A Kiloparsec-Scale Stellar Cavity in the Center of Abell402-BCG May be Caused by Dynamic Interactions with an Ultramassive Black Hole

Abstract

We present new observations from JWST NIRCam that reveal a striking kpc-wide cavity in the stellar distribution of the central galaxy in the cluster Abell402. Supporting data from HST allow us to rule out extinction due to dust as an explanation and, instead, suggest that this is a localized depression in the stellar density field corresponding to ~2x10^9 Msun in missing stars within a volume of 0.5kpc^3. On larger scales, both the JWST and HST data show evidence for a 2.2kpc flattened core in the stellar distribution (on which the smaller-scale cavity is superimposed), which implies the presence of a central ultra-massive black hole with M_BH = 6 +/- 4 x10^10 Msun. We report evidence for a mid-IR-bright point source at one edge of the cavity, suggesting that this black hole is actively accreting. MUSE spectroscopy reveal that this source is a LINER AGN and that there is a second candidate AGN on the opposite side of the cavity with a relative velocity of 370km/s -- if real, this implies the presence of a kpc-separation dual AGN with a total binary mass of 6 +/- 2 x10^10 Msun, which would make this the most massive binary black hole system discovered to date. We propose that this unique stellar cavity is the result of a short-lived dynamical interaction between at least one supermassive black hole and the background stellar density field, caused either by three-body scattering during binary hardening or the induction of a dipole instability in the stellar density field.

Figures (10)

• Figure 1: Multi-wavelength observations of the central galaxy in Abell 402.Left: Three-color image of the cluster core based on data from HST and JWST (north is up, east is left). Upper right: Zoom-in on the inner $2^{\prime\prime}\times 2^{\prime\prime}$ of the central galaxy in Abell 402. This zoomed image reveals a 0.25$^{\prime\prime}$-wide cavity in surface brightness, as well as a bright point source on the western edge of the depression. Lower right: Images of the inner region of the central galaxy in five different filters at four different epochs. The point source (depicted with a white pointer in all panels) is brightest in the redder JWST band, where the combination of the bright point source and degraded PSF obscures the presence of any cavity.
• Figure 2: Left: Comparing the dark patch in Abell1060-BCG and Abell402-BCG. In the upper row, we show F555W and F110W observations of the central galaxy in Abell1060 ($z=0.012$), which has a well-known ring of dust in the inner kpc. The F555W image has been smoothed and rebinned to match the F110W image. In the right column we show the ratio of these two images, which highlights the wavelength-dependent intensity of the dust ring. In the middle row, we have simulated how Abell1060 would appear at $z=0.3$ by adjusting the binning/smoothing based on the larger angular diameter distance -- the dust ring now appears to be an unresolved cavity in the galaxy center, similar to what is observed in Abell402. The dust feature is still highly significant in the ratio image. In the bottom panel, we show Abell402, where we have smoothed and rebinned the F150W2 data to match the F606W image. In this case, the ratio image contains no structure, suggesting that the observed cavity is missing stars, rather than dust. Right: Intensity of the dark patch compared to the expectation value from a symmetric model, as a function of wavelength for Abell402-BCG (diamonds) and Abell1060-BCG (orange circles). This figure shows that the constant depth of the cavity in Abell402 with wavelength (dashed line) is inconsistent with predictions from extinction models. We compare to the standard models for the Milky Way cardelli89 and for the starburst galaxies calzetti00, but also compare to eight models that span different regions in the LMC, SMC, Milky Way, M31, and M33 gordon24. All of these models predict a factor of $\sim$4 difference in rest-frame extinction between the F150W2 and F606W bands. In contrast, the feature in Abell1060-BCG is fully consistent with dust. We also compare to a simple model in which we remove a cylindrical volume of stars from the midplane of the galaxy, with spectral shape based on the measured color gradient of the galaxy -- this model (in grey) describes the data excellently.
• Figure 3: This figure shows, in red, the F150W2 surface brightness profile extracted from the central galaxy in Abell 402 BCG along a wedge as shown in the inset. For comparison, we show the galaxy with the largest core measured to date, IC 1101 dullo17, and a sample of cored galaxies from dullo19. All profiles have been corrected for cosmological dimming and k-corrected to the rest-frame V band assuming an old stellar population. The vertical red dotted line shows the location of the break radius measured from the two-dimensional fit described in Appendix A.1. This figure highlights the similarities between IC 1101 and Abell 402 BCG, and suggest that Abell 402 BCG likely harbors an ultramassive black hole.
• Figure 4: Left: Integrated spectra from a 1$^{\prime\prime}$-wide region centered on the stellar cavity, showing the brightness of various lines. The two velocity components are clearly offset, with the blue-shifted source having LINER-like emission line ratios (low [O iii]/H$\beta$ ratio, high [N ii]/H$\alpha$ ratio), while the red-shifted source has very bright [O iii] emission but no detectable [N ii]. In the H$\beta$ panel, the rest-frame velocity of the galaxy is shown for comparison. In all panels, the green line represents the empirical continuum model, extracted from a nearby region free of emission lines and rescaled in intensity to fit the central region. Right: Zoomed-in image of the central galaxy in Abell 402 in the F150W2 band. The white rectangle shows the extraction area for the MUSE spectra shown on the left. Red and blue contours show the intensity of the two kinematic components of [O iii]$\lambda$5007 shown on the left. This figure demonstrates the presence of two highly-ionized, point-like sources on either side of the stellar cavity, with high relative velocities. The eastern (red-shifted) source, while compact, appears to be embedded in a larger ($\sim$10 kpc) blob of highly-ionized gas.
• Figure 5: Line ratio diagram, comparing high-ionization ([O iii]/H$\beta$) and low-ionization ([N ii]/H$\alpha$) line ratios bpt, which separates star forming regions from active galactic nuclei and is insensitive to the effects of dust. The black/grey points in the background represent 85,224 galaxies with spectra from the Sloan Digital Sky Survey kewley06. Where the point density becomes sufficiently high we transition to a 2D histogram. There is a dense locus of points that are consistent with predictions for H ii regions kewley01 (purple grid). The blue-shifted source on the western side of the stellar cavity has line ratios consistent with LINER AGN heckman80allen08 (green grid) and is inconsistent with models of star formation. The red-shifted source on the eastern side of the stellar cavity has no detectable [N ii] emission, and so appears here as an upper limit in [N ii]/H$\alpha$. This source is fully consistent with models of Seyfert-like AGN with low-metallicity gas groves06 (pink grid), but is also marginally consistent with a very young and energetic starburst. We rule out the latter scenario based on the lack of blue continuum emission, which would have been clearly visible in the archival HST data at F606W.