Annual Report 1997  

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-1 Radio Galaxies
-2 Evolution of Galaxy and Field Clusters
-3 QSO Absorption Lines
-4 Stellar Population modelling
-5 Semianalytic modelling
-6 Simulations of galaxies and clusters

TMR Research Network Contract No. ERBFMRX--CT96--0086

The following report is organised scientifically according to the six subject areas set out in the original proposal. In addition to the subject workshops mentioned in the individual sections, an annual meeting of the entire network was held over the period 19 to 24 May, 1997 at Ringberg Castle, Bavaria. 63~scientific participants in the network attended this meeting. Further details of this and other activities can be found on the network webpage.

1 Radio galaxies top

The `Westerbork Northern Sky Survey' (WENSS) has now been completed, and has enabled the selection and study of samples of low luminosity radio galaxies. In addition, in conjunction with other radio surveys, it has been used as the basis for a successful search for the most distant radio galaxies. Optical and infrared imaging using the HST has been carried out on distant radio galaxies with giant ionised gas halos, showing these to consist of many distinct clumps, possibly subunits coming together to form a massive central galaxy.

Scientific highlights

  • Demonstrating, using powerful radio galaxies from a Cambridge survey, that distant radio galaxies lie at the centre of the most massive structures in the early Universe.
  • A detection of powerful extended X-ray emission from a very distant radio galaxy, indicating an extremely large mass of material.
  • The use of WENSS to show that the comoving number density of radio galaxies falls beyond a redshift of two, thus defining the extent of the `quasar epoch'.
  • The discovery of the most distant radio galaxy known in the southern hemisphere.

Networking and coordination activities:

A network meeting on ``The K$-z$ relationship, observations and theory'' was held in Leiden on 31st January 1997, involving participants from 5 network sites. The report is available on the network WWW site. A number of Leiden scientists have travelled to network meetings at other nodes of the network. In addition, Christian Kaiser (Cambridge) visited Leiden from one week in July 1997 for collaborative work with Philip Best and Huub Röttgering, and with Guinevere Kauffmann (Munich) who was working in Leiden for two weeks at the time.

Appointment of young researchers:

Two young researchers are employed by the network on this topic. One is Philip Best. He has travelled to meetings at other sites of the network, and is now involved in active collaborations with researchers in Leiden, Cambridge, Paris and Munich. The second is Malcolm Bremer in Paris who maintains active collaborations with Cambridge and Leiden.

2 Evolution of field and cluster galaxies: top

A key objective in the network programme is the understanding of how galaxies evolve in rich cluster environments. Detailed comparisons of a very large imaging and spectroscopic dataset (from ground-based telescopes and HST) have allowed us link the morphological and stellar population properties of cluster galaxies at large look-back-times with those in similar environments at low redshift. A strong morphological evolution has been found for the S0 and spiral populations, as well as for the morphology-density relation in irregular clusters. Moreover, the brightest cluster galaxies have increased their mass in stars by factors $\simeq2$--$4$ in the last $\simeq8\,$billion years. Detailed semi-analytic galaxy formation models linked with spectro-photometric galaxy evolution models have provided a good understanding of the origin of these evolutionary phenomena within the context of the hierarchical scenario of structure formation in the Universe.

To understand the strong evolution found in the field galaxy populations at high redshift a key issue is how galaxy interactions (collisions, mergers, etc) affect the gaseous interstellar medium (the raw material from which stars will form) and the consequent formation of stars. We have found that gas is expelled during the process of star formation. Moreover, in a close interaction between a small and a large galaxy, the cannibalism of the interstellar medium of the smaller system by the larger may be almost 100\% efficient in a single crossing of the galaxies ---this is something not previously considered possible.

The commissioning of the new SCUBA bolometer array receiver at the James Clerk Maxwell Telescope (JCMT) has improved the performance of submillimetre-wave continuum observations by between one and three orders of magnitude. This has made observations of dust ---a key ingredient of star-formation--- in extremely distant galaxies practical for the first time. Exciting recent observations in the fields of rich clusters have provided our first look at the population of distant ($z>1$) star-forming galaxies in the submillimetre waveband.

The detailed study of the field galaxy population in the crucial $1


A remarkable new result is the discovery with the Ryle Telescope of a cluster candidate via the Sunyaev--Zel'dovich decrement in the cosmic microwave background radiation towards the quasar pair PC1643+4631A\&B at redshift $z=3.8$. Very deep optical, infrared and X-ray images have revealed no cluster at $z<2$ that is similar to known galaxy clusters, implying that the system must be a massive cluster either at $z>2$, or at $z<2$ and dark. Either way, this system is a challenge for models for the formation of galaxies and larger structures.

Networking and coordination activities:

Cambridge hosted a working group meeting on the subject "Evolution of Field and Cluster Galaxies" (February 28 - March 1, 1997). The primary aim of the meeting was to discuss and set up collaborative projects, and to produce work plans and ideas for exchange of personnel within the network. The following scientific topics were discussed: evolution of galaxies in clusters, redshift surveys of field galaxies, star formation rate history of the Universe, HII and starburst galaxies at different redshifts, chemical evolution and IGM enrichment, relation starbursts-AGNs, star formation and stellar populations in nearby galaxies and high redshift galaxies. The meeting was attended by 16 researchers from other network institutions, and 24 Cambridge-based researchers.
An Internet WWW home page with scientific and practical information on Network activities has been established. It contains links to the rest of the network institutions. The URL is:

external link

Appointment of young researchers:

In October 1st, 1996, Dr. Bianca M. Poggianti was appointed as a research fellow in Cambridge, funded by the Network. The appointment has a duration of two years. She had been awarded a PhD from the University of Padova, and had a post-doctoral research appointment at the Kapteyn Instituut in Groningen before moving to Cambridge. Her main research activities are related to the evolution of the stellar populations and spectral properties of galaxies. She actively collaborates with researchers in Cambridge, Padova and Durham within the Network. At the same time Dr.\ Itziar Aretxaga was appointed for two years in Garching. She works on starbursts in galactic nuclei and collaborates actively with Cambridge researchers.

3 QSO absorption lines top

The study of the gas in and around galaxies as seen in quasar absorption lines has progressed in the direction of understanding the extent, structure, metallicity and ionization state of galaxy halos at high redshift. A collaboration between IAP and IoA has detected for the first time CO emission at very high redshift (4.4 and 4.7), in correspondence of strong associated absorption systems and revealed the presence of dust. A collaboration including IAP and Padova has successfully obtained a spectrum of one of the galaxies showing these remarkable properties, observing a strong Lyman-$\alpha$ emission at $z=4.702$ and inferring a star formation rate of about 20 solar masses per year. Suitable groups of quasars for testing with multiple lines of sight (LOS) the characteristic sizes and clustering properties of the high-z absorbers have been searched for in a collaboration between Padova and IAP. A very interesting group of 4 relatively bright QSOs ($V \leq 19.5$), all at $z>2$ and within a square field of 10 arcmin side, has been located in the SGP and will allow to probe multiple LOS separations between 0.6 and 3.4 Mpc with observations planned for fall 1998. A collaboration for comparing this type of observations with hydro-dynamical simulation is starting between Garching and Padova. A search for high-z ($z>3$) relatively bright ($18.51.8$ and $z>1.2$ for the detection of Ly-alpha and CIV absorbers, respectively). The second the distribution with redshift of the faint galaxies close to the quasar sight-line and in the surrounding field. The Network has proposed successfully to begin follow--up of HDF--South on ESO's telescopes.

4 Stellar Population modellingtop

The primary objectives in this area are to match the full observed ranges of wavelengths, metallicities, and elemental abundance ratios in population synthesis models and to identify reliable indicators of star formation. At this time, Padova and Cambridge team members have calibrated with unprecedented accuracy the effects of varying metallicities and elemental abundance ratios on stellar evolution and on the spectral signatures of early-type galaxies (Tantalo et al. 1997; Poggianti \& Barbaro 1997; Rampazzo et al. 1997; Garcia-Vargas et al. 1997). Accurate calibrations of stellar infrared colors by the Padova and Paris team members and the inclusion of circumstellar dust emission in the Padova synthesis models also opens new possibilities to disentangle age and metallicity effects in observed stellar populations (Ng et al. 1997; Bressan et al. 1997). In addition, Cambridge and Padova team members have developed nebular emission models that will now be used to calibrate the star formation rates of distant galaxies (Poggianti \& Barbaro 1997; Barbaro \& Poggianti 1997). In Paris, population synthesis programs were completed for the full observed range of metallicities (Bruzual \& Charlot 1997) and implemented in the Munich semi-analytic models. Highly encouraging first results show that hierarchical galaxy formation is compatible with all observed properties of early-type galaxies (Kauffmann et al. 1996; Kauffmann \& Charlot 1997). Paris team members also developed new models for the infrared emission of galaxies to interpret current and future IR/submm observations (Guiderdoni et al. 1997a, b). This prescription, is being implemented into the Durham semi-analytic models and combined with the Padova scheme of AGN/QSO formation to make detailed comparisons with observations (Baugh et al. 1997; Franceschini et al. 1997). In particular, the models will be tested against a successful Paris-Padova survey achieved with the Infrared Space Observatory (Puget et al. 1997).

Networking and coordination activities:

A workshop on this topic took place in Paris on Feb. 7/8 1997 and was attended by 11 researchers from the other 5~sites.

Employment of young researchers:

Dr. Y. Ng has been employed for two years in Padova as a network postdoc in this field. He has active collaborations with Paris and Leiden.

5 Semianalytic modelling top

There has been considerable progress this year in using semi-analytic techniques to understand the formation and evolution of elliptical galaxies in a scenario where they are formed from mergers of disks. G.Kauffmann a (MPA) and S.Charlot (IAP) included a prescription for chemical evolution into their modelling and demonstrated that it is possible to obtain a good fit to the slope and scatter colour-magnitude relation of cluster ellipticals, both at present and at high redshift. G.Kauffmann (MPA), S. Charlot (IAP) and S. White (MPA) analyzed the reddest galaxies from a large, deep redshift survey and showed that only one third of present-day ellipticals existed in their present form at redshift 1, lending important new support to the merger picture. A. Aragon-Salmanca (IoA), C.Baugh (Durham) and G. Kauffmann (MPA) have studied the evolution of brightest cluster galaxies from a redshift of one to the present day. They also find that the semi-analytic merger model provides a good fit to the data.

An ongoing collaboration between G. Kauffmann (MPA) and S. Charlot (IAP) aims to explain the observed properties of globular cluster systems of elliptical galaxies. P. Best (Leiden), H. Rottgering (Leiden) and G. Kauffmann (MPA) are also modelling the evolution of environments and luminosity functions of radio galaxies and other AGN using semi-analytic techniques.

S. Cole, C. Baugh \& C. Frenk (Durham) in collaboration with C. Lacey (Copenhagen) have rewritten their code for studying semi-analytic galaxy formation. More comprehensive stellar population models and chemical enrichment has been incorporated into the model. A treatment of galaxy sizes is also made. The new code has been applied to study the properties of the recently discovered population of Lyman break galaxies at $z=3$, leading to the interpretation that these objects mark the first significant episode of galaxy formation in the universe. The amplitude predicted for the clustering of these objects has been tested in a project with Governato (Durham) and Quinn \& Stadel (Seattle), in which the semi-analytic model was used to identify halos in an N-body simulation that should contain Lyman break galaxies.

Collaborations have been started with the Padova group (Bressan, Granato, Silva) and the Paris group (Guiderdoni, Devrient, Buchet) to study the effects of dust on galaxy SEDs, with an aim to make predicitons for ISO/SCUBA counts.


The demonstration that most observed ellipticals attained their observed form after redshift one, but that such a history is nevertheless consistent with the systematic properties of the elliptical population.

Networking and coordination activities:

A workshop on semianalytic was held in Garching on 20/21 March 1997 and was attended by about 25~scientists from all 6~sites.

6 Simulations of galaxies and clusters top

Progress in this area has been substantial. The year 1 milestone (characterising the expected ``core" structure of dark halos in different cosmologies) was reached on schedule (see ``highlights" below) and significant progress was achieved against our remaining objectives. Activity took place at Durham, Munich and Padova, resulting in 24 joint publications involving researchers in the first two of these. Network researchers (Colberg, Frenk, Jenkins, Kay, Pearce, Springel and White, in collaboration with external scientists in some cases) carried out simulations of the clustering evolution of dark matter in different cosmogonies. These were performed on supercomputers in the UK and Germany and are amongst the largest ever attempted. In collaboration with network partners at Durham and Munich, one of the young researchers funded by the network (Dr. Governato) began an extensive programme of N-body/gasdynamical simulations of the formation of galactic disks, working towards the year 2 milestone. Another young researcher at Padova (Dr. Carraro) performed similar simulations, in this case applied to the formation of elliptical galaxies. Finally, researchers at Munich and Durham completed the first phase of development of a new fully parallel N-body/gasdynamical simulation code specifically designed to work on the massively parallel supercomputers to which network researchers have substantial access. An interim (N-body only) version of this code is now operational and will soon be used to perform the largest ever cosmological simulation of the evolution of dark matter over virtually the entire visible universe.


1) The discovery that the radial density profiles of dark matter halos formed in hierarchical clustering cosmologies have a universal form, independent of mass and cosmological parameters. This form accurately describes the density profile over approximately two orders of magnitude in radius. The characteristic density (or radial scale) of a halo is well correlated with its total mass, reflecting the later formation epochs of halos of increasing mass.

2) The largest ever supercomputer simulations of the formation of cosmic structures. These were used to derive high precision determinations of the clustering statistics of cold dark matter as a function of time in large cosmological volumes, and of the abundance and internal properties of galaxy and galaxy cluster halos.

Networking and coordination activities:

Durham organized a 2-day meeting to coordinate research in cosmological simulations across the network. The meeting, held on 2-3/May/1997, was attended by 17 participants from 5 network institutions. Several collaborative projects were advanced or initiated at this meeting and some have already led to publications (see below). A programme and minutes of the meeting were posted on the network webpage.

Appointment of young researchers:

Dr. Fabio Governato was appointed to a postdoctoral position at Durham funded by the network. His main responsibility is to carry out N-body/gasdynamical simulations of the formation of galaxy disks in a cosmological context, including prescriptions to model star formation and feedback. The post was advertised alongside those at the other network institutions. In order to integrate Dr. Governato into the network programme, he undertook visits to Padova, Paris and Munich.

Comments to: Naoki naoki@mpa-garching.mpg.detop