The Host Galaxy (If Any) of the Little Red Dots

TL;DR

This work presents a multi-band, morphology–SED analysis of eight JWST UNCOVER LRDs using GalfitS to simultaneously fit nuclear and host components across seven NIRCam bands. A hybrid PSF approach and rigorous mock simulations yield a robust host detection only for MSAID38108 at $z=4.96$ with $ obreakspace \\log (M_*/M_\odot)=8.66^{+0.24}_{-0.23}$, $R_e=0.66^{+0.08}_{-0.05}$ kpc, and $n=0.71^{+0.07}_{-0.08}$; for the other seven LRDs, stringent $M_*$ upper limits are established, typically an order of magnitude below expectations from local BH–host scaling relations based on $M_{\rm BH}$ from broad H$\alpha$. The analysis also finds extended, off-centered emission in half the sample, with uncertain origins in several cases. Collectively, the results suggest that LRDs host disproportionately massive BHs (or undermassive hosts) at high redshift, challenging simple BH–galaxy coevolution scenarios and illustrating the value of spatially resolved, multi-band modeling for disentangling AGN and host contributions.

Abstract

We investigate the host galaxy properties of eight little red dots (LRDs) selected from the JWST UNCOVER survey, applying a new technique ({\tt\string GalfitS}) to simultaneously fit the morphology and spectral energy distribution using multi-band NIRCam images covering $\sim 1-4\,μ{\rm m}$. We detect the host galaxy in only one LRD, MSAID38108 at $z = 4.96$, which has a stellar mass of $\log (M_*/M_{\odot}) = 8.66^{+0.24}_{-0.23}$, an effective radius $R_e=0.66^{+0.08}_{-0.05}$ kpc, and a Sérsic index $n=0.71^{+0.07}_{-0.08}$. No host emission centered on the central point source is found in the other seven LRDs. We derive stringent upper limits for the stellar mass of a hypothetical host galaxy by conducting realistic mock simulations that place high-redshift galaxy images under the LRDs. Based on the black hole masses estimated from the broad H$α$ emission line, the derived stellar mass limits are at least a factor of 10 lower than expected from the $z \approx 0$ scaling relation between black hole mass and host galaxy stellar mass. Intriguingly, four of the LRDs (50\% of the sample) show extended, off-centered emission, which is particularly prominent in the bluer bands. The asymmetric emission of two sources can be modeled as stellar emission, but the nature of the other two is unclear.

Figures (15)

• Figure 1: PSF models for the seven NIRCam bands, constructed empirically using PSFEx (top) and using the hybrid technique described in Section \ref{['hyb_psf']} (middle). The bottom row shows the 1D surface brightness profiles for the PSFEx (red) and hybrid (blue) PSF models.
• Figure 2: Simultaneous multi-band image fitting results using a single point-source model for MSAID2008 at $z=6.74$. The first seven rows, from top to bottom, show the results for the seven NIRCam bands (F115W, F150W, F200W, F277W, F356W, F410M, F444W). In the left-most column, the upper panel of each row shows the radial surface brightness distribution (open circles with error bars), the PSF model (blue dotted line), as well as the median profile of the mock point source (blue solid line) and its standard deviation (blue shaded region). The background noise level is denoted by the black horizontal dashed line. The $\chi^2$ for each band is given in the lower-left corner of each panel, while that for all seven bands is given in the upper-right corner the panel for F444W. The lower subpanels give the residuals between the data and the best-fit model (data$-$model). The imaging columns, from left to right, display the original data, best-fit model, data minus the nucleus component, and residuals normalized by the errors (data$-$model$/$error), which are stretched linearly from $-5$ to 5. The bottom row illustrates the results of one of the mock images created for the target LRD, selected to reveal the host galaxy detection limit with the lowest stellar mass and smallest effective radius. Section \ref{['upper_limit']} describes the mock simulations and detection limit calculation.
• Figure 3: As with Figure \ref{['2008_fitting_demo']}, but for MSAID38108, which has detected extended emission centered on the point source.
• Figure 4: As with Figure \ref{['2008_fitting_demo']}, but for MSAID10686, which has off-centered emission.
• Figure 5: The SED (top) and the distribution of rest-frame optical mass-to-light ratio (bottom) for the galaxies used in mock simulations for three LRDs in our sample. All results are plotted in four mass bins. In the top row, thinner lines are SEDs for individual galaxies normalized by their total flux in seven NIRCam bands, while their median SED is plotted as dots connected by thicker lines, and the error bars represent standard deviation. The total number of galaxies within $\Delta z<0.5$ selected for each LRD is labeled on the top-left corner of each panel. The bottom row plots the probability distribution function (PDF) of rest-frame optical mass-to-light ratio for the same group of galaxies, plotted with the same color as in the top ros. Verticle dashed lines gives the median value in each mass bin.