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The origin of compact galaxies with anomalously high black hole masses

Barber, C and Schaye, J and Bower, RG and Crain, RA and Schaller, M and Theuns, T (2016) The origin of compact galaxies with anomalously high black hole masses. Monthly Notices of the Royal Astronomical Society, 460 (1). pp. 1147-1161. ISSN 0035-8711

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Abstract

Observations of local galaxies harbouring supermassive black holes (BHs) of anomalously high mass, M_BH, relative to their stellar mass, M_star, appear to be at odds with simple models of the co-evolution between galaxies and their central BHs. We study the origin of such outliers in a Lambda cold dark matter context using the EAGLE cosmological, hydrodynamical simulation. We find 15 'M_BH(M_star)-outlier' galaxies, defined as having M_BH more than 1.5 dex above the median M_BH(M_star) relation in the simulation, M_{BH,med}. All M_BH(M_star)-outliers are satellite galaxies, typically with M_star ~ 10^10 M_sun and M_BH ~ 10^8 M_sun. They have all become outliers due to a combination of tidal stripping of their outer stellar component acting over several Gyr and early formation times leading to rapid BH growth at high redshift, with the former mechanism being most important for 67 per cent of these outliers. The same mechanisms also cause the M_BH(M_star)-outlier satellites to be amongst the most compact galaxies in the simulation, making them ideal candidates for ultracompact dwarf galaxy progenitors. The 10 most extreme central galaxies found at z=0 (with log_{10}(M_BH/M_{BH,med}) in [1.2, 1.5]) grow rapidly in M_BH to lie well above the present-day M_BH-M_star relation at early times (z > 2), and either continue to evolve parallel to the z=0 relation or remain unchanged until the present day, making them 'relics' of the high-redshift universe. This high-z formation mechanism may help to explain the origin of observed M_BH(M_star)-outliers with extended dark matter haloes and undisturbed morphologies.

Item Type: Article
Additional Information: This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record MNRAS (July 21, 2016) 460 (1): 1147-1161. is available online at: http://dx.doi.org/10.1093/mnras/stw1018
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Related URLs:
Date Deposited: 08 Jun 2016 09:02
Last Modified: 08 Jun 2016 09:02
DOI or Identification number: 10.1093/mnras/stw1018
URI: http://researchonline.ljmu.ac.uk/id/eprint/3720

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