ASPH 611 Term Project
A University of Calgary Department of Physics and Astronomy
Graduate Course in Radio Astronomy
Radio Sources of Interest
Annalisa De Cia, December 12, 2005
Once we detected the HI line, the goal became observing the 1420 MHz line emission from different sources in the sky; for some given position (i.e. RA and Dec, or Galactic latitude and longitude) we are going to output a spectrum (i.e. flux vs frequency) and integrate it over time.
Therefore, our resulting data will be a cube that can be represented on different sky planes centered at different frequencies (to consider the motion of the emitting source that shift the HI line). This gives a velocity distribution of the source, and on each plane the image will represent the intensity of the flux; we will have a spectrum for each position and time, and integrate it over time.
The quality of our results will depend on our capability to:
The most powerful source of HI is the Galaxy itself; its arms, full of gas and dust, especially Hydrogen, can produce a strong (Tb,~ 100 K) and diffuse (~30 Galctic degrees) HI emission along the Galactic plane. The velocity distribution of the Galactic neutral Hydrogen (i.e. the shift, the width and the profile of the HI line) depends on three main factors:
If the source we are considering is extragalactic, we have to consider the cosmological redshift. On the other hand, if we detect an ``injustified'' negative velocity for the source, it could be claimed that it is due to the cosmological redshift, and declared as an extragalactic source. However, this is not very likely to occur in our case...
The intensity of the HI flux we receive depends:
We are going to focus on some of the strongest radio sources. These sources were the most important to early radio astronomy, and thus have been well studied: CasA, CygA and Cygnus Loop, that are pretty close to the Galactic plane, and TauA, Orion region and VirA, that are farther from the "noisy" Galactic plane. The Moon's strong 1420 continuum emission seems to be a good candidate for such an observation since it is large and therefore suffers less apature dialation than a point source.
The continuum radio emission of CasA decreases not only with the frequency, but even with the time; the source loses 1.7% of its flux per year1,6 at 1420 MHz, indicating other values for the current flux and making it a bad source for calibration; however, our sensitivity will probably not be good enough to appreciate the change of intensity, and CasA ramains the strongest and moststudied extra-solar radio source. It was discovered in 1948, only three years before the first detection of the HI line (already predicted by Von de Hulst in 1945).
CasA is strongly believed to be a Super Nova Remnant (SNR) with the radio source is identified as 3C 461. Here is an image of CasA in continuum and its spectrum, taken from the Canadian Galactic Plane Survey:
The flux of CygA is variable and the position is frequency dependent, but again our telescope performance will not enable us to appreciate these fluctuations. The radio emission is significantly polarized, meaning that synchrotron processes are probably responsible for the radiation.
As the redshift (z) suggests, CygA is an extra-Galactic source known as 3C 405. The morphology of this source is still not certain; however, progress has been made since in the 1950s, when people thought it was a merging of two galaxies heating up Hydrogen. Today we know CygA is a radio loud active galaxy, with two quite symmetric radio lobes (one expanding toward us and one away from us), probably an FR2.
Here is an image of CygA at in continuum and its spectrum, taken from the Canadian Galactic Plane Survey:
Cygnus Loop (or Cygnus X?)
The source TauA can be identified as SN1054 (the Supernova observed in 1054 A.D. by Chinese astronomers), 3C 144, and M1 - the famous Crab Nebula.
The fact that "crab" is sometimes used as a unit makes us confident to use this source as calibrator (if it will be detected); however, TauA is also known as PSR 0531, the pulsar probably in the center of the SNR, the remnant of the implosion, and has a variable flux. Again, we will not be able to detect these small fluctuations, since our telescope response to this total flux will be 1.3 K.
The Orion Region is an extended strong HI emitting region that contains many HI and HII features, like the Orion nebula and Barnard's ring, and is far from the Galactic plane:
Its large Galactic latitude makes the Orion Region likely to be observed, because of the smaller diffuse HI background off the Galactic plane. Its extended structure makes the source a poor candidate for calibration; however, our resolution of ~4o will allow us to look at the Orion Region no more as a point source, integrating the flux for a little more time and maybe obtain some information about its rotation structure, or at least appreciate the broadening of the line from random internal motions.
VirA is an isolated HI source, very far from the Galactic plane, where the background is HI poor and measurments can be made easier. We hope that its position will help us in detecting it, even if our antenna response will be only ~0.28K if the noise is not that high.
VirA is also known as 3C 274, or M87, the quasar famous for its jet, because it is visible in so many wavelengths, from radio to X-Rays; however, even if in the radio frequencies of the sources look broader, our resolution will not be enough to appreciate the structure of the "Smoking gun"...but it will be a target.
Here is an image of VirA in continuum and its spectrum, taken from the Canadian Galactic Plane Survey:
The strongest radio source after the Sun is the Moon. It produces a continuum thermal radio emission (with low polarization), that embraces even the HI frequency.
Since its angular size is about half a degree, we will still see it as a point source, but we could integrate its flux over about 15 minutes (the time it will take to cross our 4 degree beam) and its strong HI emission will increase our antenna temperature by ~8K.
Its flux changes only slightly in a Lunation, and this change increases with wavelength. In our case, for the HI frequency, the Moon phase is completely negligible.
The only velocity component that will influence the shift of the line will be the rotation of the Earth, since the Moon has only a very small radial velocity component with respect to the Earth (its eccentricity is only 0.0167).
Last modified: 10:22 am July 17, 2014