MULTIWAVELENGTH ATLAS OF GALAXIES

Emission Mechanisms

• OPTICAL: The optical emission from galaxies tells us about the structure and type of stellar (star) population. We show this information in the atlas as optical B or I band images. The B and I refer to the type of optical filter used in front of the detector.

  The B filter is centred at a wavelength of 450 nanometers (nm, where 1nm = 10^-9 m) and I is centred at 820 nm.
  

  As well some galaxies have regions of 10⁴ degree Kelvin (K) ionized gas, that astronomers call warm gas . This gas is usually associated with regions of star formation, and hence will be seen in many spiral galaxies. These warm gas regions will be seen in the optical Halpha+[NII] images of the atlas.

• X-ray: The X-ray emission in spirals is attributed to individual sources such as X-ray binary star systems (XBs) in which two stars orbit around a common centre of gravity. Source variability in some bright X-ray sources in nearby spirals strongly suggest that gas transfer onto an accretion disk is occuring in the XB systems. Many such XB systems may harbour Black Holes.
  

  Thermal emission (10⁷ K hot gas) associated with supernova remnants (SNRs) is also a component of X-ray emission.
  
  

  Chandra X-ray Observatory image of the supernova remnant Cassiopeia A (Cas A)

  
  

  Many elliptical galaxies possess hot gas halos (10⁷ K) and appear to contain emission from XBs as well. The X-ray emission shown in this atlas has energies of 0.2-2.0 keV, whose emission wavelength ranges from 6 - 0.6 nm.

• RADIO: In the radio wavelength region, the distribution of neutral hydrogen (HI) (a normal hydrogen atom) can be detected. This emission depicts the cold phase (10-100 K) of the interstellar medium (ISM). Neutral hydrogen can be detected because the electron orbiting in the hydrogen atom will occasionally change its axis of spin and in doing so emit radiation at a wavelength of 21 cm. Spiral galaxies typically possess large amounts of HI.
  
  

  The atlas also shows 20 cm images, which has radiation at a frequency of 1.49 GigaHertz. [Radio astronomers tend to quote both wavelength and frequency of observations!]

  This continuum emission can consist of both synchrotron emission (radiation emitted by electrons accelerated by a magnetic field) ...
  

  
  

  ... and emission from ionized hydrogen (HII) regions. In spirals the synchrotron emission can be the dominant component at 1.49 GHz and arise from relativistic electrons whose origin is probably SNRs. The radio emission at 20 and 21 cm occurs at 20 and 21 x 10⁷ nm respectively.

• INFRARED: The far infrared (FIR; approx. 10-100 microns) is dominated by thermal emission from dust, warmed by starlight. Two component models now exist that describe the warmer emission originating in star forming molecular clouds, and the cool dust associated with HI. The equivalent wavelengths for 10-100 microns is 10⁴ - 10⁵ nm.
  Infrared image of the centre of the Galaxy