Engineering topics

Devereux, Nicholas A.

Publications and source records attributed to Devereux, Nicholas A..

11 records

NASA NTRS1994

Two views of the Andromeda Galaxy H-alpha and far infrared

A complete H-alpha image of the Andromeda Galaxy (M31) is presented allowing the first direct measurement of the total H-alpha luminosity which is (7.3 +/- 2.4) x 10(exp 6) solar luminosity. The H-alpha emission is associated with three morphologically distinct components; a large scale star-forming ring, approximately 1.65 deg in diameter, contributing 66% of the total H-alpha emission, a bright nucleus contributing 6% of the total H-alpha emission with the remaining 28% contributed by a previously unidentified component of extended and filamentary H-alpha emission interior to the star forming ring. The correspondence between the H-alpha image and the Infrared Astronomy Satellite (IRAS) far-infrared high resolution image is striking when both are convolved to a common resolution of 105 arcsec. The close correspondence between the far-infrared and H-alpha images suggests a common origin for the two emissions. The star-forming ring contributes 70% of the far-infrared luminosity of M31. Evidence that the ring emission is energized by high mass stars includes the fact that peaks in the far-infrared emission coincide identically with H II regions in the H-alpha image. In addition, the far-infrared to H-alpha luminosity ratio within the star-forming ring is similar to what one would expect for H II regions powered by stars of spectral types ranging between O9 and B0. The origin of the filamentary H-alpha and far-infrared luminosity interior to the star-forming ring is less clear, but it is almost certainly not produced by high mass stars.

Devereux, Nicholas A.↗

NASA NTRS1994

The origin of the far-infrared luminosity within the spiral galaxy M101

High resolution 60 and 100 micron images obtained with the Infared Astronomical Satellite (IRAS) are compared with H alpha images in order to investigate the origin of the far-infrared luminosity within the late-type spiral galaxy M101. There is a good correspondence between the far-infrared and H-alpha morphology. The far-infrared and H-alpha luminosities have been measured at 129 independent locations on the star forming disk of M101. After correcting the H-alpha luminosity for extinction and extrapolating the IRAS (40-120 microns) luminosity to 1000 microns we find that the far-infrared luminosity is commensurate with that expected from the O and B stars which are required to ionize the hydrogen gas, at all locations within M101. Additionally, the IRAS HiRes 60 and 100 micron images reveal that the dust temperature peaks coincide identically with the location of H II regions. The far-infrared luminosity of M101 is radiated primarily by dust with temperatures well in excess of that expected for cirrus, but similar to that observed for Galactic and extragalactic H II regions.

Devereux, Nicholas A.↗

NASA NTRS1992

The origin of the far-infrared luminosity within the spiral galaxy M51

High angular resolution 170 micron and H-alpha maps have been recombined in order to elucidate the origin of the far-IR luminosity within the late-type spiral galaxy M51. New quantitative evidence is presented supporting a common origin for the far-IR and H-alpha luminosities is the striking correspondence, at all locations measured within M51, between the luminosity in the far IR and that expected from the O and B stars which are required to ionize the hydrogen gas. The warm 33 K dust temperature measured for M51 is similar to that measured for galactic and extragalactic star formation regions. It is concluded that O and B stars can easily generate both the far-IR luminosity and the warm dust temperature at all locations within M51. The dust mass required to radiate the IRAS 60 and 100 micron emission measured for M51 represents only a small percentage of the total dust content of M51. Constraints are placed on the mass and temperature of the cold dust.

Devereux, Nicholas A.↗

NASA NTRS1991

Molecular clouds in starburst galaxy nuclei

New CO observations of a complete sample of starburst galaxies are presented in order to determine the range of molecular gas masses and star-formation efficiencies that characterize these systems. The properties of the galaxies in the sample are given, and the H-alpha and R-band CCD images for six of the galaxies are shown. The novel feature of this study is that the galaxies in this study are infrared selected as opposed to optically selected. They exhibit levels of nuclear star formation activity which are comparable to the starburst galaxies M82 and NGC 253. A subset of galaxies with unusually narrow CO line widths, high star-formation efficiencies, and compact 10-micron distributions is revealed. It is proposed that, if gas distributions in these galaxies are also compact, as the narrow linewidths suggest, the high yields of massive stars per unit mass of molecular gas may be attributed to a star-formation mechanism which becomes more efficient as the gas density increases.

Young, Judith S.↗

NASA NTRS1991

The rate and efficiency of high-mass star formation along the Hubble sequence

Data obtained with IRAS are used to compare and contrast the global star formation rates for a galactic sample which represents essentially all known noninteracting spiral and lenticular galaxies within 40 Mpc. The distribution of 60 micron luminosity is similar for spirals of types Sa-Scd inclusively, although the luminosities of the very early and very late types are, on average, one order of magnitude lower. High-mass star formation rates are similar for early, intermediate, and late type spirals, and the average high-mass star formation rate per unit molecular gas mass is independent of type for spiral galaxies. A remarkable homogeneity exists in the high-mass star-forming capabilities of spiral galaxies, particularly among the Sa-Scd types. The Hubble sequence is therefore not a sequence in the present-day rate or production efficiency of high-mass stars.

Devereux, Nicholas A.↗

NASA NTRS1990

The gas/dust ratio in spiral galaxies

IRAS data are used here to calculate warm dust masses, which are then compared with the molecular and atomic gas masses for 58 spiral galaxies in order to constrain the fraction and the phase of the interstellar medium in spiral galaxies that contributes to the emission measured by IRAS. The dispersion in the ratio of dust mass to total gas mass is larger than expected on the basis of measurement errors. The dispersion in the ratio of dust mass to inner disk gas mass is less than the dispersion in the ratio of IR to radio emission. The inner gas to warm dust mass ratio for spiral galaxies is 1080 + or - 70, indicating that 80-90 percent of the dust mass in spiral galaxies is radiating at over 100 microns and has a temperature less than about 30 K. However, the bulk of the dust in spiral galaxies is less than about 15 K regardless of the phase of the ISM. Both H I and H2-associated dust contributes to the warm 30 K emission.

Devereux, Nicholas A.↗

NASA NTRS1990

The FCRAO extragalactic CO survey: Global properties of galaxies

Since stars form in molecular clouds, a critical element in studies of galaxy evolution is knowledge of the molecular content of a large sample of galaxies. To this end, researchers undertook a survey of CO emission from galaxies using the Fire College Radio Astronomy Observatory (FCRAO) 14-m millimeter telescope at 115 GHz. The aim was to better understand the differences found among and within galaxies with regard to the efficiency of star and cloud formation. The galaxies observed as part of the FCRAO Extragalactic CO Survey were selected on the basis of their optical or infrared properties. The galaxies observed thus far are (1) brighter than B sub T (sub o)=12.5 in the blue, or (2) brighter than 20 Jy at 100 microns. From major axis CO observations at 45 seconds resolution and spacing in over 200 galaxies, researchers determined the CO radial distributions, and derived global CO fluxes (cf. Kenney and Young 1988); H2 masses were derived using the conversion factor N(H2)/I sub CO=2.8 times 10 to the 20th power cm(-2)/K km s(-1)) (Bloemen et al. 1986). Here, researchers concentrate on the global galaxy properties within the sample. Neutral hydrogen (HI) masses for the sample galaxies were taken from Huchtmeier et al. (1983), blue luminosities and morphological types were taken from RC2. IR luminosities, colors, dust temperatures and dust masses were determined from coadded Infrared Astronomy Satellite (IRAS) data (Young et al. 1989). They have chosen to first compare absolute luminosities and masses in order to determine the slope and scatter in each correlation; next they investigate luminosity independent ratios in order to intercompare large and small galaxies.

Young, J. S.↗

NASA NTRS1990

Evidence against a simple two-component model for the far-infrared emission from galaxies

Two of the first Infrared Astronomy Satellite (IRAS) results were that galaxies have a wide range of values for the ratio of 60 micron to 100 micron flux density (0.2 less than or equal to S sub 60/S sub 100 less than or equal to 1.0) and that this ratio is correlated with L sub fir, L sub b, L sub fir being the total far-infrared luminosity and L sub b being the luminosity at visible wavelengths (de Jong et al. 1984; Soifer et al. 1984). From these results arose the following simple model for the far-infrared emission from galaxies (de Jong et al. 1984), which has remained the standard model ever since. In this model, the far-infrared emission comes from two dust components: warm dust (T approx. equals 50 K) intermingled with, and heated by, young massive OB stars in molecular clouds and HII regions, and colder dust (T approx. equals 20 K) associated with the diffuse atomic hydrogen in the interstellar medium and heated by the general interstellar radiation field. As the number of young stars in a galaxy increases, S sub 60/S sub 100 increases, because there is a greater proportion of warm dust, and so does L sub fir/L sub b, because most of the radiation from the young stars is absorbed by the dust, leading to a swifter increase in far-infrared emission than in visible light. Although this model explains the basic IRAS results, it is inelegant. It uses two free parameters to fit two data (the 60 and 100 micron flux densities)-and there are now several observations that contradict it. Despite these major problems with the two-component model, it is not clear what should be put in its place. When considering possible models for the far-infrared emission from galaxies, the observational evidence for our own galaxy must be considered. Researchers suspect that the study by Boulanger and Perault (1988) of the far-infrared properties of the local interstellar medium may be particularly relevant. They showed that molecular clouds are leaky - that most of the light from OB stars in molecular clouds does not heat the dust in the clouds, but instead leaks out. The consequence of this is that that while most of the far-infrared emission from the solar neighborhood is from dust associated with diffuse HI, this dust is mostly heated by young stars.

Eales, Stephen A.↗

NASA NTRS1990

A comparison of the radial distribution of molecular gas and non-thermal radio continuum in spiral disks

The present study includes 65 spiral galaxies selected from the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey for which the major axis distributions of CO emission and 1.49 GHz radio continuum emission are well determined. The radial distribution of the CO emission has been measured with the FCRAO at positions along the major axis that are spaced by one half power beam width (HPBW) (45 seconds). The radial profile of the 1.49 GHz radio continuum emission was constructed by determining the radio emission at the location of the CO measurements from the 1.49 GHz maps of Condon (1987). Large, greater than a factor of ten, radially decreasing gradients in the star formation efficiency are observed for a small percentage, approx. 10 percent, of the spirals in this sample. The majority of spirals, however, are associated with small gradients in the star formation efficiency that do not systematically increase or decrease with radius. That the star formation efficiency does not systematically decrease with radius tends to argue against a global dynamical mechanism, such as a spiral density wave, for being the dominant mechanism triggering disk star formation for the majority of spirals in this sample. The results tend to support the view that the star formation in spiral disks is dominated by a local process that depends more on the molecular cloud properties than the dynamical structure of a galaxy.

Devereux, Nicholas A.↗

NASA NTRS1990

The origin of the far-infrared luminosity from spiral galaxies

The relationship between the far-infrared and H-alpha luminosity has been investigated for a sample of 124 spiral galaxies. After correcting the H-alpha luminosities for an average 1 mag of extinction and extrapolating the IRAS 40-120 micron luminosity to 1000 microns, it is found that the mean ratio of H-alpha to far-infrared luminosity is comparable to that expected from H II regions powered by stars with masses larger than 6 solar masses. This result constitutes strong evidence in support of the view that high-mass O and B stars are responsible for both the H-alpha and far-infrared emission in spiral galaxies of high, billion solar luminosities or greater, far-infrared luminosity.

Devereux, Nicholas A.↗

NASA NTRS1989

A reevaluation of the infrared-radio correlation for spiral galaxies

The infrared radio correlation has been reexamined for a sample of 237 optically bright spiral galaxies which range from 10 to the 8th to 10 to the 11th solar luminosities in far-infrared luminosity. The slope of the correlation is not unity. A simple model in which dust heating by both star formation and the interstellar radiation field contribute to the far-infrared luminosity can account for the nonunity slope. The model differs from previous two component models, however, in that the relative contribution of the two components is independent of far-infrared color temperature, but is dependent on the far-infrared luminosity.

Devereux, Nicholas A.↗