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Showing votes from 2017-09-01 12:30 to 2017-09-05 11:30 | Next meeting is Friday Sep 19th, 11:30 am.
It is widely accepted that black holes with masses greater than a million solar masses (M⊙) lurk at the centres of massive galaxies. The origins of such ‘supermassive’ black holes (SMBHs) remain unknown1, although those of stellar-mass black holes are well understood. One possible scenario is that intermediate-mass black holes (IMBHs), which are formed by the runaway coalescence of stars in young compact star clusters2, merge at the centre of a galaxy to form a SMBH3. Although many candidates for IMBHs have been proposed, none is accepted as definitive. Recently, we discovered a peculiar molecular cloud, CO–0.40–0.22, with an extremely broad velocity width, near the centre of our Milky Way galaxy. Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of about 105M⊙ is lurking in this cloud4. Here we report the detection of a point-like continuum source as well as a compact gas clump near the centre of CO–0.40–0.22. This point-like continuum source (CO–0.40–0.22*) has a wide-band spectrum consistent with 1/500 of the Galactic SMBH (Sgr A*) in luminosity. Numerical simulations around a point-like massive object reproduce the kinematics of dense molecular gas well, which suggests that CO–0.40–0.22* is one of the most promising candidates for an intermediate-mass black hole.
The object CO–0.40–0.22 is a compact cloud (~5 pc) with an extremely broad velocity width (~100 km s−1) and very high intensity ratio (≥1.5) for CO J = 3–2/J = 1–0 detected at a projected distance of ~60 pc away from the Galactic nucleus5. It belongs to a peculiar category of molecular clouds called high-velocity compact clouds (HVCCs) that were originally identified in the CO J = 1–0 survey data6,7,8. The CO–0.40–0.22 cloud is the only dense cloud with a negative velocity detected in the HCN J = 4–3 line within the 0.3° × 0.3° area including it4. It has a continuous and roughly straight entity in the position–velocity maps, seeming not to be an aggregate of unrelated clouds with narrower velocity widths. The kinematical structure of CO–0.40–0.22 can be explained as being due to a gravitational kick experienced by the molecular cloud caused by an invisible compact object with a mass of about 105M⊙. The compactness and absence of a counterpart at other wavelengths suggest that this massive object is an inactive IMBH, which is not currently accreting matter. This is the second-largest black hole candidate in the Milky Way galaxy after Sgr A*, as well as the second IMBH candidate in the Galaxy after that in the nuclear subcluster IRS13E (MBH ≈ 1,300M⊙)9,10.
Observations with the Atacama Large Millimeter/submillimeter Array (ALMA) band 6 towards CO–0.40–0.22 have provided high-resolution HCN J = 3–2 (265.9 GHz) and CO J = 2–1 (230.5 GHz) images. Dense molecular gas traced by HCN J = 3–2 emission seems to concentrate near the centre of CO–0.40–0.22, as previously determined by the coarse-resolution HCN J = 4–3 map from the Atacama Submillimeter Telescope Experiment (ASTE) (Fig. 1a). The displacement of 0.2 pc from the centre is within the ASTE beamwidth (22″ = 0.9 pc). This dense gas clump is very compact (~0.3 pc) and has a broad velocity width (~100 km s−1). We also see about 20 rather faint clumps. These faint clumps and the central dense clump occupy around 10% of the field of view. The faint clumps have velocity widths less than 20 km s−1. Thus, the chance probability of their alignment over a width of about 100 km s−1 is less than (0.1/(100/20))100/20 = 10−8.5. The main body of the central clump appears at line-of-sight velocities from −80 to −40 km s−1 with respect to the local standard of rest (LSR), being associated with high-velocity components that reach VLSR = −105 and −5 km s−1 (Fig. 2). It is slightly elongated in roughly the same direction as the major axis of CO–0.40–0.22, with a steep velocity gradient from southeast to northwest. The mass of the clump was estimated from the HCN J = 3–2 integrated intensity to be 40M⊙, according to an excitation model at the kinetic temperature Tk = 60 K and number density n(H2) = 106.5 cm−3. On the other hand, the size parameter S = 0.15 pc and velocity dispersion σV = 22 km s−1 give a virial theorem mass of Mvir ≈ 4.1 × 103M⊙, which indicates that the clump must not be bound by its self-gravity.
Dark matter in the halos surrounding galaxy groups and clusters can annihilate to high-energy photons. Recent advancements in the construction of galaxy group catalogs provide many thousands of potential extragalactic targets for dark matter. In this paper, we outline a procedure to infer the dark matter signal associated with a given galaxy group. Applying this procedure to a catalog of sources, one can create a full-sky map of the brightest extragalactic dark matter targets in the nearby Universe ($z\lesssim 0.03$), supplementing sources of dark matter annihilation from within the Local Group. As with searches for dark matter in dwarf galaxies, these extragalactic targets can be stacked together to enhance the signals associated with dark matter. We validate this procedure on mock $\textit{Fermi}$ gamma-ray data sets using a galaxy catalog constructed from the $\texttt{DarkSky}$ $N$-body cosmological simulation and demonstrate that the limits are robust, at $\mathcal{O}(1)$ levels, to systematic uncertainties on halo mass and concentration. We also quantify other sources of systematic uncertainty arising from the analysis and modeling assumptions. Our results suggest that a stacking analysis using galaxy group catalogs provides a powerful opportunity to discover extragalactic dark matter and complements existing studies of Milky Way dwarf galaxies.
Hawking black hole (BH) radiation is emitted when particles tunnel from the inside through the event horizon to the "outside world". We here show that atoms falling from outside through a Schwarzschild horizon emit Unruh acceleration radiation which to a distant observer looks much like Hawking radiation. In particular, we find the entropy of the acceleration radiation via a simple laser-like analysis. We call this entropy Horizon Brightened Acceleration Radiation (HBAR) entropy to distinguish it from the BH entropy of Bekenstein and Hawking.