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Register a new userFading AGN Candidates: AGN Histories and Outflow Signatures
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We consider energy budgets and radiative history of 8 fading AGN, identified from mismatch between the ionizion of very extended (>10 kpc) ionized clouds and the luminosity of the nucleus viewed directly. All show significant fading on ~50,000-year timescales. We explore the use of minimum ionizing luminosity Q derived from photoionization balance in the brightest pixels in H-alpha at each projected radius. Tests using PG QSOs, and one target with detailed photoionization modeling, suggest that we can derive useful histories of individual AGN; the minimum ionizing luminosity is always an underestimate and subject to fine structure in the ionized material. These tests suggest that the underestimation from the upper envelope of Q values is roughly constant for a given object. These AGN show rapid drops and standstills; the common feature is a rapid drop in the last 20,000 years before our view of the nucleus. E-folding timescales are mostly thousands of years, with a few episodes as short as 400. In the limit of largely obscured AGN, we find additional evidence for fading, comparing lower limits from recombination balance and the maximum luminosities derived from from infrared fluxes. We compare these long-term light curves to simulations of AGN accretion; the strongest variations on these timespans are seen in models with strong and local feedback. Gemini integral-field optical spectroscopy shows a very limited role for outflows in these structures. While rings and loops of emission are common, their kinematic structure shows some to be in regular rotation. UGC 7342 exhibits local signatures of outflows <300 km/s, largely associated with very diffuse emission. Only in the Teacup AGN do we see outflow signatures of order 1000 km/s. Clouds around these fading AGN consist largely of tidal debris being externally illuminated but not displaced by AGN outflows. (Abridged)
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We present narrow- and medium-band HST imaging, with additional supporting ground-based data, for 8 galaxies identified as hosting fading AGN. These have AGN-ionized gas projected >10 kpc from the nucleus, and significant shortfall of ionizing radiation between the distant gas and the AGN, indicating fading AGN on ~50,000-year timescales. Every system shows evidence of ongoing or past interactions; a similar sample of obscured AGN with extended ionized clouds shares this incidence of disturbances. Several systems show multiple dust lanes in different orientations, broadly fit by differentially precessing disks of accreted material ~1.5 Gyr after initial arrival. The gas has lower metallicity than the nuclei; three systems have abundances uniformly well below solar, consistent with an origin in tidally disrupted low-luminosity galaxies, while some systems have more nearly solar abundances (accompanied by such signatures as multiple Doppler components), which may suggest redistribution of gas by outflows within the host galaxies themselves. These aspects are consistent with a tidal origin for the extended gas in most systems, although the ionized gas and stellar tidal features do not always match closely. In contrast to clouds near radio-loud AGN, these are dominated by rotation, in some cases in warped disks. Outflows are important only in localized regions near some of the AGN. In UGC 7342 and UGC 11185, luminous star clusters are seen within projected ionization cones, potentially triggered by outflows. As in the discovery example Hannys Voorwerp/IC 2497, some clouds lack a strong correlation between H-alpha surface brightness and ionization parameter, indicating unresolved fine structure. Together with thin coherent filaments spanning several kpc, persistence of these structures over their orbital lifetimes may require a role for magnetic confinement. (Abridged)
The relationship between active galactic nuclei activity and environment has been long discussed, but it is unclear if these relations extend into the dwarf galaxy mass regime -- in part due to the limits in both observations and simulations. We aim to investigate if the merger histories and environments are significantly different between AGN and non-AGN dwarf galaxies in cosmological simulations, which may be indicative of the importance of these for AGN activity in dwarf galaxies, and whether these results are in line with observations. Using the IllustrisTNG flagship TNG100-1 run, 6,771 dwarf galaxies are found with 3,863 ($sim$57 per cent) having some level of AGN activity. In order to quantify `environment, two measures are used: 1) the distance to a galaxys 10th nearest neighbour at 6 redshifts and 2) the time since last merger for three different minimum merger mass ratios. A similar analysis is run on TNG50-1 and Illustris-1 to test for the robustness of the findings. Both measures yield significantly different distributions between AGN and non-AGN galaxies; more non-AGN than AGN galaxies have long term residence in dense environments while recent ($leq 4 text{ Gyr}$) minor mergers are more common for intermediate AGN activity. While no statements are made about the micro- or macrophysics from these results, it is nevertheless indicative of a non-neglible role of mergers and environments.
Warm absorber (WA) is an ionised gas present in the line of sight to the AGN central engine. The effect of the absorber is imprinted in the absorption lines observed in X-ray spectra of AGN. In this work, we model the WA in Seyfert 1 galaxy Mrk 509 using its recently published shape of broad band spectral energy distribution (SED) as a continuum illuminating the absorber. Using the photoionization code {sc Titan}, recently we have shown that the absorption measure distribution (AMD) found for this object can be successfully modelled as a single slab of gas in total pressure (radiation+gas) equilibrium, contrary to the usual models of constant density multiple slabs. We discuss the transmitted spectrum that would be recorded by an observer after the radiation from the nucleus passes through the WA.
A new class of quiescent galaxies harboring possible AGN-driven winds has been discovered using spatially resolved optical spectroscopy from the ongoing SDSS-IV MaNGA survey. These galaxies, termed red geysers, constitute $5-10%$ of the local quiescent population and are characterized by narrow bisymmetric patterns in ionized gas emission features. Cheung et al. argued that these galaxies host large-scale AGN-driven winds that may play a role in suppressing star formation at late times. In this work, we test the hypothesis that AGN activity is ultimately responsible for the red geyser phenomenon. We compare the nuclear radio activity of the red geysers to a matched control sample with similar stellar mass, redshift, rest frame $NUV-r$ color, axis ratio and presence of ionized gas. We have used the 1.4 GHz radio continuum data from VLA FIRST survey to stack the radio flux from the red geyser and control samples. In addition to a 3 times higher FIRST detection rate, we find that red geysers have a 5$sigma$ higher level of average radio flux than control galaxies. After restricting to rest-frame $NUV-r$ color $>$ 5 and checking mid-IR WISE photometry, we rule out star formation contamination and conclude that red geysers are associated with more active AGN. Red geysers and a possibly-related class with disturbed H$alpha$ emission account for 40% of all radio-detected red galaxies with $rm log~(M_star/M_odot) < 11$. Our results support a picture in which episodic AGN activity drives large-scale-relatively weak ionized winds that may provide a feedback mechanism for many early-type galaxies.
We used data from the QUEST-La Silla Active Galactic Nuclei (AGN) variability survey to construct light curves for 208,583 sources over $sim 70$ deg$^2$, with a a limiting magnitude $r sim 21$. Each light curve has at least 40 epochs and a length of $geq 200$ days. We implemented a Random Forest algorithm to classify our objects as either AGN or non-AGN according to their variability features and optical colors, excluding morphology cuts. We tested three classifiers, one that only includes variability features (RF1), one that includes variability features and also $r-i$ and $i-z$ colors (RF2), and one that includes variability features and also $g-r$, $r-i$, and $i-z$ colors (RF3). We obtained a sample of high probability candidates (hp-AGN) for each classifier, with 5,941 candidates for RF1, 5,252 candidates for RF2, and 4,482 candidates for RF3. We divided each sample according to their $g-r$ colors, defining blue ($g-rleq 0.6$) and red sub-samples ($g-r>0.6$). We find that most of the candidates known from the literature belong to the blue sub-samples, which is not necessarily surprising given that, unlike for many literature studies, we do not cut our sample to point-like objects. This means that we can select AGN that have a significant contribution from redshifted starlight in their host galaxies. In order to test the efficiency of our technique we performed spectroscopic follow-up, confirming the AGN nature of 44 among 54 observed sources (81.5% of efficiency). From the campaign we concluded that RF2 provides the purest sample of AGN candidates.
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