THE OUTER DISK OF THE MILKY WAY SEEN IN λ21 cm ABSORPTION

Dickey, John M.; Strasser, S.; Gaensler, B. M.; Haverkorn, M.; Kavars, D. W.; McClure-Griffiths, N. M.; Stil, J. M.; Taylor, A. R.

doi:10.1088/0004-637x/693/2/1250

Cited by 93 publications

(135 citation statements)

References 47 publications

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“…Evidence for Galactic warp has recently been observed again from HI absorption (Dickey et al 2009), red clump giants (López-Corredoira et al 2010;Bobylev 2010), extended low density warm ionized medium (Cersosimo et al 2009), 2MASS infrared stars (Reylé et al 2009), young open clusters (Vázquez et al 2008), molecular gas (Nakagawa et al 2005), Galactic HII regions , and also star forming complexes .…”

Section: The Galactic Warpmentioning

confidence: 90%

The observed spiral structure of the Milky Way

Hou

Han

2014

A&A

227

233

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Context. The spiral structure of the Milky Way is not yet well determined. The keys to understanding this structure are to increase the number of reliable spiral tracers and to determine their distances as accurately as possible. HII regions, giant molecular clouds (GMCs), and 6.7 GHz methanol masers are closely related to high mass star formation, and hence they are excellent spiral tracers. The distances for many of them have been determined in the literature with trigonometric, photometric, and/or kinematic methods. Aims. We update the catalogs of Galactic HII regions, GMCs, and 6.7 GHz methanol masers, and then outline the spiral structure of the Milky Way. Methods. We collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7 GHz methanol masers. If the photometric or trigonometric distance was not yet available, we determined the kinematic distance using a Galaxy rotation curve with the current IAU standard, R 0 = 8.5 kpc and Θ 0 = 220 km s −1 , and the most recent updated values of R 0 = 8.3 kpc and Θ 0 = 239 km s −1 , after velocities of tracers are modified with the adopted solar motions. With the weight factors based on the excitation parameters of HII regions or the masses of GMCs, we get the distributions of these spiral tracers.Results. The distribution of tracers shows at least four segments of arms in the first Galactic quadrant, and three segments in the fourth quadrant. The Perseus Arm and the Local Arm are also delineated by many bright HII regions. The arm segments traced by massive star forming regions and GMCs are able to match the HI arms in the outer Galaxy. We found that the models of three-arm and four-arm logarithmic spirals are able to connect most spiral tracers. A model of polynomial-logarithmic spirals is also proposed, which not only delineates the tracer distribution, but also matches the observed tangential directions.

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Section: The Galactic Warpmentioning

confidence: 90%

The observed spiral structure of the Milky Way

Hou

Han

2014

A&A

227

233

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“…In the Solar neighborhood, this is ∼ 0.6 (Heiles & Troland 2003), and in external galaxies the presence of both narrower and broader components of 21 cm emission suggests that both warm and cold gas are present (de Blok & Walter 2006), with some indication based on "universality" in line profile shapes that the warm-to-cold mass ratio does not strongly vary with position (Petric & Rupen 2007). In the outer Milky Way, the ratio of HI emission to absorption appears nearly constant out to ∼ 25 kpc, indicating that the warm-to-cold ratio does not vary significantly (Dickey et al 2009). In dwarf galaxies as well, observations indicate that both a cold and warm HI component is present (Young & Lo 1996).…”

Section: Vertical Dynamical Equilibrium Of Diffuse Gasmentioning

confidence: 99%

Regulation of Star Formation Rates in Multiphase Galactic Disks: A Thermal/Dynamical Equilibrium Model

Ostriker¹,

McKee²,

Leroy³

2010

ApJ

373

544

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We develop a model for the regulation of galactic star formation rates Σ SFR in disk galaxies, in which ISM heating by stellar UV plays a key role. By requiring that thermal and (vertical) dynamical equilibrium are simultaneously satisfied within the diffuse gas, and that stars form at a rate proportional to the mass of the self-gravitating component, we obtain a prediction for Σ SFR as a function of the total gaseous surface density Σ and the midplane density of stars + dark matter ρ sd . The physical basis of this relationship is that the thermal pressure in the diffuse ISM, which is proportional to the UV heating rate and therefore to Σ SFR , must adjust until it matches the midplane pressure value set by the vertical gravitational field. Our model applies to regions where Σ < ∼ 100 M ⊙ pc −2 . In low-Σ SFR (outer-galaxy) regions where diffuse gas dominates, the theory predicts that Σ SFR ∝ Σ √ ρ sd . The decrease of thermal equilibrium pressure when Σ SFR is low implies, consistent with observations, that star formation can extend (with declining efficiency) to large radii in galaxies, rather than having a sharp cutoff at a fixed value of Σ. The main parameters entering our model are the ratio of thermal pressure to total pressure in the diffuse ISM, the fraction of diffuse gas that is in the warm phase, and the star formation timescale in self-gravitating clouds; all of these are (at least in principle) direct observables. At low surface density, our model depends on the ratio of the mean midplane FUV intensity (or thermal pressure in the diffuse gas) to the star formation rate, which we set based on Solar neighborhood values. We compare our results to recent observations, showing good agreement overall for azimuthally-averaged data in a set of spiral galaxies. For the large flocculent spiral galaxies NGC 7331 and NGC 5055, the correspondence between theory and observation is remarkably close.

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“…We adopted a uniform T S = 250 K as baseline case, which is the average spin temperature that best reproduces the blending of cold and warm atomic gas according to observations of emission-absorption H i pairs in the region covered by the CGPS (Dickey et al 2009). Other values 100 K ≤ T S < ∞ will be considered later to evaluate the related systematic uncertainties affecting the results of our analysis.…”

Section: Psrmentioning

confidence: 99%

The cosmic-ray and gas content of the Cygnus region as measured inγ-rays by theFermiLarge Area Telescope

Ackermann¹,

Ajello²,

Allafort³

et al. 2012

A&A

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Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction. Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope (LAT) in the energy range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive stellar clusters and from individual sources is addressed elsewhere. Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other pulsars and γ-ray sources, is sought. Results. The integral H i emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06 ± 0.11 (stat.), where the systematic error is dominated by the uncertainty on the H i opacity to calculate its column densities. The integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space near the solar system. The average X CO = N(H 2 )/W CO ratio is found to be [1.68 ± 0.05 (stat.) (K km s −1 ) −1 , consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass corresponding to ∼40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to 8 +5 −1 × 10 6 M at a distance of 1.4 kpc. Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus complex averaged over a few hundred parsecs is similar to that of the local interstellar space.

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THE OUTER DISK OF THE MILKY WAY SEEN IN λ21 cm ABSORPTION

Cited by 93 publications

References 47 publications

The observed spiral structure of the Milky Way

The observed spiral structure of the Milky Way

Regulation of Star Formation Rates in Multiphase Galactic Disks: A Thermal/Dynamical Equilibrium Model

The cosmic-ray and gas content of the Cygnus region as measured inγ-rays by theFermiLarge Area Telescope

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