Bibcode
Gratier, P.; Braine, J.; Schuster, K.; Rosolowsky, E.; Boquien, M.; Calzetti, D.; Combes, F.; Kramer, C.; Henkel, C.; Herpin, F.; Israel, F.; Koribalski, B. S.; Mookerjea, B.; Tabatabaei, F. S.; Röllig, M.; van der Tak, F. F. S.; van der Werf, P.; Wiedner, M.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 600, id.A27, 14 pp.
Fecha de publicación:
3
2017
Revista
Número de citas
24
Número de citas referidas
24
Descripción
Do some environments favor efficient conversion of molecular gas into
stars? To answer this, we need to be able to estimate the H2
mass. Traditionally, this is done using CO observations and a few
assumptions but the Herschel observations which cover the far-IR dust
spectrum make it possible to estimate the molecular gas mass
independently of CO and thus to investigate whether and how the CO
traces H2. Previous attempts to derive gas masses from dust
emission suffered from biases. Generally, dust surface densities, H i
column densities, and CO intensities are used to derive a gas-to-dust
ratio (GDR) and the local CO intensity to H2 column density
ratio (XCO), sometimes allowing for an additional CO-dark gas
component (Kdark). We tested earlier methods, revealing
degeneracies among the parameters, and then used a sophisticated
Bayesian formalism to derive the most likely values for each of the
parameters mentioned above as a function of position in the nearby
prototypical low metallicity (12 + log (O/H) 8.4) spiral galaxy M 33.
The data are from the IRAM Large Program mapping in the CO(2-1) line
along with high-resolution H i and Herschel dust continuum observations.
Solving for GDR, XCO, and Kdark in macropixels 500
pc in size, each containing many individual measurements of the CO, H i,
and dust emission, we find that (i) allowing for CO dark gas
(Kdark) significantly improves fits; (ii) Kdark
decreases with galactocentric distance; (iii) GDR is slightly higher
than initially expected and increases with galactocentric distance; (iv)
the total amount of dark gas closely follows the radially decreasing CO
emission, as might be expected if the dark gas is H2 where CO
is photodissociated. The total amount of H2, including dark
gas, yields an average XCO of twice the galactic value of 2
× 1020 cm-2/ K km s-1, with about
55% of this traced directly through CO. The rather constant fraction of
dark gas suggests that there is no large population of diffuse
H2 clouds (unrelated to GMCs) without CO emission. Unlike in
large spirals, we detect no systematic radial trend in XCO,
possibly linked to the absence of a radial decrease in CO line ratios.
Proyectos relacionados
Estudios Cinemáticos, Estructurales y de Composición, de los Medios Interestelares e Intergalácticos
El objetivo básico del proyecto es investigar la evolución de las galaxias mediante el entendimiento de la interacción del medio interestelar y las estrellas. La técnica principal que utilizamos es la cinemática bidimensional de galaxias enteras observada por nuestro instrumento GHaFaS, un interferometro Fabry Perot en el telescopio William
Prof.
John E. Beckman