A big red dot at cosmic noon

TL;DR

BiRD represents a bright, spectroscopically confirmed little red dot at cosmic noon ($z\sim2.33$) discovered in the JWST/J1030 field. Through NIRCam imaging and WFSS spectroscopy, BiRD shows a broad Pa$\gamma$ and He I emission with a blueshifted He I absorber, yielding a black hole mass of order $10^8\,M_\odot$ and a bolometric luminosity of $\sim3\times10^{45}\rm\ erg\ s^{-1}$, while remaining undetected in X-ray and radio bands. The analysis demonstrates a Balmer-break–driven, blue-UV/red-optical SED typical of LRDs and places BiRD in a small class of cosmic noon BLAGN with mild evolution in space density compared to UV-selected quasars. The inferred space density of LRDs at $z\sim2.5$ is comparable to that of traditional quasars, implying a non-negligible role for LRDs in BH growth and challenging models that suppress seed BH formation by $z\sim5$. Together with two analogous LRDs at similar redshift, BiRD suggests common, outflowing gas and a dust-poor environment that persists into cosmic noon, highlighting LRDs as a valuable probe of early black hole growth and AGN evolution.

Abstract

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$λ10830$ and PaG line, both showing a narrow and a broad ($FWHM\gtrsim 2000\ \rm kms^{-1}$) component. The HeI line is affected by an absorption feature, tracing dense gas with HeI column density in the $2^3S$ level $N\sim 0.5-1.2\times 10^{14}\rm cm^{-2}$, depending on the location of the absorber, which is outflowing at the speed of $Δv \sim -830\ \rm kms^{-1}$. 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 $M_{\rm BH}\sim 10^8\rm M_{\odot}$ and $L_{\rm bol}\sim 2.9\times 10^{45}\rm ergs^{-1}$, 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.

Figures (7)

• Figure 1: $F200W-F356W$ color vs. $F356W$ magnitude distribution of JWST-detected sources in the J1030 field. The gray dots represent the extended sources, while point-like sources are shown as yellow filled circles. Stars align along a horizontal sequence at $F200W-F356W\sim -0.9$. Faint ($F356W>24$) and red ($F200W-F356W >0$) point-like objects (filled circles with red contour) are mostly LRD candidates at $z>4$ (e.g., ID4879 at $z=5.95$; see text for details). The three bright ($F356W<23$), point-like objects above the stellar sequence are two standard, X-ray detected broad line AGN (J1030 and XID10; filled circles with blue contours) and the BiRD (purple pentagon; see text for details). The dotted line shows the track followed by the BiRD if moved at different redshifts as labeled from $z = 2$ to $z = 7$. The smaller pentagons show for comparison the position of two spectroscopically confirmed LRDs at cosmic noon, discovered with JWST, namely the Rosetta Stone at $z=2.26$ (plum; juodzbalis24) and RUBIES-BLAGN-1 at $z=3.10$ (pink; wang25). The colored box shows the selection region used to compute the space density of LRDs at cosmic noon (see Section \ref{['sub:spaceden']}).
• Figure 2: Multiwavelength cutouts of BiRD. The images ($5\arcsec \times 5\arcsec$ each) were obtained with CTIO (B; MUSYC survey; gawiser06), LBT/LBC (g,r,i,z), CFHT/WIRCAM (Y,J,K), JWST/NIRCam (F115W, F200W, F356W), and Spitzer/IRAC (CH1 and CH2; annunziatella18). The JWST images are enclosed in green squares. The RGB image includes the NIRCam filters only.
• Figure 3: Two-dimensional and one-dimensional spectrum of BiRD (NIRCam/WFSS). The dotted box indicates the 5-pixel aperture used to extract the one-dimensional spectrum. We see clear emission from He i , Pa$\gamma$ and O i lines, superimposed on the continuum. The contamination from a bright source is visible at the red end of the spectrum. The flux density $F_{\lambda}$ is expressed in units of $10^{-16}\ \rm erg\ s^{-1}\ cm^{-2} \mu m^{-1}$. The $1\sigma$ error spectrum is reported as a purple shaded area.
• Figure 4: Continuum and line fit of BiRD. The NIRCam/WFSS spectrum is shown in purple with the corresponding errors (shaded area). We modeled both the He i (pink) and the Pa$\gamma$ (violet) emission with a narrow and a broad Gaussian component. An absorption term was applied to both continuum emission and broad He i line to account for the feature at $3.60\ \mu \rm m$
• Figure 5: Broad band, rest-frame SED of BiRD (purple symbols) compared to that of the Rosetta Stone (plum; juodzbalis24), RUBIES-BLAGN-1 (pink; wang25) and to the median SED of LRDs at $3 \lesssim z \lesssim 9$ (red; casey24). The green contour highlights the JWST data-points at $1.15$, $2.0$, and $3.56\ \rm \mu m$, while the inverted triangles represent flux upper limits. The photometric errors for BiRD are also reported and, for some data-points, are smaller than the symbol size. The position of the Balmer break is marked by the vertical line at 3645 Å.