Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...303..851b&link_type=abstract
Astronomy and Astrophysics, v.303, p.851
Astronomy and Astrophysics
Astrophysics
49
Interstellar Medium: Clouds, Interstellar Medium: Molecules, Galaxy: General, Radio Lines: Ism
Scientific paper
The properties of star-forming clouds at the edge of the galactic molecular disk (far-outer Galaxy: FOG) are analyzed. By adding data from the literature, we compare the inner- and outer Galaxy (separated at R=R_0_=8.5kpc) cloud populations. Applying a radiation transfer model to multi-line CO observations of two FOG clouds, T_kin_'s between about 6K and 15K are derived, comparable to those of clouds at smaller R. A statistical analysis of T^*^_A_ both of clouds without embedded heating sources, and of clouds associated with IRAS sources, confirms there is no evidence for a gradient in cloud temperatures across the Galaxy for R>8.5kpc. The column density of H_2_, as derived from N(H_2_)=XxW_CO_(={Integral}T^*^_R_dv), and the H_2_ LTE column density show a reasonable agreement for X=2.3x10^20^cm^-2^(Kxkm/s)^-1^ (derived for the inner Galaxy), if the LTE analysis takes into account a galactic gradient of (H_2_/^13^CO). The average and median values of X=N_lte_/{Integral}T^*^_R_dv, 3.3+/-1.7 [1σ] and 2.9x10^20^ respectively, then are within 30-45% of the inner Galaxy value. Due to saturation effects the average X increases from 3x10^20^ (W_CO_<=20) to 6x10^20^cm^-2^(Kxkm/s)^-1^ (W_CO_>35Kkm/s). Including data from the literature we have a sample of 204 molecular clouds with 3<~R<~20kpc. Assuming X=2.3x10^20^ for all inner- and outer Galaxy clouds we find separate, nearly parallel relations in diagrams of logM_W_CO__ versus logr_A_ and log{DELTA}v versus logr_A_. Although part of the offset of a factor of 2 in r_A_ may be due to the difficulties related to the identification of inner Galaxy clouds, it may be mainly explained by inner Galaxy clouds having a higher density, or by a variation of X with R (or with mass). We find M_W_CO__{prop.to} r_A_^2.0 +/- 0.1^ for all clouds, and {DELTA}v {prop.to} r_A_^0.48 +/-0.04^ (inner) and {prop.to} r_A_^0.53 +/-0.03^ (outer). Inner- and outer Galaxy clouds define a single relation in a diagram of logL_CO_ versus log{DELTA}v: L_CO_{prop.to} {DELTA}v^3.91 +/-0.12^. This is in contrast to published results where an offset is found between clouds inside and outside the solar circle. We conclude that a diagram of CO luminosity versus line width is not a good instrument to determine whether there is a dependence of X on R. The mass spectrum for all outer Galaxy molecular clouds in the extended sample with M_W_CO__>3.75x10^4^ is dN/dM_W_CO__{prop.to} M_W_CO__^-1.62+/-0.04^. For all 204 clouds, the slope is -1.79+/-0.03. The average ratio of virial- to W_CO_-mass of all clouds is 1.4+/-1.3 (1σ), with a possible dependency on R and/or mass: there are indications that the ratio increases for clouds at larger R, or for clouds of smaller mass, but the cloud-cloud variations are large. Most clouds in the sample could be in equilibrium through confinement by external pressure. All clouds can be made to be in virial equilibrium, by allowing X to change by a factor of 4 between R =~4 and 20kpc. This would require the galactic abundance gradient to be steeper than what is expected from an extrapolation of the presently available data.
Brand Jan
Wouterloot Jan G. A.
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