Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Federica Loiacono, Roberto Gilli, Marco Mignoli, Giovanni Mazzolari, Roberto Decarli
| Summary:
We report the discovery of a little red dot (LRD), dubbed BiRD (‘big red
dot’), at $z=2.33$ in the field around the $z=6.3$ quasar SDSSJ1030+0524. Using
NIRCam images, we identified it as a bright outlier in the $F200W-F356W$ color
vs $F356W$ magnitude diagram of point sources in the field. The NIRCam/WFSS
spectrum reveals the emission from HeI$lambda 10830$ and PaG line, both
showing a narrow and a broad ($FWHMgtrsim 2000 rm kms^{-1}$) component. The
HeI line is affected by an absorption feature, tracing dense gas with HeI
column density in the $2^3S$ level $Nsim 0.5-1.2times 10^{14}rm cm^{-2}$,
depending on the location of the absorber, which is outflowing at the speed of
$Delta v sim -830 rm kms^{-1}$. As observed in the majority of LRDs, BiRD
does not show X-ray or radio emission. The BH mass and the bolometric
luminosity, both inferred from the PaG broad component, amount to $M_{rm
BH}sim 10^8rm M_{odot}$ and $L_{rm bol}sim 2.9times 10^{45}rm
ergs^{-1}$, respectively. Intriguingly, BiRD presents strict analogies with
other two LRDs spectroscopically confirmed at cosmic noon, GN-28074 ("Rosetta
Stone") at $z=2.26$ and RUBIES-BLAGN-1 at $z=3.1$. The blueshifted HeI
absorption detected in all three sources suggests that gas outflows may be
common in LRDs. We derive a first estimate of the space density of LRDs at
$z<3$ based on JWST data, as a function of $L_{rm bol}$ and BH mass. The space
density is only a factor of $sim 2-3$ lower than that of UV-selected quasars
with comparable $L_{rm bol}$ and $z$, meaning that the contribution of LRDs to
the broader AGN population is also relevant at cosmic noon. A similar trend is
also observed in terms of BH masses. If, as suggested by recent theories, LRDs
probe the very first and rapid growth of black hole seeds, our finding may
suggest that the formation of black hole seeds remains efficient at least up to
cosmic noon.
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