February 7, 2017
Gemini follows up on candidate galaxies with fading active galactic nuclei (AGN) first identified thanks to the Galaxy Zoo citizen science project. Researchers find that these galaxies show a significant reduction in ionizing photons within the last 20,000 years. Additionally, the gas clouds around these fading AGN are dominated by rotation, unlike those around radio-loud AGN, which are outflows coming from the nuclei.
Figure 1: Minimum ionizing luminosity of extended AGN-ionized clouds along the projected radius. These Hubble Space Telescope data show a luminosity drop in the last 20,000 years before our direct view of the nucleus, characteristic for all AGN of this study.
William C. Keel (University of Alabama) and his collaborators used Hα narrowband filters on the Hubble Space Telescope (HST) in conjunction with multi-object spectroscopy with the Gemini Multi-Object Spectrograph (GMOS) integral-field unit (IFU) on the Gemini North telescope on Maunakea to observe a set of fading active galactic nuclei (AGN). These fading AGN, identified in the Galaxy Zoo project, appear to have experienced a significant reduction in luminosity within 20,000 years or less based on this research.
This work focused on nine AGN which are accompanied by extended ionized gas clouds larger than 10 kiloparsecs from these galaxies’ nuclei. Because these clouds span galaxy scales (or even larger) they can implicitly tell us about the luminosity history of the AGN. A common feature in this subset of AGN is a radial drop in luminosity within 20,000 year timeframes which can be observed in Figure 1, where rapid drops in the number of ionizing photons is shown.
Figure 2: [O III] emission-line profiles from the GMOS IFU spectra overlaid on the HST [O III] images for Mkn 1498, one of the galaxies studied in this work. This galaxy displays a ringlike emission feature dominated by rotation with a velocity range of ±175 km/sec, (the 700 km/sec referenced in the legend refers to the entire velocity range shown in each miniature line profile plot).
The research team also used the GMOS IFU spectra to measure line ratios in these regions to probe their ionization mechanism and look for kinematic evidence of outflows – marked by large velocity ranges and often bipolar patterns in velocity – or other phenomena.
The team’s results confirm what was hinted at by earlier, and less complete data (by the same team), that these fading AGN are structurally different from radio-loud AGN which are dominated by outflows. Instead, these fading AGN are dominated by rotation and consist largely of externally illuminated tidal debris. The rotation can be observed in Figure 2, based on Gemini data which shows shifting of the [O III] emission line due to rotation of the gas cloud.
In summary, these results support the idea that AGN with extended emission regions are bright for periods 10,000-100,000 years, interspersed with substantially fainter episodes. Further work by the group will examine the ionization of circumnuclear gas, where direct AGN radiation may no longer be the most important source, and more detailed modeling of the gas motions in these galaxies.
This work is accepted in The Astrophysical Journal and the paper can be found here.
Also read this Galaxy Zoo blog posting describing this work.
See the full article here .
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Gemini’s mission is to advance our knowledge of the Universe by providing the international Gemini Community with forefront access to the entire sky.
The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai’i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.
The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country’s contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.