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Register a new userThe WMAP haze from the Galactic Center region due to massive star explosions and a reduced cosmic ray scale height
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One important prediction of acceleration of particles in the supernova caused shock in the magnetic wind of exploding Wolf Rayet and Red Super Giant stars is the production of an energetic particle component with an E^-2 spectrum, at a level of a few percent in flux at injection. After allowing for transport effects, so steepening the spectrum to E^-7/3, this component of electrons produces electromagnetic radiation and readily explains the WMAP haze from the Galactic Center region in spectrum, intensity and radial profile. This requires the diffusion time scale for cosmic rays in the Galactic Center region to be much shorter than in the Solar neighborhood: the energy for cosmic ray electrons at the transition between diffusion dominance and loss dominance is shifted to considerably higher particle energy. We predict that more precise observations will find a radio spectrum of u^-2/3, at higher frequencies u^-1, and at yet higher frequencies finally u^-3/2.
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Recent results from multi-wavelength observations of the inner few hundred pc of the Galactic center have added two new characteristics to the ISM in this unique region. One is the cosmic ray ionization rate derived from H$_3^+$ measurements is at least two orders of magnitudes higher than in the disk of the Galaxy. The other is the bipolar thermal X-ray and synchrotron emission from this region, suggesting a relic of past activity. We propose that the high cosmic ray pressure drives a large-scale wind away from the Galactic plane and produces the bipolar emission as well as highly blue-shifted diffuse gas detected in H$_3^+$ absorption studies. We then discuss the interaction of large-scale winds with a number of objects, such as cloudlets and stellar wind bubbles, to explain the unusual characteristics of the ISM in this region including the nonthermal radio filaments. One of the implications of this scenario is the removal of gas driven by outflowing winds may regulate star formation or black hole accretion.
We report the discovery of a $1^circ$ scale X-ray plume in the northern Galactic Center (GC) region observed with Suzaku. The plume is located at ($l$, $b$) $sim$ ($0mbox{$.!!^circ$}2$, $0mbox{$.!!^circ$}6$), east of the radio lobe reported by previous studies. No significant X-ray excesses are found inside or to the west of the radio lobe. The spectrum of the plume exhibits strong emission lines from highly ionized Mg, Si, and S that is reproduced by a thin thermal plasma model with $kT sim 0.7$ keV and solar metallicity. There is no signature of non-equilibrium ionization. The unabsorbed surface brightness is $3times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ arcmin$^{-2}$ in the 1.5-3.0 keV band. Strong interstellar absorption in the soft X-ray band indicates that the plume is not a foreground source but is at the GC distance, giving a physical size of $sim$100 pc, a density of 0.1 cm$^{-3}$, thermal pressure of $1times10^{-10}$ erg cm$^{-3}$, mass of 600 $M_odot$ and thermal energy of $7times10^{50}$ erg. From the apparent association with a polarized radio emission, we propose that the X-ray plume is a magnetized hot gas outflow from the GC.
We analyse WMAP 7-year temperature data, jointly modeling the cosmic microwave background (CMB) and Galactic foreground emission. We use the Commander code based on Gibbs sampling. Thus, from the WMAP7 data, we derive simultaneously the CMB and Galactic components on scales larger than 1deg with sensitivity improved relative to previous work. We conduct a detailed study of the low-frequency foreground with particular focus on the microwave haze emission around the Galactic center. We demonstrate improved performance in quantifying the diffuse galactic emission when Haslam 408MHz data are included together with WMAP7, and the spinning and thermal dust emission is modeled jointly. We also address the question of whether the hypothetical galactic haze can be explained by a spatial variation of the synchrotron spectral index. The excess of emission around the Galactic center appears stable with respect to variations of the foreground model that we study. Our results demonstrate that the new galactic foreground component - the microwave haze - is indeed present.
We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free-free absorption along the line of sight. We focus on one cloud, G0.13--0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (alpha^{74MHz}_{327MHz}=1.3pm0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l~0.2^0 and is a strong source of 74 MHz continuum, SiO (2-1) and FeI Kalpha 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13--0.13 molecular cloud and that low energy cosmic ray electrons are responsible for the FeI Kalpha line emission. The high cosmic ray ionization rate ~10-13 s-1 H-1 is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high velocity dispersion. LVG modeling of multi-transition SiO observations of this cloud implies H2 densities ~104-5 cm-3 and high temperatures. The lower limit to the temperature of G0.13-0.13 is ~100K, whereas the upper limit is as high as 1000K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.
The Galactic Center is an excellent laboratory for studying phenomena and physical processes that may be occurring in many other galactic nuclei. The Center of our Milky Way is by far the closest galactic nucleus, and observations with exquisite resolution and sensitivity cover 18 orders of magnitude in energy of electromagnetic radiation. Theoretical simulations have become increasingly more powerful in explaining these measurements. This review summarizes the recent progress in observational and theoretical work on the central parsec, with a strong emphasis on the current empirical evidence for a central massive black hole and on the processes in the surrounding dense nuclear star cluster. We present the current evidence, from the analysis of the orbits of more than two dozen stars and from the measurements of the size and motion of the central compact radio source, Sgr A*, that this radio source must be a massive black hole of about 4.4 times 1e6 Msun, beyond any reasonable doubt. We report what is known about the structure and evolution of the dense nuclear star cluster surrounding this black hole, including the astounding fact that stars have been forming in the vicinity of Sgr A* recently, apparently with a top-heavy stellar mass function. We discuss a dense concentration of fainter stars centered in the immediate vicinity of the massive black hole, three of which have orbital peri-bothroi of less than one light day. This S-star cluster appears to consist mainly of young early-type stars, in contrast to the predicted properties of an equilibrium stellar cusp around a black hole. This constitutes a remarkable and presently not fully understood paradox of youth. We also summarize what is known about the emission properties of the accreting gas onto Sgr A* and how this emission is beginning to delineate the physical properties in the hot accretion zone around the event horizon.
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