Black Hole Mass Estimation of a Background Quasar in the Vicinity of the cD Galaxy Holm 15A

INTRODUCTION

We present observations of the central region of the galaxy cluster Abell 85: the cD galaxy Holm 15A and SDSS J004150.75-091824.3 (hereafter, J004150). Holm 15A was reported by O. López-Cruz et al. [3] as the galaxy with the largest core ($r_{\gamma} = 4.57 \pm0.06$ kpc) and that could host an Ultramassive Black Hole (UMBH) with $M_{\mathrm{BH}} \sim10^{10}\ M_{\odot}$ (even $M_{\mathrm{BH}} \sim10^{11}\ M_{\odot}$ with some scaling relations). But K. Mehrgan et al. [5] reported $M_{\mathrm{BH}} = 4.0 \pm0.01 \times10^{10}\ M_{\odot}$ using the stellar velocity field.

J004150, with $z_{\mathrm{phot}} \sim0.9$ (G. T. Richards et al. [8]), lies within the halo of Holm 15A. Then, O. López-Cruz et al. proposed that J004150 may be a rogue SMBH ejected from the nucleus of Holm 15A ($z = 0.05536 \pm0.00002$) due to the scouring action of SMBH triples (e.g., L. Hoffman & A. Loeb [2]; Y. Ni et al. 2022). Also, the present study estimates the SMBH mass of J004150 using single-epoch virial mass estimators based on broad emission lines of active galactic nucleus.

Data and Methodology

We observed the central region of Abell 85 (top-left panel of Figure 1) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS Multi-Object Spectrograph (MOS) under the programme GTC5-17AMEX (PI: ERL). We carried out the observations in service mode using the R1000B grism, slit width of 1.2″, covering a spectral range from ∼3700-7500 Å, with a spectral resolution of ∼11 Å.

To estimate the BH mass of J004150, we used the BH virial mass estimator based on the Mg II $\lambda2798$ emission line:

where $L$ represents the continuum luminosity at $\lambda= 3000$ Å, $\lambda L_{\lambda}$, or Mg II line luminosity, L(Mg II); FWHM is the full width at half maximum of the MgII line; coefficients values are $a = 1.933$, $b = 0.63$ and $c = 2.0$ for L(Mg II) (Y. Shen & X. Liu [9]), and $a = 0.86$, $b = 0.5$ and $c = 2.0$ for $\lambda L_{\lambda}$ (M. Vestergaard & P. S. Osmer [11]). To find these parameters, we performed the spectral decomposition of the Mg II line (bottom-right panel of Figure 1) similarly to D. Zhu et al. [13]. We fit a pseudo-continuum model consisting of a power-law and the UV Fe II template (M. Vestergaard & B. M. Peterson [12]). We subtracted the pseudo-continuum from the spectrum and fitted the Mg II line with broad and narrow components in the $[2690,2910]$ Å range. We modeled the broad component with multiple Gaussians, using three Gaussians (each with FWHM $\ge900$ km/s); for the narrow component, we used two Gaussians and checked both possibilities of doublet ratio, (2:1) for optically thin and (1:1) for optically thick. We used Astropy (A. M. Price-Whelan et al. [7]) to fit the continuum and the Mg II line profile.

To estimate the uncertainties in the parameters of Eq. (1), we performed a Monte Carlo analysis in which we randomly perturbed the original spectrum to generate 50 mock spectra. The resulting values are $\lambda L_{\lambda} = 0.9097 \pm0.0072 \times10^{45}$ erg/s, $L(\mathrm{Mg\ II}) = 8.720 \pm0.069 \times10^{42}$ erg/s and $\mathrm{FWHM}\,(\mathrm{Mg\ II}) = 4896.9 \pm452.9$ km/s. To estimate the uncertainty of the BH mass, we propagated the errors using Eq. (1) as:

where $\sigma_{\mathrm{int}} = 0.25$ or 0.33 dex is the intrinsic scatter of the calibration from M. Vestergaard & P. S. Osmer [11] or Y. Shen & X. Liu [9], respectively; $\sigma_{\log L}$ is the uncertainty in $\lambda L_{\lambda}$ or $L(\mathrm{Mg\ II})$ and $\sigma_{\log\mathrm{FWHM}}$ is the FWHM(Mg II) uncertainty.

Results and Discussion

We found that J004140 is a background quasar at $z = 1.56386 \pm0.00397$², measured using CIV $\lambda1549$, HeII $\lambda1640$, CIII] $\lambda1909$ and MgII $\lambda2798$ lines (bottom-left panel of Figure 1); our redshift results improved over J. P. Madrid [4].

For Holm 15A, we report $z = 0.05565 \pm0.00052$ using $H_{\alpha}$, $H_{\beta}$, Mg, and Na D lines (top-right panel of Figure 1), in agreement with O. López-Cruz et al. [3]. These results clearly rule out O. López-Cruz et al. suggestion that J004140 is a BH ejected from the nucleus of Holm 15A.

We obtained a SMBH mass for J004150 of $\log(M_{\mathrm{BH}}/M_{\odot}) = 8.72 \pm0.26$ utilizing the M. Vestergaard & P. S. Osmer [11] calibration. Since our spectrum ends at $\lambda_{\mathrm{rest}} \sim2930$ Å, which prevented a direct measurement of the continuum at 3000Å, we implemented an extrapolation according to Y. Shen et al. [10]; thus, we independently calculated the mass using the Y. Shen & X. Liu [9] relation, obtaining $\log(M_{\mathrm{BH}}/M_{\odot}) = 8.65 \pm0.34$. The agreement between both estimates confirms the consistency in the SMBH masses.

Given recent observations reporting overmassive BHs at high redshift (e.g., [e.g. 1]), BH mass estimates in quasars at intermediate redshift become relevant for constraining evolution models of galaxies.

Acknowledgments

This work is based on observations using GTC at the Observatorio del Roque de los Muchachos of the IAC.