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Register a new userMassive relic galaxies challenge the co-evolution of SMBHs and their host galaxies
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Added by
Anna Ferr\\'e-Mateu
Publication date
2015
fields
Physics
and research's language is
English
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We study a sample of eight massive galaxies that are extreme outliers (3-5$sigma$) in the M$_{bullet}$-M$_mathrm{bulge}$ local scaling relation. Two of these galaxies are confirmed to host extremely large super massive black holes (SMBHs), whereas the virial mass estimates for the other six are also consistent with having abnormally large SMBHs. From the analysis of their star formation histories and their structural properties we find that all these extreme outliers can be considered as relic galaxies from the early (z$sim$2) Universe: i.e. they are compact (R$_{mathrm{e}}$$<$2 kpc) and have purely old stellar populations (t$gtrsim$10 Gyr). In order to explain the nature of such deviations from the local relations, we propose a scenario in which the hosts of these uber-massive SMBHs are galaxies that have followed a different evolutionary path than the two-phase growth channel assumed for massive galaxies. Once the SMBH and the core of the galaxy are formed at z$sim$2, the galaxy skips the second phase, remaining structurally untouched and without further mass and size increase. We show that if the outliers had followed the normal evolutionary path by growing in size via merger activity, the expected (mild) growth in mass would place them closer to the observed local relations. Our results suggest that the SMBH growth epoch for the most massive galaxies stopped $sim$10Gyr ago.
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We study the preferred environments of $z sim 0$ massive relic galaxies ($M_star gtrsim 10^{10}~mathrm{M_odot}$ galaxies with little or no growth from star formation or mergers since $z sim 2$). Significantly, we carry out our analysis on both a large cosmological simulation and an observed galaxy catalogue. Working on the Millennium I-WMAP7 simulation we show that the fraction of today massive objects which have grown less than 10 per cent in mass since $z sim 2$ is ~0.04 per cent for the whole massive galaxy population with $M_star > 10^{10}~mathrm{M_odot}$. This fraction rises to ~0.18 per cent in galaxy clusters, confirming that clusters help massive galaxies remain unaltered. Simulations also show that massive relic galaxies tend to be closer to cluster centres than other massive galaxies. Using the New York University Value-Added Galaxy Catalogue, and defining relics as $M_star gtrsim 10^{10}~mathrm{M_odot}$ early-type galaxies with colours compatible with single-stellar population ages older than 10 Gyr, and which occupy the bottom 5-percentile in the stellar mass-size distribution, we find $1.11 pm 0.05$ per cent of relics among massive galaxies. This fraction rises to $2.4 pm 0.4$ per cent in high-density environments. Our findings point in the same direction as the works by Poggianti et al. and Stringer et al. Our results may reflect the fact that the cores of the clusters are created very early on, hence the centres host the first cluster members. Near the centres, high-velocity dispersions and harassment help cluster core members avoid the growth of an accreted stellar envelope via mergers, while a hot intracluster medium prevents cold gas from reaching the galaxies, inhibiting star formation.
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