AGN in massive galaxies identified via optical broadband variability: lessons from VST-COSMOS for future LSST science

Abstract

We study the properties of 56 massive (M$_{\rm{\star}}$ > 10$^{10}$ M$_{\odot}$) galaxies at $z<1$ that host AGN, detected via their broadband optical variability in the VST-COSMOS survey. VST-COSMOS provides a nearly-identical single visit depth ($r$ $\sim$ 24.6 mag) and temporal baseline (eleven years) as the forthcoming Legacy Survey of Space and Time (LSST), albeit in a much smaller 1 deg$^2$ footprint (four orders of magnitude smaller than that of the LSST). We compare the properties (morphologies, the presence of interactions, rest-frame colours and environment) of our AGN to galaxies in a control sample, which are drawn from the non-variable population and matched in redshift and stellar mass to their AGN counterparts. The fraction of AGN with early-type morphology ($\sim$55 per cent) and the fraction that is interacting ($\sim$23 per cent) are similar to what is observed in the controls, suggesting that these AGN are not primarily triggered by interactions. Similarly, the AGN and controls do not show strong differences in their rest-frame $(u-z)$ colours or local environment, suggesting that neither the recent star formation histories nor the surroundings of the AGN are strongly atypical of the general galaxy population. This study provides a glimpse into forthcoming AGN science using the LSST. With vastly improved statistics, LSST will offer unprecedented insights into AGN demographics, host-galaxy evolution and the processes that fuel supermassive black holes, potentially reshaping our understanding of their place in the Universe.

Figures (6)

• Figure 1: Mid-infrared diagram where colours are obtained as ratios of the fluxes from the four channels of the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH; Weaver2022). The blue dots indicate the sources in our control sample. The solid line defines the AGN locus based on Donley2012.
• Figure 2: Example images of galaxies in our study (each image is 10 x 10 arcsec$^2$). The images are taken using the HST-ACS F814W ($I$-band) filter. The top and bottom rows show early-type galaxies (ETGs) and late-type galaxies (LTGs) respectively. Interacting systems are indicated using an orange filled square in the lower right-hand corner of the image. Galaxy 4 is involved in an ongoing merger with a companion to the north and exhibits a tidal bridge. Galaxy 5 is a post-merger system which shows tidal features to the west. Galaxies 9 and 10 both appear visibly asymmetric due to a recent or ongoing interaction.
• Figure 3: Distributions of redshift (top) and stellar mass (bottom) of our variability-selected AGN (red) and massive galaxies from the parent COSMOS2020 catalogue (black). Both samples have been normalised to a peak value of 1.
• Figure 4: Redshift vs stellar mass for our galaxies. The heatmap indicates the parent COSMOS2020 galaxy population. AGN are shown using filled symbols, with the different morphological classes shown colour-coded (see legend). Interacting galaxies are indicated using crosses. The dashed orange line indicates the redshift at which a galaxy population of a given stellar mass is likely to be mass-complete at the depth of the VST-COSMOS survey (see text in Section \ref{['sec:completeness']} for details).
• Figure 5: Left: Rest-frame $(u-z)$ colour vs stellar mass. The heatmap indicates the parent COSMOS2020 galaxy population. AGN are shown using filled symbols, with the different morphological classes shown colour-coded (see legend). Interacting galaxies are indicated using crosses. Right: Distributions of the rest-frame $(u-z)$ colours of our AGN and control samples. The dashed vertical lines represent median values while the error bars indicate the uncertainty on the median, calculated via bootstrapping.