Query : 1998A&A...331..451C

1998A&A...331..451C - Astronomy and Astrophysics, volume 331, 451-462 (1998/3-2)

Molecular gas in spiral galaxies.


Abstract (from CDS):

The molecular hydrogen content of a galaxy is a key parameter for its activity and future evolution. Its variations with basic properties such as size, mass, morphological type, and environment, the ratio of molecular to atomic gas masses, should provide us with a better view of galaxy evolution. Such studies have been done in the past by Sage (1993A&A...272..123S) or the FCRAO group (e.g. Young & Knezek 1989), and have led to controversial results, for example about the MH2/MHI ratio. While Sage (1993A&A...272..123S), using a distance-limited sample of 65 galaxies and the CO(1-0)line emission as a tracer of the H2 mass, finds that most galaxies have MH2/MHI lower than 1, Young & Knezek (1989ApJ...345L..55S) and Young et al. (1995ApJS...98..219Y), from a different sample of 178 objects, claim equal amounts of gas in the molecular and atomic phase. Here we again tackle this problem, by gathering a much larger sample of 582 objects, not only from the literature but also from several CO(1-0)surveys that we have completed and which are largely unpublished. Our sample is clearly not complete and contains a large number of cluster galaxies as well as many more massive objects than a distance-limited sample. Contrary to previous analyses, we have taken into account the non-detections by using the survival analysis method. Our sample includes 105 isolated galaxies, observed by us, that we use as a reference sample in order to determine whether cluster galaxies are CO-deficient. We find that the ratio of H2 and HI masses is on the average lower than 1, with <log(MH2/MHI)≥log(0.20)±0.04 (median=log(0.27)±0.04). For spirals with types Sa to Sc, we have slightly higher values: log(0.28) and log(0.34) respectively. The actual H2 masses and MH2/MHI ratios could be lower than given above if, as suggested by recent γ-ray and 1.3mm continuum data, the conversion factor between CO(1-0)emissivities and H2 masses for large spiral galaxies is lower than the value adopted here (X=2.3x1020cm–2/(K.km/s)). The molecular to atomic gas ratio shows a constant value from Sa to Sbc's, and a factor of 10 decrease for late-types, beginning at Sc's. This effect can be attributed to the low CO emission of late-type, low-mass galaxies; we find no such decrease for objects with a dynamical mass larger than 1011M. These high-mass objects actually show an increase of their normalized atomic and molecular gas content towards late-types, while for low-mass objects, this is seen on HI only. Several authors have tried to search for galaxies deficient in H2 in the core of clusters such as Virgo or Coma, but these studies were hampered by the lack of a suitable reference sample (Kenney & Young 1989ApJ...344..171K, Casoli et al. 1991A&A...249..359C, Horellou et al. 1995A&A...303..361H). Using isolated galaxies and galaxies in the outer regions of clusters as a reference sample, we give a predictor for the normalized H2 mass of a galaxy MH2i/D252, which depends upon its normalized far-infrared emission LFIR/D252 and its morphological type. This predictor allows us to define a ``CO deficiency factor'', CODEF, analogous to what has been defined for the HI emission. We find that there is no significant CO deficiency of galaxies in the cores of rich clusters.

Abstract Copyright:

Journal keyword(s): galaxies: evolution - galaxies: spiral - ISM: molecules - galaxies: statistics

Simbad objects: 9

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Number of rows : 9
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
1 M 31 G 00 42 44.330 +41 16 07.50 4.86 4.36 3.44     ~ 12387 1
2 NGC 891 H2G 02 22 32.907 +42 20 53.95 11.08 10.81 9.93 7.86   ~ 1643 2
3 ACO S 373 ClG 03 38 29.4 -35 27 08           ~ 1782 0
4 M 81 Sy2 09 55 33.1726556496 +69 03 55.062505368   7.89 6.94     ~ 4359 3
5 NAME Coma Supercluster SCG 11 23 +23.9           ~ 265 0
6 ACO 1367 ClG 11 44 44.6 +19 41 59           ~ 1063 1
7 NAME Virgo Cluster ClG 12 26 32.1 +12 43 24           ~ 6509 0
8 NGC 4565 LIN 12 36 20.804 +25 59 14.61   13.61 12.43     ~ 941 0
9 M 51 GiP 13 29 52.698 +47 11 42.93   9.26 8.36 8.40   ~ 4248 4

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