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Carbon monoxide emission as a precise tracer of molecular gas in the Andromeda galaxy

N. Neininger tex2html_wrap_inline1113 , M. Guélin tex2html_wrap_inline1115 , H. Ungerechts tex2html_wrap_inline1117 , R. Lucas tex2html_wrap_inline1115 , and R. Wielebinski tex2html_wrap_inline1121 .
tex2html_wrap_inline1123 Radioastronom. Inst. der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
tex2html_wrap_inline1115 IRAM, 300, rue de la piscine, 38406 St.Martin d' Hères, France
tex2html_wrap_inline1121 MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
tex2html_wrap_inline1117 IRAM, Divina Pastora 7, 18012 Granada, Spain
Abstract: We have mapped the CO 1-0 line emission over the soutwest half of M 31 with the IRAM 30-m telescope. The observations were made on-the-fly. The small beamsize, low sidelobe level and tight sampling yield a resolution (23'') and sensitivity (3 Jy kms tex2html_wrap_inline1027 ) unprecendented for a map of this size ( tex2html_wrap_inline1135 armin tex2html_wrap_inline1137 ). In addition, we have surveyed with the Plateau de Bure interferometer a few selected fields with a resolution of 2'' (9 pc).

The most striking results are i) the excellent correlation between CO and the dark lanes visible on the optical image (see figure on cover page), ii) the thinness of the molecular arms, iii) the good agreement between the CO and HI arm crests, iv) the detection of large streaming motions in the immediate vicinity of bright HII regions.

Figure: a- Relation between the CO and HI integrated intensities, I(CO) and I(HI), inside a 6 kpc-long, 1.5 kpc-wide lane centred on D 39 and following the southern R= 11 kpc arm of M 31. Each point corresponds to a tex2html_wrap_inline1143 arcsec tex2html_wrap_inline1137 cell (5 10 tex2html_wrap_inline1147 pc tex2html_wrap_inline1137 in the plane of M 31). b- Relation between the total hydrogen column density, estimated from I(CO) and I(HI), and the dust apparent opacity in the red.

It is often argued that the optically thick mm emission of tex2html_wrap_inline1151 CO does not trace the molecular gas properly and could be biased toward warm regions. The rich pattern of dust lanes visible throughout M 31's bright stellar disk yields another way to detect interstellar clouds, which does not depend on temperature or chemistry. Our high resolution CO map and the interferometric HI map of Brinks and Shane (A&AS 55, 1984) allow for the first time a detailed comparison of CO, HI and dust as tracers. All dark lanes with galactocentric radii tex2html_wrap_inline1153 kpc are detected in CO. CO is always brighter where the extinction is larger, while HI can be bright in regions with moderate visual extinction.

This is illustrated in Fig. 4a which compares point by point the CO and HI velocity-integrated intensities I(CO) and I(HI) in the conspicuous tex2html_wrap_inline1159 kpc arm. Whereas HI is bright in many CO-dim regions with moderate visual extinction, CO is never observed where HI is faint, or the extinction is low. We have plotted in Fig 4b the total hydrogen column density, derived from the expression tex2html_wrap_inline1161 , as a function of the apparent visual opacity, tex2html_wrap_inline1163 . N(H) is seen to scale with tex2html_wrap_inline1163 : the slope is tex2html_wrap_inline1169 H-atom cm tex2html_wrap_inline1171 , close to that for the clouds in the solar neighbourhood. The linear correlation coefficient is tex2html_wrap_inline1173 , significantly larger than that obtained by plotting I(HI) or I(CO) alone ( tex2html_wrap_inline1179 ). The good CO-dust correlation is a very strong indication that CO traces the H tex2html_wrap_inline1181 column density. It also implies that a sizable fraction of the gas in the arm is in molecular form.

Nature in press. Pre-/reprints: and

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