The Sudden Death of the Nearest Quasar

Review of paper by Schawinski et al. 2009, ApJ, 724, L30 (http://labs.adsabs.harvard.edu/adsabs/abs/2010ApJ...724L..30S/)

Journal Club talk, Astrophysics Research Institute (ARI), Liverpool John Moores University (LJMU), 19th November, 2010

Transcript

1. The sudden death
   of the nearest quasar Schawinski et al. published 2010 October 26, ApJ, 724, L30 astro-ph November 1st ARI Journal Club, Nov 19, 2010
2. “Hanny’s Voorwerp” - What is it? DISCOVERY AND FOLLOW ON OBSERVATIONS 11 x 16 kpc 45,000 - 70,000 light years 1C 2497 Discovered by Hanny van Arkel (a Dutch school teacher), in the Galaxy Zoo project (visual inspection of SDSS images - morphological classification by the general public) (Lintott et al .2008, 2009) Large cloud of ionized gas embedded in a larger reservoir of atomic hydrogen with mass ~ 109 M⦿ 45,000-70,000 lyr from the nucleus of the spiral galaxy, IC 2497 (z ~0.05, Mr = - 22.1 mag) [O III] λ4949, λ5007 is the dominant emission line (i.e. bright in SDSS g only). Other high-ionization emission lines, [He II] and [Ne V] suggest photoionization from an AGN, rather than SF or shocks. No ionizing source in the cloud. Radial velocity (from [OIII] line) of Hanny’s Voorwerp is 269 ± 20 km s-1 less than IC 2497 (Vsys ~15,000 km s-1) . NOAO - WIYN (B,V & I) image
3. IC 2497 DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) Radio Source: VLA First Survey (Becker et al. 1995) L1.4 GHz = 1.00 ± 0.05 x 1023 WHz-1 Point source (VLBI resolution) and extended emission from WSRT. (Jozsa et al. 2009) S1.65GHz, VLBI = 1.09 ± 0.14 mJy [Jet**] Infrared Source: IRAS source @ 25, 60 and 100 μm LIR = 3.9 x 1011 L⦿ (LIRG? confusion?) L42-122μm = 6 x 1044 erg s -1 “FIR energy distribution suggests emission from a source which is colder than most AGN-donated sources” strong Hα emission (resolved) - double nucleus? - minor merger?
4. DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) Optical Imaging: Significant detection only in SDSS g-band Mg = 18.84, and narrowband Hα imaging. “Hanny’s Voorwerp” 4.9 kpc ‘bubble’ strong Hα emission
5. DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) “Hanny’s Voorwerp” Optical Imaging: Significant detection only in SDSS g-band Mg = 18.84, and narrowband Hα imaging. WHT Spectroscopy:
6. DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) “Hanny’s Voorwerp” Optical Imaging: Significant detection only in SDSS g-band Mg = 18.84, and narrowband Hα imaging. WHT Spectroscopy: “presence of [NeV] and [HeII] lines indicates the gas is more highly ionized than can be accounted for by starlight.”
7. DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) “Hanny’s Voorwerp” Optical Imaging: Significant detection only in SDSS g-band Mg = 18.84, and narrowband Hα imaging. WHT Spectroscopy: spectrum comparable to a low-ionization nuclear emission line region (LINER) or the narrow line region of a Seyfert galaxy
8. DISCOVERY AND FOLLOW ON OBSERVATIONS (Lintott et al. 2009) “Hanny’s Voorwerp” Optical Imaging: Significant detection only in SDSS g-band Mg = 18.84, and narrowband Hα imaging. SWIFT UV /X-ray data (contours): Continuum dominated emission? “Strong emission line in the Swift UV filter might be [CIII], but emission from this line is strongly weighted to higher density gas such as that found in AGN broad-line emission regions rather than the low- density gas in the Voorwerp.” Dust lane in IC 2497 does not obscure the bright emission from the core. NO X-ray emission. (limit of 3.3 x 1041 erg s-1 between 2 and 10 keV) Possible that the AGN emission is obscured towards us, but not towards Hanny’s Voowerp??
9. Te from [OIII] = 13500 ± 1300 K Internal reddening difficult to determine. Hα and Hβ are from different detectors. Line widths are < 100km s-1. Voorwerp has a global velocity gradient of 100 km s-1 [N II] / Hα [S II] / Hα [Ne V] / [Ne III] [O II] / [O III] line ratios, velocity widths + additional radio data + lots of other things I don’t really understand.. “Hanny’s Voorwerp” Emission-line ratios and diagnostics from WHT spectroscopy (Lintott et al. 2009) “...suggest that Hanny’s Voorwerp is a low-density gas-rich object, illuminated by a hard ionizing radiation field.... ...The source of gas may be IC 2497 itself, or the Voorwerp may be an independent dwarf galaxy.” - low derived metallicity “Voorwerp is embedded in a large cloud complex of HI, possibly being the remnant of an interaction of IC2497 with a galaxy group... or massive companion [warp]” (Jozsa et al. 2009)
10. Shock velocities of 400 km s-1 are needed to produce strong He II and [Ne V] emission, far beyond the observed radial velocity range of 90 km s -1. Shock models also predict much higher Te ~ 2 x 104 K The increase in ionization level across the Voorwerp, decreasing with distance from IC 2497 supports the hypothesis that the neighboring galaxy is the direct of indirect source of ionization. An AGN in IC 2497? This hypothesis is supported by the observed strength of high-ionization species such as He II and [Ne V], which distinguish this object from typical star-forming regions. Preliminary investigation using the Swift X-ray data (no X-ray emission), and FIR data ( LFIR =1.5 x 1044 erg s-1, order of magnitude less than the required energy) from IRAS concluded that; Possible Sources of Ionization SHOCKS: Could IC 2497 be the source of ionization? An AGN resides in IC 2497 and although heavily obscured, is still able to ionize the Voorwerp. OR The ionizing source is is no longer present and the Voorwerp represents the first instance of a light echo being seen from a quasar-luminous AGN.
11. More Recent Analysis - this paper Archival Infrared Data: Suzaku and XMM-Newton Data: Light from an obscured Lbol ~ 1046 erg s-1 (NH = 1024 cm-2) quasar at the center of IC 2497 should be re-emitted at mid- and far-IR wavelengths. A quasar with high obscuration should still be detected in hard X-rays (>10keV) where photoelectric absorption is minimal. IC 2497 observed with Suzaku X-ray Telescope: 75 ks observation. IC 2497 observed with XMM-Newton: 11ks observation. Sensitivity between 0.1- 7 keV. Source detected at 0.1-5 keV. L2-10keV = 4.2 x 1040 erg s-1 DON’T SEE THIS NO SIGNIFICANT DETECTION Two component fit to data: (1) thermal emission from a warm interstellar medium (2) power law from a low-luminosity AGN BUT ALSO consistent with emission from star formation and X-ray binaries.
12. Assuming a Compton-thick AGN both data limit the present-day hard X-ray luminosity to L15-30keV = 3.5 x 1042 erg s-1 , roughly 2 orders of magnitude more luminous than the observed soft X-ray power law, but not sufficient enough to ionize the Voorwerp. Assuming a low-luminosity AGN (radio and HST images support this) ---> XMM consistent with a central black hole with low accretion rate ---> must also be unobscured along our line of sight, apart from Galactic extinction (??) This relatively short shutdown timescale of a quasar may be due to a sharp increase in the fuel supply or a change in accretion rate, driven by disk instabilities. More Recent observations - this paper CONCLUSION: “the galaxy’s central engine has decreased it’s radiative output by at least two, and more likely by over four orders of magnitude since being in a much more luminous phase within the last 70,000 years.”