Final published version including significant revisions. Twenty four pages, fourteen figures. Original version April 2006; final version published in MNRAS August 2007

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The UKIRT Infrared Deep Sky Survey (UKIDSS)

Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges

Galactic Winds

We present results of our Chandra observation with the ACIS-I instrument centered on the position of Sagittarius A* (Sgr A*), the compact nonthermal radio source associated with the massive black hole (MBH) at the dynamical center of the Milky Way Galaxy. We have obtained the first high-spatial-resolution (~1 arcsec), hard X-ray (0.5-7 keV) image of the central 40 pc (17 arcmin) of the Galaxy and have discovered an X-ray source, CXOGC J174540.0-290027, coincident with the radio position of Sgr A* to within 0.35 arcsec, corresponding to a maximum projected distance of 16 light-days for an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222 +/-17 (1 sigma) net counts from the source in 40.3 ks. Due to the low number of counts, the spectrum is well fit either by an absorbed power-law model with photon index Gamma = 2.7 (1.8-4.0) and column density NH = (9.8 [6.8-14.2]) x 10^22 cm^-2 (90% confidence interval) or by an absorbed optically thin thermal plasma model with kT = 1.9 (1.4-2.8) keV and NH = (11.5 [8.4-15.9]) x 10^22 cm^-2. Using the power-law model, the measured (absorbed) flux in the 2-10 keV band is (1.3 [1.1-1.7]) x 10^-13 ergs cm^-2 s^-1, and the absorption-corrected luminosity is (2.4 [1.8-5.4]) x 10^33 ergs s^-1. We also briefly discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane and present morphological evidence that Sgr A* and Sgr A West lie within the hot plasma in the central cavity of Sgr A East.

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Chandra X-ray Spectroscopic Imaging of Sgr A* and the Central Parsec of the Galaxy

We report the results of the first-epoch observation with the ACIS-I instrument on the Chandra X-Ray Observatory of Sagittarius A* (Sgr A*), the compact radio source associated with the supermassive black hole (SMBH) at the dynamical center of the Milky Way. This observation produced the first X-ray (0.5- 7 keV) spectroscopic image with arcsecond resolution of the central 17 0 � 17 0 (40 pc � 40 pc) of the Galaxy. We report the discovery of an X-ray source, CXOGC J174540.0� 290027, coincident with Sgr A* within 0>27 � 0>18. The probability of a false match is estimated to be d0.5%. The spectrum is well fitted either by an absorbed power law with photon index � � 2:7 or by an absorbed optically thin thermal plasma with kT � 1:9 keV and column density NH � 1 � 10 23 cm � 2 . The observed flux in the 2-10 keV band is � 1:3 � 10 � 13 ergs cm � 2 s � 1 , and the absorption-corrected luminosity is � 2:4 � 10 33 ergs s � 1 . The X-ray emission at the position of Sgr A* is extended, with an intrinsic size of � 1>4 (FWHM), consistent with the Bondi accretion radius for a 2:6 � 10 6 Mblack hole. A compact component within the source flared by up to a factor of 3 over a period of � 1 hr at the start of the observation. The search for Kline emission from iron was inconclusive, yielding an upper limit on the equivalent width of 2.2 keV. Several potential stellar origins for the X-ray emission at Sgr A* are considered, but we conclude that the various properties of the source favor accretion onto the SMBH as the origin for the bulk of the emission. These data are inconsistent with '' standard '' advection-dominated accretion flow (ADAF) models or Bondi models, unless the accretion rate from stellar winds is much lower than anticipated. The central parsec of the Galaxy contains an � 1.3 keV plasma with electron density ne � 26� � 1=2 f cm � 3 , wheref is the filling factor. This plasma should supply � 10 � 6 Myr � 1 of material to the accretion flow at the Bondi radius, whereas measurements of linear polar- ization at 150 GHz and above limit the accretion rate near the event horizon to d10 � 8 Myr � 1 , assuming an equipartition magnetic field. Taken together, the X-ray and radio results imply that outflows or convection are playing a role in ADAF models and subequipartition magnetic fields in Bondi models, or else the X-ray emission must be generated predominantly via the synchrotron self-Compton (SSC) process. The measured extent of the source and the detection of short timescale variability are evidence that the emission from Sgr A* contains both thermal and nonthermal emission components at comparable levels. We also discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane. Mor- phological evidence is presented that Sgr A* and the H ii region Sgr A West lie within the hot plasma in the central cavity of Sgr A East, which we interpret as a supernova remnant that may have passed through the position of the SMBH, leading to a period of increased activity that ended within the past � 300 yr. Similarly, we have discovered bright clumps of X-ray emission located on opposite sides of the Galactic plane, along a line passing through the central parsec of the Galaxy. The arrangement of these lobes suggests that Sgr A* may have experienced an earlier period of increased activity lasting several thousand years during which it expelled hot gas in a bipolar outflow oriented roughly perpendicular to the Galactic plane. Additionally, we present an analysis of stellar emission within the central parsec of the Galaxy. Subject headings: accretion, accretion disks — black hole physics — galaxies: active — Galaxy: center — X-rays: ISM — X-rays: stars

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Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy

We present a comprehensive description of the population synthesis code StarTrack. The original code has been significantly modified and updated. Special emphasis is placed here on processes leading to the formation and further evolution of compact objects (white dwarfs, neutron stars, and black holes). Both single and binary star populations are considered. The code now incorporates detailed calculations of all mass transfer phases, a full implementation of orbital evolution due to tides, as well as the most recent estimates of magnetic braking. This updated version of StarTrack can be used for a wide variety of problems, with relevance to observations with many current and planned observatories, e.g., studies of X-ray binaries (Chandra, XMM-Newton), gravitational radiation sources (LIGO, LISA), and gamma-ray burst progenitors (HETE-II, Swift). The code has already been used in studies of Galactic and extragalactic X-ray binary populations, black holes in young star clusters, Type Ia supernova progenitors, and double compact object populations. Here we describe in detail the input physics, we present the code calibration and tests, and we outline our current studies in the context of X-ray binary populations.

https://iopscience.iop.org/article/10.1086/521026/pdf

Compact Object Modeling with the StarTrack Population Synthesis Code

The origin of the X-ray emission for the central region of our Galaxy has remained a mystery1,2,3,4. In particular, the relative spectral contributions of the diffuse emission and discrete sources, which are critical to understanding the high-energy phenomena in this environment, have been unclear because of the lack of sufficient spatial resolution. Here we report the results of a large-scale imaging survey of the Galactic Centre that resolves these components. We find that the Kα emission from iron that has been highly ionized (so that it has only two electrons left), which has previously been attributed to the diffuse component1, actually arises mainly from discrete sources. This suggests that the presence of a large amount of hot gas (T ≈ 108 K) is no longer required to explain the iron line emission. The spectra of the discrete sources indicate the presence of numerous accreting white dwarfs, neutron stars, and/or black holes in the region. The diffuse emission dominates over the contribution from the faint point sources, and is shown to be associated globally with interstellar features that have been observed at radio and mid-infrared wavelengths, suggesting that it is the product of recent massive star formation.

Q. D. Wang

Papers

A faint discrete source origin for the highly ionized iron emission from the Galactic Centre region.