Tuesday, 05/14/13

Two papers today. The first is what I gather is the overview paper on Suzanne Madden’s recent survey with Herschel using PACS/SPIRE photometry/spectroscopy. The second is a little more outside of what any of us does, but interesting. Miller & Bregman report on a observation-based calculation of the mass in the Milky Way’s hot gas halo using X-ray absorption lines toward several AGN. They claim that 10%-50% of the "missing" baryons in the Milky Way can be attributed to this hot gas component. Enjoy!

Title: An Overview of the Dwarf Galaxy Survey
Authors: Madden, S. C.; Remy Ruyer, A.; Galametz, M.; Cormier, D.; Lebouteiller, V.; Galliano, F.; Hony, S.; Bendo, G. J.; Smith, M. W. L.; Pohlen, M.; Roussel, H.;Sauvage, M.; Wu, R.; Sturm, E.; Poglitsch, A.; Contursi, A.; Doublier, V.; Baes, M.; Barlow, M. J.; Boselli, A.; Boquien, M.; Carlson, L. R.; Ciesla, L.; Cooray, A.;Cortese, L.; De Looze, I.; Irwin, J. A.; Isaak, K.; Kamenetzky, J.; Karczewski, O. L.; Lu, N.; MacHattie, J. A.; Halloran, B. O; Parkin, T. J.; Rangwala, N.;Schirm, M. R. P.; Schulz, B.; Spinoglio, L.; Vaccari, M.; Wilson, C. D.; Wozniak, H.
Publication: eprint arXiv:1305.2628
Comment: Article accepted for PASP (april 25 2013)
Bibliographic Code: 2013arXiv1305.2628M

Abstract

The Dwarf Galaxy Survey (DGS) program is studying low-metallicity galaxies using 230h of far-infrared (FIR) and submillimetre (submm) photometric and spectroscopic observations of the Herschel Space Observatory and draws to this a rich database of a wide range of wavelengths tracing the dust, gas and stars. This sample of 50 galaxies includes the largest metallicity range achievable in the local Universe including the lowest metallicity (Z) galaxies, 1/50 Zsun, and spans 4 orders of magnitude in star formation rates. The survey is designed to get a handle on the physics of the interstellar medium (ISM) of low metallicity dwarf galaxies, especially on their dust and gas properties and the ISM heating and cooling processes. The DGS produces PACS and SPIRE maps of low-metallicity galaxies observed at 70, 100, 160, 250, 350, and 500 mic with the highest sensitivity achievable to date in the FIR and submm. The FIR fine-structure lines, [CII] 158 mic, [OI] 63 mic, [OI] 145 mic, [OIII] 88 mic, [NIII] 57 mic and [NII] 122 and 205 mic have also been observed with the aim of studying the gas cooling in the neutral and ionized phases. The SPIRE FTS observations include many CO lines (J=4-3 to J=13-12), [NII] 205 mic and [CI] lines at 370 and 609 mic. This paper describes the sample selection and global properties of the galaxies, the observing strategy as well as the vast ancillary database available to complement the Herschel observations. The scientific potential of the full DGS survey is described with some example results included.

Title: The Structure of the Milky Way’s Hot Gas Halo
Authors: Miller, Matthew; Bregman, Joel
Publication: eprint arXiv:1305.2430
Comment: 33 pages, 10 figures, accepted to ApJ
Bibliographic Code: 2013arXiv1305.2430M

Abstract

The Milky Way’s million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton RGS archival data and measure OVII K alpha absorption-line strengths towards 26 active galactic nuclei (AGN), LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a beta-model as the underlying gas density profile and find best-fit parameters of n_o = 0.46^{+0.74}_{-0.35} cm^-3, r_c = 0.35^{+0.29}_{-0.27} kpc, and beta = 0.71^{+0.13}_{-0.14}. These parameters result in halo masses ranging between M(18 kpc) = 7.5^{+22.0}_{-4.6} x 10^8 M_sun and M(200 kpc) = 3.8^{+6.0}_{-0.5} x 10^{10} M_sun assuming a gas metallicity of Z = 0.3 Z_sun, which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of f_b = 0.07^{+0.03}_{-0.01} is significantly smaller than the universal value of f_b = 0.171, implying the mass contained in the Galactic halo accounts for 10 – 50% of the missing baryons in the Milky Way. We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5 – 2 keV band, the Milky Way’s star formation rate, spatial and thermal properties of cooler gas (~10^5 K) and the observed Fermi bubbles towards the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the LMC. We find that Z >~ 0.2 Z_sun based on the pulsar dispersion measure towards the LMC.

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