HSTIMAGING OF FADING AGN CANDIDATES. I. HOST-GALAXY PROPERTIES AND ORIGIN OF THE EXTENDED GAS

Abstract

We present narrow-and medium-band HST imaging, with additional supporting ground-based imaging, spectrophotometry, and Fabry-Perot interferometric data, for eight galaxies identified as hosting fading AGN. These are selected to have AGN-ionized gas projected > 10 kpc from the nucleus, and energy budgets with a significant shortfall of ionizing radiation between the requirement to ionize the distant gas and the AGN as observed directly, indicating fading of the AGN on ≈ 50, 000-year timescales. This paper focuses on the host-galaxy properties and origin of the gas. In every galaxy, we identify evidence of ongoing or past interactions, including tidal tails, shells, and warped or chaotic dust structures; a similarly-selected sample of obscured AGN with extended ionized clouds shares this high incidence of disturbed morphologies. Several systems show multiple dust lanes in different orientations, broadly fit by differentially precessing disks of accreted material viewed ∼ 1.5 Gyr after its initial arrival. The host systems are of early Hubble type; most show nearly pure de Vaucouleurs surface-brightness profiles and Sersic indices appropriate for classical bulges, with one S0 and one SB0 galaxy. The gas has systematically 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 outfows 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. Unlike extended emission regions around many radio-loud AGN, these clouds are kinematically dominated by rotation, in some cases in warped disks. Outflows can play important kinematic roles only in localized regions near some of the AGN. We find only a few sets of young star clusters potentially triggered by 1 Visiting astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.2 SARA Observatory 2 AGN outflows. In UGC 7342 and UGC 11185, multiple luminous star clusters are seen just within the projected ionization cones, potentially marking star formation triggered by outflows. As in the discovery example Hanny's Voorwerp/IC 2497, there are regions in these clouds where lack of a strong correlation between Hα surface brightness and ionization parameter indicates that there is unresolved fine structure in the clouds. Together with thin coherent filaments spanning several kpc, persistence of these structures over their orbital lifetimes may require a role for magnetic confinement. Overall, we find that the sample of fading AGN occurs in interacting and merging systems, that the very extended...

Fading AGN Candidates: AGN Histories and Outflow Signatures∗

“…These data and our processing are described in detail by Keel et al (2015). For brevity, we truncate the SDSS designations of two systems without more common catalog names, SDSS J151004.01+074037.1 and SDSS J220141.64+115124.3.…”

“…Flux calibration used a single observation of HZ 44; as the observations occurred on dates from 13 August 2013 to 27 August 2014, this flux calibration will be only approximate. We do have external checks on line ratios based on previous long-slit spectroscopy (Keel et al 2012a, Keel et al 2015. Both UGC 11185 data runs were truncated by deteriorating conditions; we have only a single grating tilt for each IFU position, with 2 × 1200-second exposures, and these are affected significantly by cosmetic issues, so coverage of some spectral lines is incomplete and their flux scales are unreliable.…”

“…As listed in Table 3, we have observations centered on the nuclei of Mkn 1498 and NGC 5972, and sets of 2 overlapping offset fields for UGC 7342 and 11185; the table includes the requested offsets from the galaxy nucleus to the observed IFU center point. For the Teacup AGN, three offset fields cover virtually all the emission-line regions seen in the HST ACS images (Keel et al 2015). Similar data for this galaxy taken with the ESO VIMOS system have been shown by Harrison et al (2015); they describe results only for the [O III] emission lines, and the spatial scale of 0.67 per pixel limits their spatial resolution compared to these GMOS For straightforward comparison of emission-line properties over wide ranges in signal-to-noise ratio (SNR), we fit Gaussian profiles to the lines, constraining Hα and the adjacent [N II] lines to have the same full width at half-maximum (FWHM), with the same constraint on the [S II] doublet.…”

“…The points at ∆T = 0 in both versions of this figure are from the Hα fluxes at the nuclei (operationally within a projected radius of 1 from slit spectroscopy), in the approximation that they represent full covering of the instantaneous ionizing flux (strictly, a lower limit since we see that ionizing photons do escape the nuclear region). The flux values are taken from our spectroscopy in Keel et al (2015), as given in Table 5, including only the narrow component of the composite profile in Mkn 1498 for consistency. As a rough numerical description of these reconstructed histories, we collect in Table 4 a set of exponential-fit properties to segments of the smoothed version in Fig.…”

Concluding Remarks

“…Now many light echoes have been discovered, from episodic activity emanating from the compact regions in the center of galaxies, such as a SMBH (Keel et al 2012(Keel et al , 2015Gagne et al 2014). A burst of ultraviolet light emitted in a direction almost sideways to the line of sight, and coming from a central region of the galaxy often obscured to view, excites an echo in ionized Oxygen (O ++ ).…”