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:

• point to and resolve the source (resolution: our beamsize will be ~4^o)
• detect and measure the correct flux (sensitivity: our antenna temperature for a strong source of 1000 Jy will be ~1.3 K)
• distinguish between different frequencies and map the velocity distribution (energy resolution and actual width of the HI line detected)
• separate the source HI line emission from the continuum, from the background, from the RFI and from the noise: since we cannot manage the human and stocastical influences, we would like to find strong compact HI isolated objects, where the background emission is much smaller than the source.

The most powerful source of HI is the Galaxy itself; its arms, full of gas and dust, especially Hydrogen, can produce a strong (T_b,~ 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:

1. the Earth and Solar System motions: a standard correction will be needed to take out our relative motion within the galaxy (Local Standard of Rest (LSR correction)
2. the Galactic rotation: the line is blueshifted for the arms that are relatively coming toward us (i.e. with a negative velocity) and redshifted for the branches that are going away (positive velocity)
3. the random internal motion of the emitting source (the particles of dust and gas composing an HI nebula, for instance) makes the width of the line larger, as a result of a sum of blueshifts and redshifts, making the source visible in a larger range of velocities
4. the peculiar motions of the source, like expansion or rotation along the sky plane, for example, can distort the line profile. In the case of rotation, one side of the source makes the line redshifted, and the other makes it blueshifted. This gives a double peaked line profile in which the distance between the peaks is proportional to the rotational velocity; the source will be stronger at two different velocity planes in the cubic data.

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:

1. on the continuum radiation that produces the line heating the Hydrogen: in most cases the sources of the continuum are in the background, sometimes they are even extragalactic sources, as in the cases of CygA and VirA
2. on the density of the Hydrogen, the quantity of gas that is heated to produce the line: once a telescope is well calibrated, the intensity of the line can give useful information about the mass of the emitting region
3. on the absortion that occurs, and depends not only on the nature of the dust, but even on the Hydrogen density and continuum flux: sometimes the sources on the background are so strong that they make the nebulas become thick, i.e. the absortion is stronger than the emission (its zero is calibrated on the continuum, and the signal we receive in these cases is less because it is absorbed). This is the case, in different velocity planes, of the strongest sources.

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.

Cassiopeia A

• R.A.(J2000): 23h 23m 25.4s
• Dec: +58^o 48m 38s
• Glat(b): -2.13468
• Glon(l): 111.73610
• Angular size: ~2'
• Flux: 2720 Jy @1GHz (2400 Jy @1420MHz)

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 year^1,6 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:

http://www.seds.org/~spider/spider/Vars/casA.html

Cygnus A

• R.A.(J2000): 19h 59m 28.3s
• Dec: +40^o 44m 02s
• Glat(b): +5.7554756
• Glon(l): 76.1898379
• Angular size: ~2'
• z: 0.0561
• Flux: 1600 Jy @1400MHz (1500 Jy @1420MHz)

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:

http://www.seds.org/messier/photos/photos.html

Cygnus Loop (or Cygnus X?)

• R.A.(J2000): 20h 51m 00s
• Dec: +30^o 40m
• Glat(b): -8.5
• Glon(l): 74.0
• Angoular size: 230' x 160'
• Flux: 210 Jy @1Ghz

Taurus A

• R.A.(J2000): 05h 34m 31.97s
• Dec: 22^o 00m 52.1s
• Glat(b): -5.78
• Glon(l): 184.56
• Angoular size: 7' x 5'
• Flux: 1040 Jy @1GHz

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.

http://www.seds.org/messier/m/m001.html

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.

Orion Region

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:

• -15.4 < Glat(b) <-22.4
• 191 < Glon(l) < 223
• 5h 05m < R.A. < 5h 58m
• Angular size: ~13 deg Orion Nebula (M42) is centered on
• R.A.: 05h 35m 24s
• Dec: -05^o 27m 00
• Glat: -18
• Glon: 176.5
• Flux: 360 Jy @1420MHz

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 ~4^o 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.

Virgo A

• R.A.(J2000): 12h 30m 49.42s
• Dec: +12^o 23m 28s
• Glat(b): +74.49
• Glon(l): 283.78
• Flux: 212 Jy @1420MHz

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:

http://www.seds.org/messier/m/m087.html

The Moon

• Mean distance: 384,000 Km
• Radius: 1,738 Km
• Angular size: ~0.52^o
• Brightness temperature: 250 K @1420MHz (~6000 Jy)

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).

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