Exploring Active Galactic Nuclei and Little Red Dots with the Obelisk simulation

TL;DR

This study develops a spatially resolved framework to predict AGN photometry in galaxies from the Obelisk simulation and uses JWST-aligned color cuts to investigate color-selected Little Red Dots (cLRDs). By combining realistic AGN SEDs, dust attenuation with MW- or SMC-like dust, and a line-of-sight radiative-transfer scheme, the authors show that cLRDs arise only when the AGN is sufficiently luminous relative to its host and the attenuation is intermediate, enabling the AGN to dominate red JWST bands while the blue bands remain host-dominated. The work also explores MBH population variants (true, distr, distr overm) and super-Eddington regimes, finding that cLRDs preferentially occur for high $f_{ m Edd}$ and/or high $M_{ m BH}/M_\ast$, with cLRD fractions strongly modulated by line-of-sight geometry and dust laws. These results place cLRDs within the broader AGN population, quantify the selection biases of UNCOVER-like colors, and highlight the importance of spatially resolved attenuation modeling for interpreting high-redshift AGN and their role in MBH growth. The findings imply that super-Eddington accretion and intermediate attenuation could account for many observed red, compact AGN, providing insight into the co-evolution of MBHs and their host galaxies in the early Universe.

Abstract

The James Webb Space telescope has discovered an abundant population of broad line emitters, typical signposts for Active Galactic Nuclei (AGN). Many of these sources have red colors and a compact appearance that has led to naming them `Little Red Dots'. In this paper we develop a detailed framework to estimate the photometry of AGN embedded in galaxies extracted from the \Obelisk{} cosmological simulation to understand the properties of color-selected Little Red Dots (cLRDs) in the context of the full AGN and massive black hole population. We find that using realistic spectral energy distributions (SEDs) and attenuation for AGN we can explain the shape of the cLRD SED as long as galaxies host a sufficiently luminous AGN that is not too much or too little attenuated. When attenuation is too low or too high, AGN do not enter the cLRD selection, because the AGN dominates over the host galaxy too much in blue filters, or it does not contribute to photometry anywhere, respectively. cLRDs are also characterized by high Eddington ratios, possibility super-Eddington, and/or high ratios between black hole and stellar mass.

Figures (18)

• Figure 1: Properties of simulated galaxies with $M_{\rm star}>10^8 \,{\rm M_\odot}$ at $z=6$, including galaxies that do not host MBHs. From top to bottom: star formation rate averaged over 10 Myrs in black, and over 100 Myrs in gray; mass-weighted stellar age, mass-weighted stellar metallicity, where solar metallicity is assumed to be 0.02, dust mass within twice the effective radius. $R_{\rm eff}$ is defined as the half-mass projected radius in the plane orthogonal to the stellar angular momentum, computed as the (2D) radius such that 50% of the mass is within that radius. The violet line in the top panel shows the fit from 2014ApJS..214...15S.
• Figure 2: Sketch of our approach to modeling gas and dust absorption, showing how differential attenuation affects different parts of a given galaxy and lines of sight. We highlight two lines of sight with rays shown in purple and red arrows for each emitter. Darker grey colors show gas and dust clouds with larger optical depth. Stars' colors indicate age: for instance young (blue) stars are generally still embedded in their birth clouds, while this is not necessarily the case for older (yellow and orange) stars. Since at least some star-forming regions are dusty, their contribution to the SED will be more affected by the extinction curve than other parts of the galaxy. The position of the AGN is marked in black, with thicker arrows. The example distribution of emitters and absorbers shown here has been chosen to highlight how the same source can be AGN- or stellar-dominated, depending on the line of sight. In this example, along the cumulative purple line of sight the AGN dominates over starlight: the AGN is seen through a clear line of sight, while much of the emission from the stellar population is absorbed. Vice-versa, along the cumulative red line of sight the AGN emission is sub-dominant to starlight.
• Figure 3: Examples showing how different assumptions affect the emergent SEDs at $z=6$. For the same galaxy we show different $M_{\rm BH}$, $f_{\rm Edd}$ and $N_{\rm H}$, as well as different lines-of-sight for the emission from the stellar population. The NIRCAM response for the filters analyzed in this paper is shown in gray.
• Figure 4: Bolometric luminosity function for the actual MBHs in the simulation ('true') and for the models described in Table \ref{['tab:models']}, where 'resc' means we have corrected the number densities for the overdensity bias: Obelisk is a protocluster. To obtain the cosmic average we have rescaled the Obelisk galaxy mass function to that of the NewHorizon simulation, which simulates an average region of the Universe. This figure assumes an active fraction of unity, and therefore represents an upper limit to the LF for each of the models. The results are compared to the luminosity functions derived in 2024ApJ...963..129M and 2024ApJ...964...39G from JWST data and to fits to the evolution of the bolometric luminosity function derived in 2020MNRAS.495.3252S.
• Figure 5: Relation between hydrogen column density in the ISM and galaxy mass and half-mass radius at $z=6$. Top panel: column density obtained integrating the column density from the position of the MBH to the host galaxy virial radius ('max' model). Bottom panel: column density obtained excluding the inner 80 pc around the MBH ('outmax80' model). The orange points show the mean column density for each galaxy, with size scaling with $M_{\rm star}$. For a random subset of 24 galaxies we show the column density for 12 different lines of sight, with the greyscale proportional to $M_{\rm star}$, as shown in the colorbar.