Journal Article

Cold accretion discs and lineless quasars

Ari Laor and Shane W. Davis

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 1, pages 681-688
Published in print October 2011 | ISSN: 0035-8711
Published online October 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19310.x
Cold accretion discs and lineless quasars

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The optical–UV continuum of quasars is broadly consistent with the emission from a geometrically thin optically thick accretion disc (AD). The AD produces the ionizing continuum which powers the broad and narrow emission lines. The maximum AD effective temperature is given by , where M is the black hole mass, the accretion rate and fmax is set by the black hole spin a*. For a low enough value of , the AD may become too cold to produce ionizing photons. Such an object will form a lineless quasar. This occurs for a local blackbody (BB) AD with a luminosity Lopt= 1046 erg s−1 for M > 3.6 × 109 M, when a*= 0, and for M > 1.4 × 1010 M, when a*= 0.998. Using the AD-based , derived from M and Lopt, and the reverberation-based M, derived from Lopt and the Hβ full width at half-maximum, v, gives Teff, maxL−0.13optv−1.45. Thus, Teff, max is mostly set by v. Quasars with a local BB AD become lineless for v > 8000 km s−1, when a*= 0, and for v > 16 000 km s−1, when a*= 0.998. Higher values of v are required if the AD is hotter than a local BB. The AD becoming non-ionizing may explain why line-emitting quasars with v > 10 000 km s−1 are rare. Weak low-ionization lines may still be present if the X-ray continuum is luminous enough, and such objects may form a population of weak line quasars (WLQ). If correct, such WLQ should show a steeply falling spectral energy distribution (SED) at λ < 1000 Å. Such an SED was observed by Hryniewicz et al. in SDSS J094533.99+100950.1, a WLQ observed down to 570 Å, which is well modelled by a rather cold AD SED. UV spectroscopy of z∼ 1–2 quasars is required to eliminate potential intervening Lyman limit absorption by the intergalactic medium and to explore if the SEDs of lineless quasars and some additional WLQ are also well fitted by a cold AD SED.

Keywords: accretion, accretion discs; black hole physics; galaxies: active; quasars: general

Journal Article.  7526 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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