Dr. Lia F. Sartori

::: Research :::

Below you can find a summary of my recent projects, the peer reviewed papers I am involved in, and a list of the seminars and conferences I have given and attended in the past.

:: Forward modelling AGN variability on multiple timescales ::
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I am developing a simple model to link and describe the AGN variability features observed at multiple timescales, from days to Gyr, and for different objects, based on the distribution of Eddington ratio among the galaxy population.

A model for AGN variability on multiple timescales (Sartori et al. 2018b)
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In this letter we presented a framework to test if and how variability at different timescales, and in different AGN, can be linked and explained based on the distribution of Eddington ratio (ER) among the galaxy population. Specifically, we proposed a forward modelling approach to simulate the evolution of the AGN luminosity with time based on the probability density function (PDF) and the power spectral density (PSD) of the ER distribution. Motivated by general galaxy population properties, we proposed that the PDF may be inspired by the ER distribution function (ERDF), and that a single (or limited number of) ERDF+PSD set may explain all observed variability features. This may imply that the observed AGN diversity is simply the result of generating light curves from the same set of statistical properties. After outlining the framework and the model, we presented a compilation of variability measurements from the literature and from our own measurements covering many orders of magnitude both in time lags and variability amplitude. We then combined the variability measurements on a structure function (SF) plot ranging from days to Gyr, which provides a novel and valuable overview of the AGN variabiltiy phenomenon.

:: Constraining long timescales AGN variability with multiwavelength observations ::
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I am using multiwavelength observations to study the Voorwerpjes galaxies, our best laboratory to constrain AGN variability on 10⁴-10⁵ yr timescales.

Joint NuSTAR and Chandra analysis of the obscured quasar in the IC 2497 - Hanny's Voorwerp system (Sartori et al. 2018a)
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Previous studies suggested that IC 2497, the galaxy related to the AGN photoionised cloud called Hanny's Voorwerp, hosts a quasars which significantly faded in the last ~10⁵ yr. In order to reliably determine the current intrinsic luminosity of the AGN in IC 2497, and therefore the magnitude of the claimed drop in luminosity, we requested and obtained hard X-ray observations from the NuSTAR telescope. The analysis of these new data, together with soft X-ray observations, optical longslit spectroscopy and narrowband imaging, as well as mid-IR photometry, showed that the AGN is highly obscured (Compton thick) and that its luminosity dropped by a factor ~50 in the last ~10⁵ yr. These findings confirmed the idea that IC 2497 hosts a fading AGN, although the magnitude of the drop in luminosity is significantly lower than previously thought. Based on repeated mid-IR and X-ray observations, and on the optical line rations observed in the galaxy, we argued that the AGN in IC 2497 should not be classified as a changing-look AGN. Rather, the measured change in Eddington ratio supports the idea that IC 2497 is entering a regime where it is switching from a radiatively efficient to a radiatively inefficient accretion state, in a similar way as observed in X-ray binaries.
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Extended X-ray emission in the IC 2497 - Hanny's Voorwerp system: energy injection in the gas around a fading AGN
(Sartori et al. 2016)
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We used deep Chandra soft X-ray observations to study the environment of the fading AGN in IC 2497. Thanks to the high spatial resolution of Chandra, ~0.5 arcsec, we found evidence of extended X-ray emission from hot gas in the center of the galaxy, with a teperature structure suggesting the presence of a central bubble or cavity. A possible scenario is that this bubble is inflated by the fading AGN which, after changing accretion state, is now in a kinetic mode. Other possibilities are that the bubble has been inflated by the past luminous quasar, or that the temperature gradient is due to a shock front from a superwind driven by the AGN. In the paper we discussed the possible scenarios and the implications for the AGN-host galxy interaction, as well as an analogy between AGN and X-ray binaries lifecycles. We speculated that an AGN could inject mechanical energy into the host galaxy at the end of every lifecycle, and thus provide a source for mechanical feedback which may affect star formation.

Optical, near-IR, and sub-mm IFU observations of the nearby dual active galactic nulei MRK 463 (Treister, Privon, Sartori et al. 2018)
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Treister et al. 2018 presented a multiwavelength study of the VP galaxy Mkn 463. As part of this work, I developed a photoionisation analysis method aimed at determining the past AGN luminosity, and therefore reconstructing the long timescales AGN light curves, using integral field units (IFU) spectroscopy. By comparing the optical line ratios measured in the VP cloud to photoionisation models we determined that the AGN dropped in luminosity by a factor of 3-20 over the last ~4 x 10⁴ yr. Besides being an important contribution to the published study, this analysis provides a proof of concept for my AGN light curve reconstruction method, which I will apply to IFU observations of other galaxies with extended AGN photoionised gas.

:: Other projects ::
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For my Master Thesis I applied different selection techniques to search for and investigate AGN activity in local dwarf galaxies.

The search for active black holes in nearby low-mass galaxies using optical and mid-IR data (Sartori et al. 2015)
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In this project we applied different AGN selection techniques to systematically search for active SMBHs in a sample of ~50000 nearby dwarf galaxies in the Solan Digital Sky Survey. Combining different optical emission line diagnostic diagrams and mid-IR selection cuts we measured an active SMBH occupation fraction of ~1%, significantly lower than the overall SMBH occupation fraction predicted by the different SMBH seed models (>50-90% depending on the model). This means that either SMBHs in dwarf galaxies are mostly inactive, or that current seed formation models significantly overpredict the SMBH occupation fraction in the low-mass regime. In addition, we found that different selection criteria select AGN in host galaxies with different physical properties such as stellar mass and optical colors, and caution that in the low-mass regime the standard mid-IR selection techniques) may be strongly contaminated by young stars and tehrefore unreliable. Finally, our AGN candidates sample will be very valuable for future follow-up studies aiming at understanding the relation between SMBHs and host galaxies in the low-mass, poorly studied regime.

:: Publications ::

• A Forward Modeling Approach to AGN Variability -- Method Description and Early Applications
  Sartori, L. F.; Trakhtenbrot, B.; Schawinski, K.; Caplar, N.; Treister, E.; Zhang, C.; ApJ, 883, 139 (2019)
• A model for AGN variability on multiple time-scales
  Sartori, L. F.; Schawinski, K.; Trakhtenbrot, B.; Caplar, N.; Treister, E; Koss, M. J.; Urry, C. M.; Zhang, C. E.; MNRAS, 476, 34L (2018)
• Joint NuSTAR and Chandra analysis of the obscured quasar in IC 2497 - Hanny’s Voorwerp system
  Sartori, L. F.; Schawinski, K.; Koss, M. J.; Ricci, C.; Treister, E.; Stern, D.; Lansbury, G.; Maksym, W. P.; Balokovic, M.; Gandhi, P.; Keel, W. C.; Ballantyne, D. R.; MNRAS, 474, 2444 (2018)
• Extended X-ray emission in the IC 2497 - Hanny’s Voorwerp system: energy injection in the gas around a fading AGN
  Sartori, L. F.; Schawinski, K.; Koss, M. J.; Treister, E.; Maksym, W. P.; Keel, W. C.; Urry, C. M.; Lintott, C. J.; Wong, O. I.; MNRAS, 457, 3629 (2016)
• The search for active black holes in nearby low-mass galaxies using optical and mid-IR data
  Sartori, L. F.; Schawinski, K.; Treister, E.; Trakhtenbrot, B.; Koss, M. J.; Shirazi, M.; Oh, K.; MNRAS, 454, 3722 (2015)

• BAT AGN Spectroscopic Survey - XIII. The nature of the most luminous obscured AGN in the low-redshift universe
  Baer, R. E.; Trakhtenbrot, B.; Oh, K.; Koss, M. J.; Wong, O. I.; Ricci, C.; Schawinski, K.; Weigel, A. K..; Sartori, L. F.; Ichikawa, K.; Secrest, N. J.; Stern, D.; Pacucci, F.; Mushotzky, R.; Powell, M. C.; Ricci, F.; Sani, E.; Smith, K. L.; Harrison, F. A.; Lamperti, I.; Urry, C. M.; MNRAS, 489, 3073 (2019)
• On the Prevalence of Supermassive Black Holes over Cosmic Time
  Buchner, J.; Treister, E.; Bauer, F. E.; Sartori, L. F.; Schawinski, K.; ApJ, 874, 117 (2019)
• A population of luminous accreting black holes with hidden mergers
  Koss, M. J.; Blecha, L.; Bernhard, P.; Hung, C.-L.; Lu, J. R.; Trakhtenbrot, B.; Treister, E.; Weigel, A.; Sartori, L. F.; Mushotzky, R.; Schawinski, K.; Ricci, C.; Veilleux, S.; Sanders, D. B.; Nature, 563, 214 (2018)
• PSFGAN: a generative adversarial network system for separating quasar point sources and host galaxy light
  Stark, D.; Launet, B.; Schawinski, K.; Zhang, C. E.; Koss, M. K.; Turp, M. D.; Sartori, L. F.; Zhang, H.; Chen, Y.; Weigel, A. K.; MNRAS, 477, 2513 (2018)
• Optical, Near-IR, and Sub-mm IFU Observations of the Nearby Dual Active Galactic Nuclei MRK 463
  Treister, E.; Privon, G. C.; Sartori, L. F.; Nagar, N.; Bauer, F. E.; Schawinski, K.; Messias, H.; Ricci, C.; U, V.; Casey, C.; Comerford, J. M.; Muller-Sanchez, F.; Evans, A. S.; Finlez, C.; Koss, M.; Sanders, D. B.; Urry, C. M.; ApJ, 854, 83 (2018)
• BAT AGN Spectroscopic Survey. I. Spectral measurements, derived quantities, and AGN demographics
  Koss, M.; Trakhtenbrot, B.; Ricci, C.; Lamperti, I.; Oh, K.; Berney, S.; Schawinski, K.; Balokovic, M.; Baronchelli, L.; Crenshaw, D. M.; Fischer, T.; Gehrels, N.; Harrison, F.; Hashimoto, Y.; Hogg, D.; Ichikawa, K.; Masetti, N.; Mushotzky, R.; Sartori, L. F.; Stern, D.; Treister, E.; Ueda, Y.; Veilleux, S.; Winter, L.; ApJ, 850, 74 (2017)
• Active galactic nuclei from He II: a more complete census of AGN in SDSS galaxies yields a new population of low-luminosity AGN in highly star-forming galaxies
  Baer, Rudolf E.; Weigel, A. K.; Sartori, L. F.; Oh, K.; Koss, M. J.; Schawinski, K.; MNRAS, 466, 2879 (2017)
• Fading AGN Candidates: AGN Histories and Outflow Signatures
  Keel, W. C.; Lintott, C. J.; Maksym, W. P.; Bennert, V. N.; Chojnowski, S. D.; Moiseev, A.; Smirnova, A.; Schawinski, K.; Sartori, L. F.; Urry, C. M.; ApJ, 835, 256 (2017)
• Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ∼10⁵ yr
  Schawinski, K.; Koss, M. J.; Berney, S.; Sartori, L. F.; MNRAS, 451, 2517 (2015)

:: Seminars ::

• Space Telescope Science Institute, AGN Journal Club, Baltimore, MD, September 2018
• Space Telescope Science Institute, Galaxy Journal Club, Baltimore, MD, September 2018
• Princeton University, Galread extragalactic discussion group, Princeton, NJ, September 2018
• University of California Santa Cruz, FLASH talk, Santa Cruz, CA, September 2018
• Caltech, IPAC science talk, Pasadena, CA, September 2018
• University of California Los Angeles, Lunch talk, Los Angeles, CA, September 2018
• University of Washington, DIRAC seminar, Seattle, WA, September 2018
• Vatican Observatory, Special seminar, Castel Gandolfo, Italy, May 2018
• Pontificia Universidad Católica de Chile, AGN meeting, Santiago de Chile, Chile, January 2018
• Kyoto University, Kyoto - Zurich AGN workshop, Kyoto, Japan, December 2017
• Istituto Ricerche Solari Locarno, Special seminar, Locarno Monti, Switzerland, November 2017
• ETH Zurich, Research seminar, Zurich, Switzerland, December 2016
• Pontificia Universidad Católica de Chile, Galaxy evolution discussion group, Santiago de Chile, Chile, December 2016
• Pontificia Universidad Católica de Chile, Seminar, Santiago de Chile, Chile, November 2016
• University of Bern, Astronomy seminar, Bern, Switzerland, September 2016
• University of Manchester, Group lunch, Manchester, UK, June 2016
• University of Yale, Yale BH group meeting, New Haven, CT, March 2016
• University of Yale, Yale Galaxy Journal Club, New Haven, CT, March 2016
• Harvard Center for Astrophysics, Special seminar, Cambridge, MA, March 2016
• University of Alabama, Astronomy seminar, Tuscaloosa, AL, March 2016
• Università di Milano Bicocca, Astronomy seminar, Milano, Italy, October 2015
• Università di Ferrara, Astronomy seminar, Milano, Italy, April 2015
• INAF Bologna, Astronomy seminar, Bologna, Italy, April 2015
• Gemini Observatory, AGN ionisation workshop, La Serena, Chile, March 2015
• ETH Zurich, Research seminar, Zurich, Switzerland, November 2014

:: Conferences and meetings ::

• Local hard X-ray selected AGN across the multi-wavelength spectrum, invited talk, Santiago de Chile, Chile, March 2018
• 228th American Astronomical Society meeting, poster and oral constribution for BASS special session, National Harbor, MD, January 2018
• Swiss Society for Astrophysics and Astronomy General Assembly 2017, participant, Zurich, Switzerland, October 2017
• Unveiling the physics behind extreme AGN variability, talk, St. Thomas, U.S. Virgin Islands, July 2017
• Swiss Society for Astrophysics and Astronomy General Assembly 2016, participant, Davos, Switzerland, October 2016
• AGN what’s in a name, poster, Garching, Germany, June 2016
• 2nd Young Astronomers on Galactic Nuclei, invited talk, Paris, France, November 2016
• X-ray view of BH activity in the local Universe, talk, Zurich, Switzerland, March 2016
• 1st Young Astronomer on Galactic Nuclei, invited talk, Milano, Italy, October 2015
• Demographycs and environment of AGN from multiwavelength surveys, talk, Chania, Crete, September 2015
• Sociedad Chilena de Astronomía meeting 2014, poster, Puerto Varas, Chile, March 2014
• Unveiling the AGN - galaxy evolution connection, poster, Puerto Varas, Chile, March 2014
• 225th American Astronomical Society meeting, poster, Seattle, WA, January 2015

::: Outreach :::

I am strongly involved in outreach, science communication and science promotion, especially through workshops and public talks.
In addition, I am the responsible and project manager for the Italian speaking Switzerland at the Swiss Youth in Science (Schweizer Jugend forscht) Foundation.

:: Workshops ::

• Das Ungeheuer in der Milchstrasse (in German), ETHZ Treffpunkt Sciencecity
• Mit Big Data den Geheimnissen des Universums auf der Spur (in German and English), ETHZ Scientifica
• Licht und Astronomie – Geschichte und Geheimnisse des Universums (in German and English), ETHZ Scientifica
• Vom Urknall bis in den Kosmos (in German), ETHZ Zukunftstag
• Eine kleine Reise durchs Sonnensystem (in German), boys@science, Swiss Youth in Science
• Astrophysik: Geschichte und Geheimnisse des Universums (in German), Astronomy workshop for high school students, Swiss Youth in Science
• Astrophysik: Geschichte und Geheimnisse des Universums (in German), MINT-Day high school Uster
• Galaxy Zoo: uno sguardo sull’Universo (in Italian), Astronomy workshop for high school students at Liceo di Lugano 2

:: Public Talks ::

• Sternennacht auf dem Chäserrugg - Eine Reise durch das Universum (in German), Chäserrugg, Switzerland
• Un viaggio dalla materia oscura ai buchi neri (in Italian), Bellinzona, Switzerland
• Un viaj intra galassie e bogg negar (in Ticinese Dialect), Milano, Italy
• A Ph.D. in Black Holes and Galaxy Evolution (in English), Zürich, Switzerland

:: My work in the news ::

• The AGN cake, STARtorialist
• A Black Hole Story Told by a Cosmic Blob and Bubble, Chandra website
• Here’s what really happened to Hanny’s Voorwerp, ScienceNews