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Register a new userAGN All the Way Down? AGN-like Line Ratios are Common In the Lowest-Mass Isolated Quiescent Galaxies
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We investigate the lowest-mass quiescent galaxies known to exist in isolated environments ($mathrm{M^* = 10^{9.0-9.5} M_odot}$; 1.5 Mpc from a more massive galaxy). This population may represent the lowest stellar mass galaxies in which internal feedback quenches galaxy-wide star formation. We present Keck/ESI long-slit spectroscopy for 27 isolated galaxies in this regime: 20 quiescent galaxies and 7 star-forming galaxies. We measure emission line strengths as a function of radius and place galaxies on the Baldwin Phillips Terlevich (BPT) diagram. Remarkably, 16 of 20 quiescent galaxies in our sample host central AGN-like line ratios. Only 5 of these quiescent galaxies were identified as AGN-like in SDSS due to lower spatial resolution and signal-to-noise. We find that many of the quiescent galaxies in our sample have spatially-extended emission across the non-SF regions of BPT-space. When considering only the central 1$^{primeprime}$, we identify a tight relationship between distance from the BPT star-forming sequence and host galaxy stellar age as traced by $mathrm{D_n4000}$, such that older stellar ages are associated with larger distances from the star-forming locus. Our results suggest that the presence of hard ionizing radiation (AGN-like line ratios) is intrinsically tied to the quenching of what may be the lowest-mass self-quenched galaxies.
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To gain insights into long-term Active Galactic Nuclei (AGN) variability, we analyze an AGN sample from the Sloan Digital Sky Survey (SDSS) and compare their photometry with observations from the Hyper Suprime-Cam survey (HSC) observed $langle 14.85 rangle$ years after SDSS. On average, the AGN are fainter in HSC than SDSS. We demonstrate that the difference is not due to subtle differences in the SDSS versus HSC filters or photometry. The decrease in mean brightness is redshift dependent, consistent with expectations for a change that is a function of the rest-frame time separation between observations. At a given redshift, the mean decrease in brightness is stronger for more luminous AGN and for objects with longer time separation between measurements. We demonstrate that the dependence on redshift and luminosity of measured mean brightness decrease is consistent with simple models of Eddington ratio variability in AGN on long (Myr, Gyr) timescales. We show how our results can be used to constrain the variability and demographic properties of AGN populations.
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Galaxy mergers are key events in galaxy evolution, often causing massive starbursts and fueling active galactic nuclei (AGN). In these highly dynamic systems, it is not yet precisely known how much starbursts and AGN respectively contribute to the total luminosity, at what interaction stages they occur, and how long they persist. Here we estimate the fraction of the bolometric infrared (IR) luminosity that can be attributed to AGN by measuring and modeling the full ultraviolet to far-infrared spectral energy distributions (SEDs) in up to 33 broad bands for 24 merging galaxies with the Code for Investigating Galaxy Emission. In addition to a sample of 12 confirmed AGN in late-stage mergers, found in the $Infrared$ $Array$ $Satellite$ Revised Bright Galaxy Sample or Faint Source Catalog, our sample includes a comparison sample of 12 galaxy mergers from the $Spitzer$ Interacting Galaxies Survey, mostly early-stage. We perform identical SED modeling of simulated mergers to validate our methods, and we supplement the SED data with mid-IR spectra of diagnostic lines obtained with $Spitzer$ InfraRed Spectrograph. The estimated AGN contributions to the IR luminosities vary from system to system from 0% up to 91% but are significantly greater in the later-stage, more luminous mergers, consistent with what is known about galaxy evolution and AGN triggering.
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