BEES: Quasar lifetime measurements from extended rest-optical emission line nebulae at $z\sim6$

Abstract

Measurements of quasar lifetimes at high redshift indicate that the earliest billion-solar-mass supermassive black holes (SMBHs) have only been active as luminous quasars for less than a million years. Recently, extended Ly$α$ nebulae around $z\sim6$ quasars have revealed that these short observed lifetimes are unlikely a sightline-dependent effect. However, the interpretation of Ly$α$ emission is not straightforward due to its resonant nature. In this work, we use rest-frame optical emission lines, which more directly trace photoionization by the quasar, to unambiguously validate the short line-of-sight quasar lifetimes observed at early cosmic epochs. We use deep James Webb Space Telescope/NIRSpec IFU observations of five $z\sim 6$ quasars with small proximity zones to search for their extended emission line nebulae in H$α$ and [O III]$5007$, and detect extended emission in both emission lines around four quasars in our sample. We then use the light-crossing time of these nebulae to measure quasar lifetimes along transverse sightlines. Using their H$α$ nebulae, we also confirm that recombination is likely the dominant emission mechanism behind their previously detected Ly$α$ nebulae. Our results confirm the existence of high-redshift quasars with extremely short lifetimes, $t_{\rm Q} \lesssim 10^{5}\ {\rm yr}$, hosting billion-solar-mass black holes, indicating that rapid accretion is likely responsible for the assembly of SMBHs in the early Universe.

Figures (11)

• Figure 1: Rest-optical spectra of quasars in our sample extracted over a $0.2"$ (radius) aperture from the NIRSpec IFU observations presented here. For each quasar, we display the full spectrum (top left), the fit residuals (bottom left, pink), as well as a zoom-in on the H$\beta$ and H$\alpha$ emission line regions (top and bottom right, respectively). The various colored lines represent the median of the MCMC posterior distribution for the different spectral components, with the composite model shown in red. Note that the $\pm1\sigma$ measurement uncertainties on the extracted spectra are shown as gray shaded regions in the residual panel.
• Figure 2: (contd.)
• Figure 3: The results of our H$\alpha$ (top row, orange color scheme) and [O III] (bottom row, green color scheme) nebular search for J0100+2802. Left column: From left to right, the panels display pseudo-narrowband images of 1) the data cube collapsed around the emission line in units of surface brightness, 2) the PSF extracted from the broad wings of the emission line, 3) the PSF-subtracted equivalent of panel 1, 4) the $\chi$ data, essentially representing the SNR of the PSF-subtracted data, and 5) the smoothed $\chi$ data showing the extended nebular emission. Panels 1, 3, 4, and 5 are collapsed across the wavelength range of the detected nebula, and panel 2 is collapsed across the wavelengths of the PSF extraction as displayed in Appendix \ref{['app:specregions']}. White patches correspond to masked foregrounds/artifacts, and the black circle marks the position of the quasar. Right column: The annulus-averaged surface brightness profiles extracted from panel 3, computed both using the full pseudo-narrowband image (gray data points) and only taking into account the nebular voxels (black data points). The black data points are used to identify the maximum extent of the nebula, from which the lifetimes are derived. The noise floor (gray shaded regions corresponding to $1\sigma$ and $2\sigma$) is extracted from the outermost annulus, excluding pixels identified as part of the nebula. Additionally, the hatched vertical regions mark the central region inaccessible due to PSF residuals as well as the region that lies beyond the field of view of the NIRSpec IFU.
• Figure 4: Same as \ref{['fig:nebJ0100']}, but for J158--14.
• Figure 5: Same as \ref{['fig:nebJ0100']}, but for J1335+3533.