- Additional Milky
Way Galaxey Resources
The
Milky Way Galaxy, sometimes simply called the Galaxy, is a spiral system
consisting of several billion stars, one of which is the Sun.
It takes its name from the Milky Way, the irregular luminous band of stars
and gas clouds that stretches across the sky. Although the Earth
lies well within the Galaxy, astronomers do not have as clear an understanding
of its nature as they do of some external star systems. Because a thick
layer of interstellar dust obscures much of the Galaxy from scrutiny by
optical telescopes, astronomers can only determine its large-scale structure
with the aid of radio and infrared telescopes, which can detect the forms
of radiation that penetrate the obscuring matter.
The
Milky Way Galaxy is generally considered an Sbc-type galaxy. It has a
central bulge of stars and spiral arms of gas and stars in a disk. We
view the Milky Way essentially edge-on from our perspective near the plane
of the disk and 28,000 light years from the Galactic center. In recent
decades. Galactic astronomy has benefitted from a tremendous broadening
of coverage of the electromagnetic spectrum by ground-based and spaced-based
instruments. Presented below are images of the sky near the Galactic plane
in spectral lines and continuum bands spanning a frequency range of more
than 14 orders of magnitude. The images are derived from several space
and ground-based surveys, many of which are available through the Astrophysics
Data Facility and the Astronomical
Data Center of the Space
Science Data Operations Office at NASA
Goddard Space Flight Center.
Each image
represents a 360° false color view of the Milky Way within 10°
of the plane; the images are in Galactic coordinates with the direction
of the Galactic center in the center of each. For scale, the vertical
dimension of each image is forty times the angular diameter of the full
moon on the sky; the areas shown represent about one-sixth of the
entire sky. The image in the finder chart is derived from the IRAS
100 micron map with COBE
DIRBE 3.5 micron contours overlaid.
For
users interested in quantitative examination of the maps, the actual
data files corresponding to the images are available for retrieval by
anonymous
ftp. These were derived from the surveys cited below and in most
cases were transformed and interpolated to lie on regular grids in Galactic
coordinates, with sampling appropriate to the resolution.
The files
are not known to contain errors, but the user is advised to check any
scientific results against the data from the original sources. The user
is also referred to the original references for descriptions of backgrounds,
sensitivities, resolutions, and other considerations relevant to the
interpretation of the data.
The files
are compressed and in FITS image format, a binary format commonly used
in astronomy. The infrared, near-infrared, and X-ray images contain
three planes, one for each band represented in the corresponding image
in the poster. The optical image is divided into four parts, by Galactic
quadrant, because it is very large. Descriptions of the FITS standard
and links to software for reading and displaying FITS images are available
from the FITS
Support Office at NASA/GSFC.
Notes on
the production of the individual maps are included in the headers of
the files.
Digital
versions of several of the original surveys may be accessed, and images
of selected fields constructed, online via the SkyView
service. The complete data for some of the surveys are available via
the World Wide Web; links are listed in the relevant sections.
Column
density of atomic hydrogen, derived on the assumption of optically thin
emission, from radio surveys of the 21-cm spectral line of hydrogen.
On a large scale the 21-cm emission traces the "warm" interstellar medium,
which is organized into diffuse clouds of gas and dust that have sizes
of up to hundreds of light years. Most of the image is based on the
Leiden-Dwingeloo Survey of Galactic Neutral Hydrogen, made available
by the authors in advance of publication. This survey was conducted
over a period of 4 years using the Dwingeloo 25-m radio telescope, operated
by the Netherlands Foundation for Research
in Astronomy. The data were corrected for sidelobe contamination
in collaboration with the University
of Bonn.
References:
Burton,
W. B. 1985, Astron. Astrophys. Suppl. Ser., 62, 365
Hartmann, Dap, & Burton, W. B., "Atlas of Galactic Neutral Hydrogen,"
Cambridge Univ. Press, (1997, book and CD-ROM)
Kerr,
F. J., et al. 1986, Astron. Astrophys. Suppl. Ser.
Frequency:
1.4 GHz
Column
densities: 10 x 1020 -230 x 1020 cm-2
Angular
resolution: 45-60'
Column
density of molecular hydrogen inferred from the intensity of the J =
1-0 spectral line of carbon monoxide, a standard tracer of the cold,
dense parts of the interstellar medium. Such gas is concentrated in
the spiral arms in discrete "molecular clouds" and most molecular clouds
are sites of star formation. The molecular gas is predominantly H2,
but H2 is difficult to detect directly at interstellar conditions
and CO, the second most abundant interstellar molecule, is observed
as a surrogate. The column densities were derived on the assumption
of a constant proportionality between the column density of H2
and the intensity of the CO emission. Black areas in the image indicate
regions not yet surveyed for CO.
References:
Dame,
T. M., et al. 1987, Astrophysical Journal, 322, 706
Digel, S. W., & Dame, T. M. 1995, unpublished update
Online
data access:
CO
data from ADC archives
Frequency:
115 GHz
Column
densities: 12 x 1020 -285 x 1020 cm-2
Angular
resolution: 30'
Composite
mid and far-infrared intensity observed by theInfrared
Astronomical Satellite (IRAS) in 12, 60, and 100 micron
wavelength bands. The images are encoded in the blue, green, and red
color ranges, respectively. Most of the emission is thermal, from interstellar
dust warmed by absorbed starlight, including that in star-forming regions
embedded in interstellar clouds. The image here is a mosaic of IRAS
Sky Survey Atlas plates; emission from interplanetary dust in the solar
system, the "zodiacal emission," was modeled and subtracted in the production
of the Atlas at the Infrared Processing
and Analysis Center (IPAC). The black, wedge-shaped areas indicate
gaps in the IRAS survey.
Reference:
Wheelock, S. L., et al. 1994, IRAS Sky Survey Atlas Explanatory
Supplement, JPL Publication 94-11 (Pasadena: JPL) Order:
CASI HC A08/MF A02
Online
data access:
IRAS pages at
IPAC
ADF/IRAS
interface to all released IRAS data products
Frequencies:
3.0 x 103-25 x 103 GHz
Intensities: 0.25-100 (12 microns), 1.5-750 (60 microns), 12-750
MJy sr-1 (100 microns)
Angular resolution: 5'
Composite
near-infrared intensity observed by the Diffuse Infrared Background
Experiment (DIRBE) instrument on the Cosmic
Background Explorer (COBE) in the 1.25, 2.2, and 3.5 micron
wavelength bands. The images are encoded in the blue, green, and red
color ranges, respectively. Most of the emission at these wavelengths
is from cool, low-mass K stars in the disk and bulge of the Milky Way.
Interstellar dust does not strongly obscure emission at these wavelengths;
the maps trace emission all the way through the Galaxy, although absorption
in the 1.25 micron band is evident in the general direction of the Galactic
center.
Reference:
Hauser,
M. G., Kelsall, T., Leisawitz, D., & Weiland, J. 1995, COBE
Diffuse Infrared Background Experiment Explanatory Supplement, Version
2.0, COBE Ref. Pub. No. 95-A (Greenbelt, MD: NASA/GSFC)
Online
data access:
COBE data
from the COBE Home Page at the ADF
Frequencies:
86 x 103-240 x 103 GHz
Intensities:
0.5-9 (1.25 microns), 0.35-20 (2.2 microns), 0.22-4.5 MJy sr-1
(3.5 microns)
Angular
resolution: 42'
Intensity
of visible light from a mosaic of wide-field photographs by Laustsen,
Madsen, & West (1987). Scanned images of the original prints were kindly
provided by C. Madsen (ESO). Owing to the strong obscuration by interstellar
dust the light is primarily from stars within a few thousand light-years
of the Sun, nearby on the scale of the Milky Way, which has a diameter
on the order of 100,000 light years. Nebulosity from hot, low-density
gas is widespread in the image. Dark patches are due to absorbing dust
clouds, which are evident in theMolecular Hydrogen
and Infrared maps as emission regions. The mosaic
is constructed from eight photographs. Narrow, vertical gaps are evident
between some photographs, as are slight discontinuities in brightness.
Reference:
Laustsen, S., Madsen, C., West, R. 1987, Exploring the Southern Sky,
(Berlin: Springer-Verlag)
Online
data access:
Data
files (one for each quadrant of longitude) are available in FITS format
by anonymous ftp
Frequency:
460 x 103 GHz
Intensities:
uncalibrated
Angular
resolution: 1'
Composite
X-ray intensity observed by the Position-Sensitive Proportional Counter
(PSPC) instrument on the Röntgen
Satellite (ROSAT). Images in three broad, soft X-ray bands
centered at 0.25 , 0.75, and 1.5 keV are encoded in the red, green,
and blue color ranges, respectively. In the Milky Way, extended soft
X-ray emission is detected from hot, shocked gas. At the lower energies
especially, the interstellar medium strongly absorbs X-rays, and cold
clouds of interstellar gas are seen as shadows against background X-ray
emission. Color variations indicate variations of absorption or of the
temperatures of emitting regions. The black regions indicate gaps in
the ROSAT survey.
Reference:
Snowden,
S. L., et al. 1995 Astrophys. J., 454, 643
Online
data access:
ROSAT All-Sky Survey at MPE
ROSAT data
archives at the HEASARC
Frequency:
60-360 x 106 GHz
Intensities:
0-20 (0.25 keV), 0-10 (0.75 keV), 0-10 x 10-4 photons arcmin-2
s-1 (1.5 keV)
Angular
resolution: 115'
Intensity
of high-energy gamma-ray emission observed by theEnergetic
Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton
Gamma-Ray Observatory (CGRO). The image includes all photons
with energies greater than 100 MeV. At these extreme energies, most
of the celestial gamma rays originate in collisions of cosmic rays with
hydrogen nuclei in interstellar clouds. The bright, compact sources
near Galactic longitudes 185°, 195°, and 265° indicate
high-energy phenomena associated with the Crab, Geminga, and Vela pulsars,
respectively.
References:
Hunter,
S. D., et al. 1997, Astrophys. J., 481, 205
Thompson,
D. J., et al. 1996, Astrophys. J. Suppl., 107, 227
Online
data access:
EGRET instrument team's
Home Page
EGRET data from
the Compton Observatory SSC
Frequencies:
>2.4 x 1013 GHz
Intensities: 4 x 10-5 - 93 x 10-5
photons cm-2 s-1 sr-1
Angular resolution: ~120'
Major
structural features of the Milky Way (red),
optical H II regions (blue),radio
sources (green), and OB associations (purple)
are labeled in the finder chart. The image in the finder chart is derived
from the IRAS 100 micron map of intensity with contours from
the COBE DIRBE 3.5 micron map overlaid. The axes of the finder
diagram are labelled in degrees of Galactic longitude and latitude.
[CLICK DIAGRAM TO ENLARGE IT]
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