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Ossenkopf V.,University of Cologne | Ossenkopf V.,SRON Netherlands Institute for Space Research | Muller H.S.P.,University of Cologne | Lis D.C.,California Institute of Technology | And 110 more authors.
Astronomy and Astrophysics | Year: 2010

Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin. Methods. The characteristic hyperfine pattern of the H2O+ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H2O+ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation. Results. In DR21, the velocity distribution of H2O+ matches that of the [C ii] line at 158 μm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H ii-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H 2O+ column density of 7.2 × 1012 cm -2 in NGC 6334, 2.3 × 1013 cm-2 in DR21, and 1.1 × 1015 cm-2 in Sgr B2. © 2010 ESO.

Hunt L.,NAF Osservatorio Astrofisico di Arcetri | Magrini L.,NAF Osservatorio Astrofisico di Arcetri | Galli D.,NAF Osservatorio Astrofisico di Arcetri | Schneider R.,National institute for astrophysics | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

Most galaxies follow well-defined scaling relations of metallicity (O/H), star formation rate (SFR) and stellar mass (M star). However, low-metallicity starbursts, rare in the Local Universe but more common at high redshift, deviate significantly from these scaling relations. On the 'main sequence' of star formation, these galaxies have high SFR for a given M star; and on the mass-metallicity relation, they have excess M star for their low metallicity. In this paper, we characterize O/H, M star and SFR for these deviant 'low-metallicity starbursts', selected from a sample of ∼1100 galaxies, spanning almost two orders of magnitude in metal abundance, a factor of ∼10 6 in SFR, and of ∼10 5 in stellar mass. Our sample includes quiescent star-forming galaxies and blue compact dwarfs at redshift 0, luminous compact galaxies at redshift 0.3, and Lyman break galaxies at redshifts 1-3.4. Applying a principal component analysis (PCA) to the galaxies in our sample with M star ≤ 3 × 10 10M ⊙ gives a Fundamental Plane (FP) of scaling relations; SFR and stellar mass define the plane itself, and O/H its thickness. The dispersion for our sample in the edge-on view of the plane is 0.17dex, independently of redshift and including the metal-poor starbursts. The same FP is followed by 55100 galaxies selected from the Sloan Digital Sky Survey, with a dispersion of 0.06dex. In a companion paper, we develop multi-phase chemical evolution models that successfully predict the observed scaling relations and the FP; the deviations from the main scaling relations are caused by a different (starburst or 'active') mode of star formation. These scaling relations do not truly evolve, but rather are defined by the different galaxy populations dominant at different cosmological epochs. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

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