2002 PHD PROJECT :- Alan Pedlar



Observational studies of active galactic nuclei (AGN) and starbursts have largely concentrated on studies of ionised (eg emission lines) and relativistic plasma (eg synchrotron emission). However, it has also emerged that neutral gas is intimately involved in both AGN and starbursts and plays a crucial role in their physics.

Atomic Hydrogen Absorption in Seyferts

The properties of neutral gas in the central few hundred parsecs of AGN has only recently become accessible to study. Although initially its very existence was surprising (considering the presence of shocks and copious UV emission from the AGN) the properties of the neutral gas are now proving vital in constraining the physics involved in the collimation and fuelling of the nuclear activity.

MARKARIAN 6 continuuum :- Click on the image to see H1 absorption

NGC5929 Continuum : Click on image to see H1 absorption


We have pioneered the use of MERLIN to image the H1 absorption from Seyfert nuclei with typically 0.1 arcsec resolution which correponds to linear scales of a few parsecs in nearby objects such as NGC4151. At present (and until the advent of large mm arrays- eg ALMA) MERLIN is the only instrument which can image neutral hydrogen on scales directly comparably to Hubble Space Telescope resolution. These observations constrain models involving the fuelling of the nuclear activity, test current unified schemes and set upper limits to the mass of the central engine. One of the main drivers for this research was investigating the 'standard model' in which the active galactic nucleus is surrounded by a dusty torus which neatly accounts for collimated UV emission, hidden broad lines and fuelling. Molecular hydrogen gas in this torus is dissociated to form atomic hydrogen which we can detect by absorption of the 21cm radio line.

Standard Model

For further reading :- typical papers which have arisen from earlier Phd projects include

NGC4151 :- MNRAS 272 355

Markarian 6:- A&A 333, 13

NGC5929:- MNRAS 301 782

For a good summary of H1 absorption in Seyferts see Gallimore et al. ApJ 524, 684

Atomic Hydrogen in Radio Galaxies

3C305 - Click on image to see absorption spectra

3C293 - 20cm Radio Continuum, HST image & selected H1 spectra

The science case is similar to the Seyfert case, although the radio emission is stronger which allows lower column densities of hydrogen to be investigated. Also many of these galaxies are at higher redshifts than typical Seyferts. Only a few radio galaxies have been observed, many remain to be investigated. One of the most impressive studies - 3C 293 (see above) has just been completed as part of Rob Beswick's PhD project. (MNRAS - in press)

Click for more information on 3C293

Atomic Hydrogen Absorption in Starbursts

As well as work on Seyferts & Radio galaxies the PhD project can also include work on starbursts. In the the case of starbursts we can determine the absorption spectra against individual supernova remnants which not only allows us to probe the interstellar medium on parsec scales, but also is crucial in determining the dynamics of the neutral gas which is fuelling the starburst.

Click on the image to enlarge

This figure shows the MERLIN/VLA 20cm image with the Shen & Lo CO emission superimposed as contours

Click HERE to see some of the H1 absorption spectra seen against supernova remnants in M82.

Examples of work already completed can be found in

M82:- MNRAS 298 347

NGC3628:- MNRAS 300 656 (1998) Click here to see the spectra

NGC6240:- MNRAS 325 151 Click here to see image & spectra

Arp 220:- Ap.J. 560 168


The 2002-2005 PhD work would continue to investigate H1 absorption against all three types of active galaxy. One area which work is required is in surveys of complete samples of each type of object as much of the earlier work has been heavily focussed on individual objects rather than statistics. The whole area is fairly open ended, and, as with earlier projects much of the final shape depends on the students particular interests. There is some overlap with the starburst PhD project also proposed for 2002- which may mean that some areas (eg M82 OH etc) may be required for this- however this would not be a major problem given the amount of data available. A number of observations have already been taken which would form the basis of the project. Further projects will be proposed during the PhD project.

a) A search for atomic hydrogen absorption in Seyfert nuclei and nearby radio galaxies using MERLIN & VLA

b) EVN & MERLIN studies of atomic hydrogen in individual objects

c)Studies and searches for OH and H2CO absorption (NGC3079, NGC3628, M82).

d) VLA(New Mexico) & GMRT(India) observations of H1 emission

e) CO & mm continuum work using Onsala 20m (Sweden) & SEST 15m (Chile) dishes

Previous Jodrell PhD students who have completed projects involving H1 absorption

Rob Beswick rbeswick@jb.man.ac.uk (To submit in 2002)

Gordon Cole gordonhjcole@hotmail.com

Karen Wills K.Wills@sheffield.ac.uk

Carole Mundell cgm@astro.livjm.ac.uk


  • Click here for more info on atomic hydrogen absorption (bit out of date)

  • Click here for more info on starbursts
  • Click here to download a recent review of MERLIN H1 absorption work (gzipped postscript)