11 research outputs found
Far Ultraviolet Imagery of the Edge-On Spiral Galaxy NGC 4631
Far ultraviolet FUV imagery of the edge-on, Sc/SBd galaxy, NGC 4631 reveals
very strong FUV emission, resulting from active star formation, uniformly
distributed along the galactic mid- plane. Multi-band imagery, HI and HII
position-velocity curves and extinction considerations all imply that the
emission is from the outer edges of the visible galaxy. The overall FUV
morphology of this edge-on disk system is remarkably similar to those of the
so-called "chain galaxies" evident at high redshift, thus suggesting a similar
interpretation for at least some of those distant objects. FUV, U, B and V
magnitudes, measured for 48 star forming regions, along with corresponding
H-alpha and H-beta measurements are used to construct diagnostic color-color
diagrams. Although there are significant exceptions, most of the star forming
regions are less massive and older than 30 Doradus. Comparison with the
expectations from two star formation models yields ages of 2.7 to 10 Myr for
the instantaneous burst (IB) model and star formation cut-off ages of 0 to 9
Myr for the continuous star formation (CSF) model. Interpreted in terms of the
IB model the photometry implies a total created mass in the 48 star forming
regions of 25 million solar-masses. When viewed as resulting from constant star
formation the photometry implies a star formation rate of 0.33 solar-masses/yr.
These results are compared to those derived from FIR and radio observations.
Corrections for FUV emission reprocessed by interstellar grains are estimated.Comment: 29 pages including 6 encapsulated Postscript figures; accepted for
publication in ApJ; changed table forma
A high spatial resolution X-ray and H-alpha study of hot gas in the halos of star-forming disk galaxies. I. Spatial and spectral properties of the diffuse X-ray emission
We present arcsecond resolution Chandra X-ray and ground-based optical
H-alpha imaging of a sample of ten edge-on star-forming disk galaxies (seven
starburst and three ``normal'' spiral galaxies), a sample which covers the full
range of star-formation intensity found in disk galaxies. We use the
unprecedented spatial resolution of the Chandra X-ray observatory to robustly
remove point sources, and hence obtain the X-ray properties of the diffuse
thermal emission alone. The X-ray observations are combined with
comparable-resolution H-alpha and R-band imaging, and presented as a mini-atlas
of images on a common spatial and surface brightness scale. The vertical
distribution of the halo-region X-ray surface brightness is best described as
an exponential, with the observed scale heights lying in the range H_eff = 2 --
4 kpc. The ACIS X-ray spectra of extra-planar emission from all these galaxies
can be fit with a common two-temperature spectral model with an enhanced
alpha-to-iron element ratio. This is consistent with the origin of the X-ray
emitting gas being either metal-enriched merged SN ejecta or shock-heated
ambient halo or disk material with moderate levels of metal depletion onto
dust. The thermal X-ray emission observed in the halos of the starburst
galaxies is either this pre-existing halo medium, which has been swept-up and
shock heated by the starburst-driven wind, or wind material compressed near the
walls of the outflow by reverse shocks within the wind. In either case the
X-ray emission provides us with a powerful probe of the properties of gaseous
halos around star-forming disk galaxies.Comment: To appear in April 2004 edition of ApJS. For high resolution version,
see http://proteus.pha.jhu.edu/~dks/ Accepted version, now has nuclear and
total diffuse emission fluxes and luminosities, a few other minor change
A Sino-German lambda6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region
(Abridged) Polarization measurements of the Galactic plane at lambda6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, hence enable us to investigate properties of the Galactic magnetic fields and electron density. We are conducting a new lambda6 cm continuum and polarization survey of the Galactic plane covering 10degr < l < 230degr and |b|<5degr. Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The lambda6 cm data are analyzed together with maps at other bands. We discuss some results for the first survey region, 7degr X 10degr in size, centered at (l,b)=(125.5degr, 0degr). Two new passive Faraday screens, G125.6-1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6-1.8, with a size of about 46 pc, has neither correspondence in enhanced Halpha emission nor a counterpart in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 microGauss for G124.9+0.1, and exceeding 6.4 microGauss for G125.6-1.8. We obtained an estimate of 2.5 and 6.3 mK/kpc for the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak