“Giant Broad-line Regions in Low-luminosity Active Galactic Nuclei”

Nick Devereux (Embry-Riddle Aeronautical University)

Abstract:
The nearby lenticular galaxy NGC 3998 hosts the best example of an AGN fueled by chaotic cold accretion (CCA) whereby 104 K gas condenses out of a 106 K circumgalactic halo and falls toward the central supermassive BH. Photoionization modelling of visible spectra obtained with STIS aboard HST indicate that the central UV—X-ray source of NGC 3998 has ionized a large spherical volume (~ 7pc in radius) of low density (~104 cm-3) gas of such extraordinarily low metallicity (~ 1/100 solar) as to dictate a circumgalactic origin. The gas pressure gradient in the H+ region of the best fitting photoionization model is several orders of magnitude smaller than required for hydrostatic equilibrium. Thus, an inflow at the free-fall velocity is inevitable, consistent with the distinctly triangular shape observed for the broad Balmer emission lines. In general, CCA can explain the low duty-cycle observed for AGN and, more specifically for NGC 3998, redirection of the jets powering the larger scale radio lobes. NGC 3998 is just one of several nearby, non-reverberating, low-luminosity AGN associated with an unusually large broad-line region. Other nearby examples will be discussed including M81, NGC 3227, NGC 3516, NGC 4051 and NGC 4203.

“Finding New Activity-Sensitive Spectral Lines: Combined Visual Identification and an Automated Pipeline Find a Set of 40 Activity Indicators”

Alex Wise (University of Delaware)

Abstract:
Stellar activity causes radial velocity variations that can either mimic planets or hide their existence. To verify the authenticity of newly discovered planets, observers may search for periodicity in spectral lines such as Ca H & K and H ⍺, then mask out any radial velocity signals that match the activity period or its harmonics. However, not every spectrograph includes Ca H & K, and redder activity indicators are needed for planet searches around low-mass stars. Here we show how new activity indicators can be identified by correlating spectral line depths with a well-known activity index. We apply our correlation methods to archival HARPS spectra of ε Eri and ⍺ Cen B and use the results from both stars to generate a master list of activity-sensitive lines in the visible spectrum. Our newly discovered activity indicators can in turn be used as benchmarks to extend the list of known activity-sensitive lines toward the infrared or UV. With recent improvements in spectrograph technology, stellar activity is now the biggest noise source in planet searches. Our suite of > 40 activity-sensitive lines is a first step toward allowing planet hunters to access all of the information on stellar activity contained in each spectrum.