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Star formation and the interstellar medium in M33

Citation

Wilson, Christine Diana (1990) Star formation and the interstellar medium in M33. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-09152008-110204

Abstract

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New millimeter-wave and optical data are used to study high-mass star formation and its relationship with the interstellar medium in the inner 4' radius of the nearby spiral galaxy M33. The total mass of molecular hydrogen in this region is 3.4x10[superscript 7] M[...], roughly twice the mass in atomic hydrogen. The predicted atomic hydrogen column density from atomic mantles of molecular clouds is similar to the observed mean atomic hydrogen column density. Thus probably only a small fraction of the atomic hydrogen is in a diffuse component not directly associated with molecular clouds.

The properties of 38 individual molecular clouds in M33 (velocity widths, diameters, peak brightness temperatures, and masses) are very similar to those of Galactic molecular clouds. Masses derived from the virial theorem and from the integrated CO fluxes agree to within 10%, which implies that the value of the conversion factor from CO flux to H[subscript 2] column density, α, is the same in M33 and the Galaxy. This is the first direct measurement of the value of α in an external galaxy.

The mass distribution of clouds in M33 is consistent with that derived in the Galaxy for M = 0.8 - 4 x 10[superscript 5] M[...], but shows a total lack of clouds with masses greater than 4 x 10[superscript 5] M[...]. A simple model is proposed to explain the high-mass cutoff to the mass distribution as arising from the competing processes of cloud growth through accretion and cloud destruction due to star formation. Comparison of the flux detected with the interferometer with single dish data indicates that 50% of the molecular gas resides in structures less massive than 0.8 x10[superscript 5] M[...], in contrast to the Galaxy where only 15% of the molecular gas mass is in these smaller structures.

41 OB associations each containing at least ten blue stars have been identified in this region of M33. The associations have mean radii of 40 pc, masses in stars of mass > 20 M[...] of 600 M[...], and ages of 8x10[superscript 6] yr. No evidence is found for a gradient in the ratio of blue to red supergiants in the inner two kiloparsecs of M33, despite a factor of two decrease in the metallicity over this range of radius. The blue luminosity function for stars in the field is deficient at bright magnitudes relative to the luminosity function for stars in associations. In addition, the associations in the northern arm contain no stars more massive than 20 M[...] while associations in the southern arm contain several stars with masses of 60 M[...]. These differences may be due to a sudden cessation of star formation resulting in a larger mean age for the stars or to a smaller upper mass cutoff to the initial mass function in the field and the northern arm.

The offset between the molecular and atomic gas peaks in the southern spiral arm is consistent with streaming motions expected as the gas enters a spiral density wave. In addition, the OB associations in the southern spiral arm show a weak age gradient perpendicular to the arm, consistent with the presence of a density wave. However, the northern arm has an age gradient running along the arm, as would be expected for a stochastic star formation arm.

A comparison of the spatial distributions of the Hα, CO, and HI emission peaks indicates that the atomic gas is probably formed via photo-dissociation of the molecular gas by recent high-mass star formation. Roughly two-thirds of the molecular clouds with M > 0.5 x 10[superscript 5] M[...] contain recent massive star formation with inferred high-mass star formation rates for individual clouds ranging from 5x10[superscript -6] to 5x10[superscript -4] M[...] yr[superscript -1]. The photo-dissociating flux produced by these stars is sufficient to produce the amount of atomic gas near the molecular clouds if the molecular clouds have a clumped structure.

The high-mass star formation rate and efficiency are measured by combining optical, far-infrared, and millimeter data. The high-mass star formation rate calculated from the Hα emission is 0.007 M[...] yr[superscript -1] over an area of 5 kpc[superscript 2], which corresponds to a total star formation rate of 0.04 M[...] yr[superscript -1]. The star formation rate obtained from the far-infrared emission agrees with the rate obtained from the Hα emission to within a factor of two. A lower limit to the high-mass star formation rate obtained from number counts of blue stars is consistent with the rates obtained from Hα and far-infrared emission. The southern spiral arm and the molecular complexes have similar star formation efficiencies to regions of the galaxy containing little molecular gas. Thus the large amount of star formation occurring in the spiral arms of M33 is due to the presence of large amounts of molecular gas and not to an increased star formation efficiency.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Scoville, Nicholas Zabriskie
Thesis Committee:
  • Unknown, Unknown
Defense Date:9 May 1990
Record Number:CaltechETD:etd-09152008-110204
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-09152008-110204
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3555
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:31 Oct 2008
Last Modified:26 Dec 2012 03:00

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