Home Page of Dr Radhakhrishna SOMANAH (Dinesh)


My interest is more on a "larger" scale, i.e. basically extragalactic radio sources, mainly radio galaxies, quasars and certain aspects of cosmology.

Radio galaxy samples have well defined and understood selection criteria based on their radio emission, spectra and morphology, and they generally have strong emission lines making the determination of their red-shifts far easier than for normal galaxies at comparable distances. Radio galaxies have played a crucial role in our understanding of galaxy evolution.They are very good probes for IGM parameters at different red-shifts, which is very useful for cosmological models. The radio properties of powerful extended radio sources may be used to estimate the ambient gas density in the vicinity of radio lobes.

Because of the difficulties in classifying radio galaxies, Leahy (1992) and Leahy and Laing (1993) have chosen the term DRAGN which stands for " Double Radio sources Associated with Galactic Nuclei ".


can be naively classified as Fanaroff-Riley I (FRI) or Fanaroff-Riley II (FRII) class, but many will not fit this classification.

In their classic paper, Fanaroff and Riley (1974) divided "extended" radio sources into the following two structural classes:

The remarkable thing about this simple criterion is that it correlates very closely with radio power. There is a break power of about 1025 W/Hz at 1.4 GHz below which all DRAGNs are FRIs and above which nearly all are FRIIs. Today FR sources are considered as a subtype of DRAGNs. Occasionally, a peak with relatively low flux density, which initially appears insignificant, turns out at high resolution and sensitivity to be the brightest point because its flux comes from a very small region, and this can change the classification. Fanaroff and Riley were working with relatively low resolution images, and their analysis works out best when applied to such images. The main achievement of Fanaroff and Riley was to find a simple structure property common to the various DRAGNs that were known at that time (1974).

With higher resolution and sensitivity images of DRAGNs available,the need for a more detailed scheme was felt. These include Bridged Twin Jets, Tailed Twin Jets, Narrow-Angle Tails (NATs), Head-Tails, Relaxed Doubles, Classical Doubles, Plumed Doubles, Wide-Angle Tails (WATs), Naked Jets, Jetted Doubles. Other classification schemes, like depending on sizes and spectra also exist.

Towards a unified picture?

Nearly all the structures seen in the DRAGNs can be interpreted in terms of the standard model of twin plasma beams interacting with the ambient gas (IGM) to form the extended regions of radio emission.

The appearance of the source seem to be governed by five main factors.

The most probable picture is the existence of a supermassive black hole (~ 106 solar masses)at the center of the galaxy which could be due to galaxy mergers to form ellipticals. The disk of accreted material spiraling in towards the black hole heats up; gas in its vicinity reaches temperatures of millions of degrees and expands outwards from the galactic nucleus at high speeds. This flow which resembles the solar wind can sweep up other interstellar gases and expel from the nucleus. Due to physical conditions in the nucleus and its vicinity, the material comes out as jets; most probably two oppositely directed jets. Future observations with telescopes like the SKA (Square Kilometer Array) will definitely shed more light into this issue.

Active Galactic Nuclei (AGNs) have become more popular recently with the discovery that the bipolar jets emanating from them could be responsible for the formation of billions of stars. This explanation has been strengthened with the discovery of the unusual Quasar HE0450-2958 (Nature, 14 September 2005). It has been called the "Naked Quasar" as it appears to lack a host galaxy !

Check the link:

Wikipedia website, (http://en.wikipedia.org/wiki/HE0450-2958)

The image of the object HE0450-2958 found at nearly 3 billions of light years was obtained thanks to the telescopes Hubble and the VLT. This has enabled the French astrophysicist David Elbaz to make this discovery and this has brought a new insight in the formation of Galaxies and Stars. The idea is that there is a massive black hole at the center of HE0450-2958 and when the jet coming from its center meets gaseous clouds, it can lead to the formation of stars. It is believed that creation of the Galaxy is harbored by the Quasar and after a few millions of years, the process would have been complete and the Quasar changes from a naked state to a dressed state with stars in the Galaxy ! It is believed that all supermassive black holes can produce billions of stars and hence can create Galaxies. This is very different from the standard scenario of formation of stars and galaxies, where it is believed that the massive black holes at the centers of galaxies have been formed by devouring stars in their vicinities. Thus today astrophysicists are finding it more difficult to give a coherent picture.

First Surprise

It was first believed that galaxies were slowly created 1 or 2 billion years after the big bang. With the construction of telescopes of better sensitivity, the limit of the observable universe has gone to 6, then 8, 10, 12 and finally 13 billion light years and we have been surprised to discover each time galaxies. A first revolution !!

Second Surprise

Galaxies do not actually form gradually, from small clusters of stars which become gravitationally attracted to one another.Giant galaxies have been discovered at enormous distances in the universe. This shows that giant galaxies which contain up to one thousand billion of stars have already come into existence one billion years after the big bang. How did such huge structures form so fast in the universe? None of the actual models can explain this clearly.

Third Surprise

The strange correlation between the mass of galaxies and that of the massive black holes hidden inside them. In other words, the more we observe distant galaxies (hence younger ones), the more their black hole appears massive. Inside galaxies found between 1 and 8 billion light years, the stars have a mass of about 700 times more than their respective black hole. This number falls to about 400 for galaxies between 8 and 11 billion light years and seem to fall even below 150 for more distant galaxies. This fact is quite incompatible with the story that stars were the first objects of creation.

All this make it seem that the black holes were formed before the galaxies According to David Elbaz, the jets from the massive black holes were able to expel the equivalent of a whole galaxy in 10 million years only. It is sufficient to assume that massive black holes were formed after a few tens of millions of years after the big bang, so that we can have a more coherent explanation than that which we have been told so far. This theory is not unanimously shared by all astronomers. Francoise Combes (Astronomer from the Paris Observatory) thinks that only a very small fraction of all stars have been formed in this way. She thinks that in most cases, the huge amount of energy coming from the quasar will heat the environmental gas or will repel the gas far out in space, which will decrease the rate of star formation or even stop it. Prof Joseph Silk (University of Oxford) feels that this new approach is very important.

There were one thousand times more quasars 10 billion years back. So we have to look back in the distant past to have more chance of finding black holes creating stars. The hypothesis that black holes have created stars will lead to more intriguing questions.

How were these massive black holes created after the big bang?

Have they been formed after the accretion of cosmic gas or otherwise?

Some scientific results of the Mauritius Radio Telescope (MRT)

Here is an image of one of the first radio galaxies imaged at MRT, FORNAX, first named by Abbe de la Caille who gave the name FORNAX to the FORNAX cluster.

I'm presently trying to obtain a model for the formation of the extended morphologies of radio galaxies as an interaction of beams from a central " Active Galactic Nucleus " with an ambient medium, which is the intergalactic medium here.

Here is an example of a FRII source (3C219 = B0917+458)

A complete sample of extended and low-surface-brightness sources at 151 MHz will be formed as a subset of the catalog of radio sources to be made with the MRT map.


Most radio galaxies have linear sizes in the range 150 to 250 Kpc; hence most withing the sensitivity limit of the MRT survey will be resolved up to a red shift of 0.1.

For red shifts greater of 0.5, most will be seen as point sources; but a few radio galaxies will be pretty well resolved. The minimum energy density of the sources observed will be about 2x10-15 Joules/m3; hence we should be able to see most of the radio galaxies. The FRI sources will be seen up to red shifts of about 0.1 and most of them will be resolved.( our final resolution will be about 4 arcmin) All the FRII sources will be seen, but very few will be resolved for red shifts greater than 0.1 We are expecting to see around 1 million sources including around ten thousand FRII sources. If we are lucky, we might discover hitherto unknown relic and giant radio galaxies !! We might also see some very steep spectrum sources, not seen by the higher frequencies used by other radio telescopes. Feel free to contact me if you have overlapping interests.

Bouncing ball LATEST RESULTS Bouncing ball

A dirty image of resolution of 15 arcmin in RA and 15 arcmin in dec, for the region: 06 00 h < RA < 18 00 h and -70 degrees < Dec < -10 degrees has been made by me. I'm presently analyzing them. My colleague, Kumar Golap has made similar images from 17 00 h < RA < 24 00 h, 00 00 h RA < 05 00 h for his Ph.D. thesis where he has analyzed the problems faced with wide-field imaging with non-coplanar baselines. He is presently at the VLA working on AIPS++ and CASA among many other things. We have now a 24-h image in the declination range -70 degrees < Dec < -10 degrees at a resolution of 15 arcmin in RA and 15 arcmin in dec.

Bouncing ball MRT10 Sample Bouncing ball

One of the best studies samples in the last few decades (nearly 40 years) has been the northern hemisphere Third Cambridge Catalog (Edge et al., 1959) and its revised versions the 3CR (Bennett, 1962) and 3CRR (Laing et al., 1983). Because of the high flux density limit at 178 MHz of around 11 Jy, the sample contains a large proportion of very powerful radio sources. So far, there have been many studies of high-power radio sources based on the 3CR/3CRR sample. Thus it is essential to have a comparison sample for the southern sky to test whether the 3CRR sample is really representative. This motivated us to try to generate a sample of strong southern extragalactic sources which is more or less complete with reliable flux densities for the full 24-h range in RA.

So far, the equivalent of the 3C survey has not been done for the southern sky at a similar frequency. The only sample closely equivalent to the 3CRR sample for the southern sky has been made so far is the SMS4 sample with S(178 MHz) > 10.9 Jy. The SMS4 sample is a subset of the MS4 sample which contains a sample of 228 sources at 408 MHz with integrated flux densities S(408 MHz) > 4.0 Jy for the region of Galactic Latitude |b| > 10 degrees and declination range -85 degs < dec < -30 degs (Burgess & Hunstead, 2006) The MS4 sample was made using the Molonglo Reference Catalog (MRC) as the main finding survey.

Comparison of SMS4 with 3CRR shows that the southern sample has a slightly higher source density. Burgess & Hunstead argue that there can be three possible causes for this bias.

1. As the spectra of many radio sources turn over at low radio frequency, extrapolation from high frequencies is more likely to overestimate than underestimate S(178 MHz). From the flux densities obtained at 151.5 MHz from the MRT images, we have shown that this bias does not exist. In fact many sources do turn over near 150 MHz.

2. Because of the steep slope of the radio source counts, a small systematic difference in flux density scale can strongly bias the source density.

3. The 3CRR may be missing sources of low surface brightness.

For reasons stated above, it would be more appropriate to use a sample obtained from a survey at a frequency closer to 178 MHz to compare with the 3CRR. The sample will be less biased and the flux densities are obtained from the MRT images themselves without any assumption of the spectra of the sources.

The MS4 sample has been very useful for us to achieve this besides having been very useful for the flux density calibration of the MRT images. Unlike the 3C survey which covers the whole of the northern sky, the MRT survey covers only part of the southern sky. We would need a combination of the VLSS (74 MHz), MRT (151.5 MHz) and SUMSS (843 MHZ) surveys to cover the whole of the southern sky. From the frequencies and declination coverage of the southern radio telescopes, it looks that we will not achieve the exact equivalent of the 3C survey of the southern sky in the near future, but if we use the MRT survey and the MS4 sample which lie in the declination range < -70 degrees, we can obtain a sample very close to the 3CRR sample.

Bouncing ball Selection Criteria for MRT10 sample Bouncing ball

The selection criteria for the MRT10 sample are as follows:

The MRT10 sample contains 235 sources with flux densities at 151.5 MHz. Details of the individual sources in the MRT10 sample are found in (Best et al., 1999) and (Burgess & Hunstead, 2006). Nearly 30 % of sources in the list do not either have measured redshifts or the redshifts have been estimated from incomplete spectroscopic data. Hence it would be more appropriate to compare the intrinsic properties of the MRT10 sample with the 3CRR sample when the full spectroscopic data is obtained and also the full resolution images ( 4 arcmin x 4 arcmin) of the MRT10 sample.

Bouncing ball Properties of the MRT10 sample Bouncing ball

Taking into consideration the unclassified candidates in the SMRT10 sample and the fact that there are no 100 % complete samples, the differences may be barely insignificant.

Here are some images made by me:

One of the imaging challenges we have is the paucity of good calibrators. Our main calibrators are MRC0915-118 (Hydra) and MRC1932-464. In one paper titled "New Calibrators for the Mauritius Radio Telescope"- Science and Technology Research Journal of the University of Mauritius - Vol 2 - 1998, I have shown that the following sources have very good characteristics as calibrators: MRC1954-552, MRC2104-256, MRC 2211-172, MRC0349-278, MRC 0407-658 and MRC 0442-282.

List of some of my publications

Publication of one of our B.Sc. student who graduated in the 2002-2003 batch (K Khadaroo).

Did his undergraduate project on "Dark Energy". Completed his M.Sc. Astrophysics in "Queen Mary College, London".

Download his paper

My e-mail addresses: dinesh@uom.ac.mu dineshsomanah@gmail.com

My hobbies are football, badminton, swimming, snorkeling, trekking, dancing, Tai-Chi etc. etc. etc.

My other interests are philosophy, mysticism, spirituality, environmental issues etc. etc. etc.


INGENTA ,(http://www.ingenta.com)

INIST (France),(http://www.inist.fr)

LANL ARCHIVE ,(http://xxx.lanl.gov)






ADS, Harvard (http://adswww.harvard.edu/)

IPAC, Caltech (http://nedwww.ipac.caltech.edu)

CDS, (http://cdsweb.u-strasbg.fr)

xxx.lanl.gov, (http://xxx.lanl.gov/archive/astro-ph)

EINLINE, (http://hea-www.harvard.edu/einline)

USYD, (http://www.physics.usyd.edu.au)

An Atlas of DRAGNs

International Astronomy Meetings List

Astronomy Picture of the Day

Space Time Wrinkles

Gravitational Lensing

Wayne Hu's webpage (CMB stuff etc)

Spectroscopic and Imaging Surveys for Cosmology (SISCO)

SKYVIEW (The internet's virtual telescope)

SuperCOSMOS Sky Surveys (SSS)

The Hubble Deep Field South

The Hubble Deep Field

CDS Catalogues

The NRAO VLA Sky Survey

Monthly Notices of the Royal Astronomical Society (MNRAS)

The Royal Astronomical Society

American Astronomical Society

The Ionosphere and Propagation Group of Lancaster (Prof Farideh Honary)

Astronomy Source Catalogs (ATNF)

Cavendish Astrophysics Homepage of Cambridge University

Databases, Images and Surveys (COMRAD)

CATS catalog

Heavens Above Webpage