Variability-selected AGN in dwarf galaxies: the incidence of AGN in dwarf and massive galaxies is similar

TL;DR

This paper investigates how common AGN are in nearby dwarf galaxies ( $10^{8} M_ ext{sun} < M_ ext{star} < 10^{10} M_ ext{sun}$, $z<0.4$ ) and how their incidence compares to that in massive galaxies. It employs optical broadband variability from the VST-COSMOS survey to identify a sample of 30 dwarfs hosting AGN, combining COSMOS2020 parameters, visual morphologies, and environmental metrics to contrast with mass- and redshift-matched controls. By modeling true galaxy numbers and AGN detectability, the authors derive a relative AGN incidence of $f_{ m agn,d}/f_{ m agn,m} \approx 0.65^{+0.52}_{-0.26} \times (D_{ m agn,m}/D_{ m agn,d})$, suggesting that dwarfs and massive galaxies host AGN at similar rates within about a factor of two, with hints that the fraction grows with stellar mass among dwarfs. The results imply a high black hole occupation fraction in dwarfs and highlight the need for large, deep surveys (e.g., LSST) to robustly quantify AGN activity in low-mass galaxies and its role in galaxy evolution.

Abstract

We use the VST-COSMOS survey to identify, via their optical broadband variability, 30 AGN in nearby (z<0.4) dwarf (10^8 MSun < M < 10^10 MSun) galaxies. VST-COSMOS offers a 1 deg^2 survey footprint, a single visit depth of 24.6 mag and 68 r-band visits spanning an eleven-year temporal baseline. Compared to a control sample matched in stellar mass and redshift, the dwarf AGN population shows an elevated fraction of early-type galaxies but a similar fraction of interacting objects, suggesting that interactions do not play a significant role in triggering these AGN. Dwarf AGN hosts do not show strong differences in their distances to nodes, filaments and massive galaxies compared to the controls, which indicates that AGN triggering, at least in this sample, is not strongly correlated with environment. Finally, by combining the true number of galaxies, the detectability of AGN and the measured numbers of variable sources in dwarf and massive (M > 10^10 MSun) galaxies, we estimate the relative frequency of AGN in these two mass regimes. Our results suggest that the incidence of AGN in dwarfs and massive galaxies is similar (within less than a factor of 2 of each other), with some evidence that the AGN fraction increases with stellar mass in the dwarf population.

Figures (6)

• Figure 1: Examples images of our dwarf AGN (each image has a linear size of 4 arcsec). The top and bottom rows show early-type galaxies (ETGs) and late-type galaxies (LTGs) respectively. Interacting systems are indicated using a red filled circle in the lower right-hand corner of the image. Galaxies 3 and 5 show internal asymmetries, while galaxy 4 appears to be accreting a smaller companion in its central region. This figure is better viewed online than in print.
• Figure 2: Redshift vs stellar mass of dwarfs in the COSMOS2020 catalogue (shown using the heatmap) and our dwarf AGN (filled circles). ETGs and LTGs are shown colour-coded (see legend). Interacting galaxies are indicated using crosses. The solid, dashed and dotted orange lines show the redshifts below which galaxies of a given stellar mass are complete in COSMOS2020, VST-COSMOS and the SDSS MGS, which have magnitude limits of $r=28$, 24.6 and 17.77 respectively.
• Figure 3: The standard deviation of the $r$-band magnitude in our dwarf AGN. Different morphological classes are shown colour-coded (see legend). Interacting galaxies are indicated using crosses. As described in Section \ref{['sec:flux_fractions']}, if all the variability can be attributed to the AGN, then this standard deviation is also an estimate of the fraction of $r$-band flux that is likely to be contributed by the AGN.
• Figure 4: The star formation main sequence of galaxies in the COSMOS2020 catalogue (shown using the heatmap) and our dwarf AGN (filled circles). The dashed-dotted line demarcates the lower limit of the main locus of the star formation main sequence. Different morphological classes are shown colour-coded (see legend). Interacting galaxies are indicated using crosses. Examples of other main sequence loci from Behroozi2013 and Whitaker2012 are shown overplotted and their standard deviations are indicated using the error bars.
• Figure 5: Projected distance to the nearest node (top panels), nearest filament (middle panels) and nearest massive galaxy (bottom panels) of dwarf AGN (red) and their control counterparts (black). The left-hand column presents the case where all dwarf AGN are considered, while the distributions in the right-hand column are restricted to galaxies in the VST-COSMOS completeness region (indicated by 'CR only' in the panels). Medians and their uncertainties, calculated using bootstrapping, are shown using the dashed and dotted lines respectively.