Journal Article 281108

The article this week is entitled 'Hubble Space Telescope survey of the Perseus Cluster - I: The structure and dark matter content of cluster dwarf spheroidals' .  

We present the results of a Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) study of dwarf galaxies in the core of the rich nearby Perseus Cluster, down to M_V=-12. We identify 29 dwarfs as cluster members, 17 of which are previously unstudied. All the dwarfs we examine are remarkably smooth in appearance, and lack internal features. Based on these observations, and the sizes of these dwarfs, we argue that some of the dwarfs in our sample must have a large dark matter content to prevent disruption by the cluster potential. We derive a new method, independent of kinematics, for measuring the dark matter content of dEs, based on the radius of the dwarf, the projected distance of the dwarf from the cluster centre, and the total mass of the cluster interior to it. We find that the mass-to-light ratios of these dwarfs are comparable to those of the Local Group dSphs, ranging between 1 and 120.

You can find it here: arXiv:0811.3197 

This plot comes from Komossa & Merritt (arXiv:0811.1037). It's about gravitational recoil of supermassive black holes in mergers, and the implications for unified models of AGN. There's a hint of a rumour of a possibility, I gather, that the fraction of 'type 2' quasars (ones where the central region of the quasar is obscured by a surrounding torus) doesn't match up with the fraction of type 2 Seyferts. We don't see as many type 2 quasars as we'd expect, which is odd if the obscuration in both cases is just a result of the angle at which we're viewing the object.

The authors looked at some N-body simulations and noticed that in major mergers gravitational wave emission can kick the central SMBH in a quasar a significant distance away from the nucleus of the galaxy. Far enough, perhaps, that the formerly obscured central engine will pop up above the obscuring torus, and make us think we're looking at a type 1, when we're really looking at a type 2.

Weird.

November 14th, 2008

This week's paper:

http://arxiv.org/abs/0811.0859
---
Unified Rotation Curve of the Galaxy -- Decomposition into de
Vaucouleurs Bulge, Disk, Dark Halo, and the 9-kpc Rotation Dip
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We present a unified rotation curve of the Galaxy re-constructed from
the existing data by re-calculating the distances and velocities for a
set of galactic constants R_0=8 kpc and V_0=200 km/s. We decompose it
into a bulge with de Vaucouleurs-law profile of half-mass scale radius
0.5 kpc and mass 1.8 x 10^{10}M_{sun}, an exponential disk of scale
radius 3.5 kpc of 6.5 x 10^{10}M_{sun}, and an isothermal dark halo of
terminal velocity 200 km/s. The r^{1/4}-law fit was obtained for the
first time for the Milky Way's rotation curve. After fitting by these
fundamental structures, two local minima, or the dips, of rotation
velocity are prominent at radii 3 and 9 kpc. The 3-kpc dip is consistent
with the observed bar. It is alternatively explained by a massive ring
with the density maximum at radius 4 kpc. The 9-kpc dip is clearly
exhibited as the most peculiar feature in the galactic rotation curve.
We explain it by a massive ring of amplitude as large as 0.3 to 0.4
times the disk density with the density peak at radius 11 kpc. This
great ring may be related to the Perseus arm, while no peculiar feature
of HI-gas is associated.