The Host Galaxies of AGN

TL;DR

The study uses a large SDSS galaxy sample to show that AGN, especially powerful ones, preferentially inhabit massive, high-density bulges with young stellar populations and extended star formation, while weaker AGN resemble normal early-type hosts. By leveraging [OIII]5007 as a proxy for AGN power and robust stellar population diagnostics (Dn4000, Hδ_A) alongside structural metrics (μ_*, C), the authors demonstrate a luminosity-dependent evolution in host properties, including post-starburst signatures and extended star formation beyond the nucleus. A key result is the lack of significant stellar-content differences between Type 2 AGN and QSOs at fixed L[OIII] and redshift, reinforcing AGN unification and suggesting a general link between bulge growth and black-hole fueling. The findings imply that powerful AGN trace a transient phase in massive galaxies where fueling and recent star formation co-occur, with implications for the co-evolution of galaxies and their central black holes across cosmic time.

Abstract

We examine the properties of the host galaxies of 22,623 narrow-line AGN with 0.02<z<0.3 selected from a complete sample of 122,808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [OIII]$λ$5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare to those of normal galaxies and how they depend on L[OIII]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early-types. The hosts of high- luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high- luminosity AGN have strong H$δ$ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and QSOs with the same [OIII] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [OIII] luminosities.

Figures (25)

• Figure 1: An example of a BPT (Baldwin, Phillips & Terlevich 1981) diagram in which we plot the emission line flux ratio [OIII]/H$\beta$ versus the ratio [NII]/H$\alpha$ for all the galaxies in our sample where all four lines are detected with $S/N >3$ (55,757 objects). The dotted curve shows the demarcation between starburst galaxies and AGN defined by Kewley et al (2001). The dashed curve shows our revised demarcation (equation 1). A total of 22,623 galaxies lie above dashed curve. Seyfert galaxies are often defined to have [OIII]/H$\beta > 3$ and [NII]/H$\alpha>0.6$, and LINERs to have [OIII]/H$\beta < 3$ and [NII]/H$\alpha > 0.6$. Our sample includes 2537 Seyferts and 10,489 LINERs according to this definition.
• Figure 2: The BPT diagram of Fig. 1 has been binned into fixed ranges in extinction-corrected [OIII] line luminosity. Note that luminosities are given in units of the bolometric solar luminosity ($3.826 \times 10^{33}$ erg s$^{-1}$). The dashed black curve shows our adopted demarcation between star-forming galaxies and AGN. The solid curve shows the Kewley et al demarcation. The dotted red lines mark the angle $\Phi=25^{\circ}$, which separates pure LINERs from Seyferts reasonably cleanly.
• Figure 3: Left: The [OIII] line luminosity is plotted as a function of the angle parameter $\Phi$ for all AGN lying above the Kewley et al. demarcation curve in Fig. 1. Right: The [OIII] line luminosity is plotted as a function of $D$, the distance from the locus of star-forming galaxies. In each plot the solid line shows the median relation as a function of $D$ or $\Phi$, while the dashed and dotted lines indicate the 16-84 and 2.5-97.5 percentiles in $\log$ L[OIII] respectively.
• Figure 4: A comparison of the stacked spectra of 500 AGN with [OIII] line luminosities in the range $10^8-10^9$ solar (black) with the stacked spectra of a set of matching galaxies (red). The blue line shows the residual when the galaxy spectrum is subtracted from the AGN spectrum.
• Figure 5: Top panels: The blue histogram shows the fraction of all galaxies in our sample with $S/N>3$ detections of [NII], H$\alpha$, [OIII] and H$\beta$ as a function of $\log M_*$ and concentration index $C$. The red histogram shows the fraction of all galaxies classified as AGN. The black histogram shows the fraction of emission-line galaxies classified as AGN. Middle left: The fraction of galaxies with $3\times10^{10} < M_* < 10^{11} M_{\odot}$ classified as AGN is plotted as a function of $z/z_{max}$ as a red line . The blue line is for galaxies with $3\times 10^{8} < M_* < 10^{9} M_{\odot}$. Middle right: The AGN fraction for galaxies with $3 \times 10^{10} < M_* < 10^{11} M_{\odot}$ and with $\log L[OIII] >$ 6.5 (red), 7(green) and 7.5(blue) is plotted against $z/z_{max}$. Bottom panel: The fraction of galaxies containing AGN with $\log L[OIII] >$ 7 is plotted as a function of $\log M_*$ and $C$.