is dominated by the light of a nearby star. Tighter limits on accretion onto the candidate IMBH can be placed with deeper observations, a more precise localization of the IMBH, and better measurements of the local intracluster medium density and temperature at the center of the cluster.

Figures (8)

• Figure 1: Distribution of the separation to the nearest match between PM-propagated oMEGACat and NIRCam, and NIRCam and MIRI sources.
• Figure 2: False color images of the IMBH RoI, centered on the same position (after PM propagation) as Figure 1 from H+24. Fast moving stars are labeled as in H+24, with their oMEGACat to NIRCam PMs, scaled to 120 years, shown with lines. The red and blue colors correspond to NIRCam F444W and F200W images in the top left panel, and to MIRI F1500W and F770W images in the top right panel. The RGB colors correspond to MIRI F770W, NIRCam F200W, and HST F814W images in the bottom left panel, and to NIRCAM F200W, HST F814W and HST F625W images in the bottom right panel. The first three panels show $6\arcsec \times 6\arcsec$ regions around the cluster center, while the last panel zooms in on the $2\arcsec \times 2\arcsec$ region to show the PMs of the fast moving stars. The dashed cyan circle with $r=1"$ is centered at the "AvdM10" cluster center.
• Figure 3: HST-JWST CMDs of sources within $r<1\arcsec$, $r<3\arcsec$, and $r<40\arcsec$ of the cluster center. Blue sources are all sources appearing in the $r<40\arcsec$ region. Marker colors indicate which catalogs the sources appear in the $r<3\arcsec$ regions: green sources appear in oMEGACat, NIRCam, and MIRI catalogs, purple sources in oMEGACat and NIRCam, and red sources in NIRCam and MIRI. Black sources are only detected in the catalog corresponding to the CMD where they are shown. The vertical dashed lines show the bluest color expected for an IMBH accretion model of mass 8.2e3M_⊙, $p=0.5$, and $n=\qty{0.2}{\cm^{-3}}$ (first row of Table \ref{['tab:models']}). The horizontal error bars show the $2\sigma$ color error expected for sources of color 3 at different magnitudes. The diagonal dashed lines in the second and third panels are used to select "red" sources. The SEDs of the 3 sources inside the $r<3\arcsec$ region enclosed by the ellipse in the third panel are shown in Figure \ref{['fig:ngc5139_center_miri_seds']}. Note that two of these sources appear close together on the CMD.
• Figure 4: Detected NIRCam and MIRI sources within the central $r<1\arcsec$ region are shown by circles in the top two panels. The left (right) panel uses the NIRCam F200W, F444W (MIRI F770W, F1500W) images for the blue and red channels respectively. Green (white) circles show NIRCam (MIRI) sources. The smaller green circles show fainter NIRCam sources without an oMEGACat counterpart. Numbered labels are placed next to sources discussed in the text. The two bottom panels show the same images as top row but for the larger $r<3"$ region, with markers showing "red" sources, as defined in Section\ref{['CMDs']}.
• Figure 5: SEDs of three oMEGACat-NIRCam sources with MIRI counterparts showing apparent F1500W excess in the third panel of Figure \ref{['fig:ngc5139_center_cmds']}, enclosed by an ellipse. The last panel shows the SED of a more isolated star of similar brightness, that does not have the F1500W excess. The dashed gray lines shows power laws with index -4, normalized to the F444W band, expected for a thermal source with a Rayleigh-Jeans tail. The purely statistical uncertainties in the JWST fluxes produced by DOLPHOT appear small, but do not account for crowding.