Spiral Arm Tracers Research Articles

Large-scale magnetic-field structure in the spiral galaxy M83

THE large-scale structure of magnetic fields in spiral galaxies, as deduced from the linear polarization of scattered starlight1 and from the intensity and linear polarization of continuum synchrotron radio emission2, tends to follow the visible spiral arm structure. Both axisymmetric (in the galaxies IC342 and M31; refs 3 and 4) and bisymmetric (in M81; ref. 5) patterns are seen, the latter showing a field reversal between the arms. We have measured the linear polarization at a spatial resolution of ∼2 kpc in VLA (Very Large Array) observations of 20-cm continuum radio emission from the bright, nearly face-on, spiral galaxy M83 (NGC5236). The strongest linearly polarized emission is from two giant arcs, ∼30 kpc long, situated roughly opposite each other in the dark outer regions of the galaxy, ∼12 kpc from the centre. These regions do not coincide with any prominent spiral arm tracers, and from a comparison with earlier results at 6-cm wavelength, we conclude that the low polarization in the centre of the galaxy indicates disorder in the interstellar magnetic field, probably connected with star formation activity observed there.

A study of faint young open clusters as tracers of spiral features in our galaxy paper 3: Collinder 97 (OC1 506)

Photoelectric and photographic photometry of twenty-nine stars was done in the field of the open cluster Collinder 97 ≡ OC1 506. Of these stars, a total of twenty-four have been found to be possible members. There is apparently no interstellar extinction in the direction of this cluster which is in the constellation of Monoceros: itsE(B-V) = 0.0 mag. This cluster is situated at a distance of 0.63 ± 0.01 kpc, which is well within the local arm of our Galaxy. The age of this cluster is in the range of 1 × 108 to 5.9 × l08 yr, which puts it in an older age group. Thus, it cannot be specifically considered as a spiral-arm tracer in the study of our Galaxy.

Distribution and motion of CO in M31

M31 is among the few galaxies whose solid angle is more than 100 times the beam area of currently operational millimeter-wave telescopes. Roughly 10% of its area has been mapped in the J=1→0 line of CO with the 7-m telescope at Bell Laboratories. The data lie on the major and minor axes and in a filled 20′ by 15′ field. The average CO emissivity is less than 1/5 that of the Milky Way Galaxy, and the inferred H2 column densities are less than 10% of the HI column densities at the same positions. Thus M31 has a predominantly atomic interstellar medium, whereas our Galaxy seems to have a predominantly molecular interstellar medium. The molecular emission is strongly concentrated in spiral-arm segments, with greater than 25:1 contrast between arm and interarm regions. Overall, the CO emission is well correlated with the HI emission, but shows greater concentration to the spiral arms. There is no evidence for a spatial separation of the HI and CO arms to within our resolution (~1′): all the spiral-arm tracers are spatially coincident. Almost all the gas on the minor axis exhibits non-circular velocities of about 20 km s-1. There are systematic streaming motions as large as 80 km s-1 associated with spiral arms.

Giant clouds and star-forming regions as spiral-arm tracers

A variety of observations suggest that clouds of 106-107 M⊙ and extended regions of star formation are the best tracers of spiral structure in the Milky Way.

The Young Open Cluster NGC 2384

AbstractCombined photoelectric – photographic UBV-photometry for NGC 2384 is discussed. The results achieved so far are summarized and indicate that this cluster is young enough to be used as spiral arm tracer out to a distance of 3.27 kpc. The colour excess E(B-V) is 0.31, the apparent distance modulus is 13m.50 and its earliest spectral type is BO.

THE YOUNG OPEN CLUSTERS BERKELEY 62 AND BERKELEY 86.

ABSTRACT Berkeley 62 and Berkeley 86, two previously unstudied open clusters whose brightest probable members are OB stars are discussed. Photoelectric UBV observations show Be 62 lying at a distance of 2.05 + or - 0.24 kpc with a reddening of E(B-V) = 0.86 + or - 0.04 and earliest spectral type B1. It is found that Be 86 lies 1.72 + or - 0.20 kpc from the sun with a reddening of E(B-V) = 0.96 + or - 0.07 and earliest spectral type O9. Spectroscopic observations of six stars in Be 86 indicate a ratio E(U-B)/E(B-V) = 0.77 + or - 0.05, which is in agreement with other determinations made in the Cygnus region. It is noted that both clusters are young enough to be used as spiral arm tracers.

PROBABLE OB STAR MEMBERS IN ELEVEN BERKELEY CLUSTERS

ABSTRACT The Setteducati-Weaver catalog of new open star clusters discovered on the Palomar Sky Survey plates was compared recently with the catalogs of OB stars published by the Hamburg and Warner and Swasey Observatories. This resulted in the identification of eleven clusters which appear to contain OB stars and which may thus serve as important additional spiral arm tracers in the galactic longitude range of 75 to 135 . Published photometric studies are available for only two of these clusters. Key words: open clusters - galactic structure - OB stars

Reflection nebulae and spiral structure

It is shown that stars embedded in reflection nebulosity may be used as spiral-arm tracers.

The galactic distribution of young Cepheids

The distribution of long-period Population I cepheids is studied. An age comparison of cepheids and galactic clusters shows that cepheids with periods > 11.25 d, corresponding to ages of ≲ 30 × 106 yr, should be almost as good spiral arm tracers as galactic clusters with earliest types b 2–3. The distances of cepheids are determined from a revised period-luminosity-colour relation and from colour excesses, for the determination of which a new, purely photometric method is given. The resulting distribution of cepheids shows a good correlation with the spiral arms, as traced by young clusters.

Local stellar distribution and galactic spiral structure

Aside from the well-known spiral arm tracers such as the OB associations, young galactic clusters, WR stars and possibly the long-period classical cepheids, the more common stars in the neighborhood of the sun within 2 kpc show little or no relationship to the local spiral structure of the galaxy.