Broad-Line AGN at 3.5<z<6: The Black Hole Mass Function and a Connection with Little Red Dots

TL;DR

This study uses JWST/NIRSpec data from CEERS and RUBIES to assemble a robust sample of 62 broad-line AGN at 3.5<z<6.8 identified via Hα emission. By combining forward-modeling line fits with Bayesian inference and careful flux calibration, the authors derive a rest-frame UV/optical slope distribution, construct the BH mass function down to log(M_BH/M_sun) < 7 with completeness corrections, and show broad agreement with JWST-based mass functions. They also assess the overlap with Little Red Dots, finding that about one-third of BLAGN are LRDs, and demonstrate that LRD BLAGN tend to be intrinsically reddened with more dominant broad-line emission compared to non-LRD BLAGN, based on stacked Hα profiles. The UV luminosity function of BLAGN aligns with recent JWST samples, and the results collectively support a coherent picture of early SMBH growth that is broadly consistent with theoretical expectations, while indicating that signatures of black-hole seeding may be erased by z~5–6.

Abstract

We present a sample of 50 H-alpha detected broad-line active galactic nuclei (BLAGN) at redshifts 3.5<z<6.8 using data from the CEERS and RUBIES surveys. We select these sources directly from JWST/NIRSpec G395M/F290LP spectra. We use a multi-step pre-selection and a Bayesian fitting procedure to ensure a high-quality sample of sources with broad Balmer lines and narrow forbidden lines. We compute rest-frame ultraviolet and optical spectral slopes for these objects, and determine that 10 BLAGN in our sample are also little red dots (LRDs). These LRD BLAGN, when examined in aggregate, show broader H-alpha line profiles and a higher fraction of broad-to-narrow component H-alpha emission than non-LRD BLAGN. Moreover, we find that ~66% of these objects are intrinsically reddened (beta (optical)>0), independent of the contributions of emission lines to the broadband photometry. We construct the black hole (BH) mass function at 3.5<z<6 after computing robust observational and line detection completeness corrections. This BH mass function shows broad agreement with both recent JWST/NIRSpec and JWST/NIRCam WFSS based BH mass functions, though we extend these earlier results to log(M(BH)/M(sun)) < 7. The derived BH mass function is consistent with a variety of theoretical models, indicating that the observed abundance of black holes in the early universe is not discrepant with physically-motivated predictions. The BH mass function shape resembles a largely featureless power-law, suggesting that any signature from black-hole seeding has been lost by redshift z~5-6. Finally, we compute the BLAGN UV luminosity function and find good agreement with JWST-detected BLAGN samples from recent works, finding that BLAGN hosts constitute <10% of the total observed UV luminosity at all but the brightest luminosities.

Figures (9)

• Figure 1: H$\alpha$ and [N II]$\lambda\lambda$6548,6583 line fits to the G395M spectrum for object RUBIES-EGS-15825, a confirmed $z=3.67$ broad-line object in our sample. Here, the spectrum is shown in blue with 1$\sigma$ uncertainties shown as a shaded blue region, the broad H$\alpha$ component and the narrow H$\alpha$ and [N II]$\lambda\lambda$6548,6583 components of the fitted model are shown as black dashed curves, and the combined model is shown as a red curve. This model accounts for all of the expected features in the H$\alpha$ region emitted by a BLAGN.
• Figure 2: 2" $\times$ 2" NIRCam F444W (containing the H$\alpha$ line) cutout of CEERS-2782/RUBIES-EGS-50052. The blue/red outlines show the positions of the NIRSpec slitlets (0$.\!\!^{\prime\prime}$ 20 $\times$ 0$.\!\!^{\prime\prime}$ 46 per shutter) in each observation. The RUBIES-EGS-50052 slitlet alignment is significantly better centered than the CEERS-2782 pointing. Due to this superior centering, we use the higher S/N RUBIES-EGS-50052 spectrum in this analysis.
• Figure 3: Broad+narrow and single component fits to the [O iii]$\lambda\lambda$4959,5007 doublet in object RUBIES-EGS-50052. Here, the two component fit is shown by the red curve, the single component fits is shown by the green curve, and the spectrum is shown in blue. The double component fit significantly improves the line fit near the wings of the [O iii]$\lambda\lambda$4959,5007 lines, indicating the presence of an outflow.
• Figure 4: The H$\alpha$ broad-lines (blue curves), broad-line fits (red curves), and broad and narrow components of the fits (black dashed curves) for all 63 spectra (for the 62 objects) in our BLAGN sample, sorted in order of increasing redshift. Here, we normalize each spectrum by the peak of the H$\alpha$ line fit, and plot the data and fits as a function of velocity relative to the H$\alpha$ line center. We show a 700 km s$^{-1}$ region (our FWHM cut) as a shaded region centered at zero velocity in each panel. We give the MSA_ID, redshift, broad component FWHM (in km s$^{-1}$), broad component signal-to-noise ratio, and black hole mass (in units of $\log_{10}\left(M_{\odot}\right)$) for each target in each panel. Note that CEERS-2782 and RUBIES-EGS-50052 are the same source observed by both CEERS and RUBIES.
• Figure 5: The same as Figure \ref{['fig:All_Broadlines']}, but here we plot the line fits and data on a logarithmic scale to best emphasize the broad components of each line. This helps in particular when the broad peak is much lower than the narrow peak, as can be seen in CEERS-397.