Galaxy Population and Evolution

The new generation of large redshift surveys promise to reveal the properties of the galaxy population in extraordinary detail. Some of this potential was revealed by various preliminary results emerging from the 2dFGRS. [Colless] summarised some of the recent work by members of the survey team on the local luminosity function (LF). The overall LF shows clear signs of being more complex than the standard Schechter form, with a steepening of the faint end for M_bJ>-16. Using a principal component analysis (PCA) to classify the galaxies into 5 spectral types (Folkes et al. 1999), it is clear that this is the result of a general steepening of the faint end for later-type galaxies, which dominate the population at lower luminosities.

However this type of analysis does not help explain the large variation between different determinations of the local LF, not only in the faint end but also in the normalization. A more general approach is required, which will account for surface brightness selection effects. [Colless] reported a preliminary determination (Cross et al., in prep.) of the bivariate brightness distribution (BBD), the joint distribution over absolute magnitude and surface brightness. This shows that the 2dFGRS is not severely affected by its selection limit at low surface brightness, as the galaxy number distribution falls off sharply before that limit is reached. The BBD is strongly suggestive of the existence of separate dwarf and giant populations, the former with a LF that is flat or declining at the faint end and the latter with a steeply increasing faint end.

More detailed analyses of the galaxy population can be carried out using various quantities derived from the spectra: [Colless] showed that measurement of the H equivalent width can be used to map the relative distributions of the star-forming and quiescent galaxies in the 2dFGRS (Lewis et al., in prep.), while [Deeley] used the Balmer-line and [OII] equivalent widths to identify post-starburst E+A galaxies, finding an incidence of 0.25%.

A very powerful application of the redshift surveys is in the identification of sources detected in sky surveys at other wavelengths. [Cannon] reported on a program to identify the NVSS and SUMSS radio galaxies in the 2dFGRS (Sadler et al. 1999). About 5% of NVSS sources are found in the 2dFGRS, and about 1.5% of 2dFGRS sources are in NVSS; in the full redshift survey there should be about 4000 radio galaxies. The 700 sources identified so far, with roughly equal numbers of star-forming galaxies, optical AGN and radio AGN, already make up the largest homogeneous sample of optical spectra for radio galaxies. The first result emerging from this sample is a much-improved determination of the radio luminosity function, which has a double-humped appearance. By using the PCA spectral types, [Cannon] showed that this structure is due to different luminosity functions for the AGN (which dominate at high luminosity) and the star-forming galaxies (which have a steeper slope and dominate at lower luminosities).

[Jackson] described the potential for future work combining radio surveys such as NVSS, SUMSS and FIRST with large redshift surveys such as the 2dF galaxy and QSO surveys, SDSS and the 6dF survey. Understanding the properties of radio galaxy populations is very much hindered by the lack of large, homogeneous redshift samples. Cross-identification with the 2dFGRS allows the definition and detailed studies of the local radio galaxy populations: the luminosity functions (see above), environmental effects, clustering and low-redshift evolution. Cross-identification with the 2dF QSO survey allows similar studies of radio-loud QSOs. Limitations arise from the joint radio/optical selection criteria, nonetheless such studies promise to greatly improve our understanding of radio galaxies.

[Boyle] showed the results obtained on QSO evolution using a homogeneous sample of 3265 QSOs from the 2dF QSO Redshift Survey. He finds that the luminosity function is fitted by a double power law (i.e. it has a break), and that the change in the LF with redshift is consistent with a 0.3,

0.7 universe and pure luminosity evolution with the approximate form

out to z=2.3. He noted that the SDSS QSO survey, which is not limited by UVX selection, may be able to see whether this evolution turns around for redshifts between 2.3 and 3, with decreasing numbers of QSOs at fixed luminosity. He also reported results from HST and ground-based imaging of a sample of 76 low-redshift, X-ray selected AGN (Schade et al., in prep.). Host galaxies with -23<<-17 were detected in all cases, with 55% of AGN residing in bulge-dominated galaxies. A weak correlation is found between the AGN and host luminosities. Interestingly, 10% of AGN hosts do not show a point source (in agreement with HST studies of fainter AGN). The luminosities, sizes and morphologies of the AGN hosts are completely consistent with the `normal' galaxy population, except that AGN preferentially reside in bulge-dominated systems.

[Giavalisco] presented some recent results from surveys of Lyman-break galaxies (LBGs) at z3 and z4 (Giavalisco et al. 1998, Steidel et al. 1999). The colour-selection of LBGs is a highly efficient method for generating samples of star-forming galaxies at high redshift. Follow-up spectroscopic observations have now provided redshifts for more than 750 galaxies with z=2-5. The selection biases are believed to be well-understood, so that it is possible to compute the far-ultraviolet luminosity function (LF) for the LBGs. Combining ground-based and Hubble Deep Field samples, the z3 LF (measured in the R band, which corresponds to 1700Å in the rest-frame) is found to be well-represented by a Schechter function with a steep faint end (-1.6), although the slope does depend on the incompleteness correction that is applied. With assumptions about the LBG spectral energy distribution, and using the Calzetti (1997) reddening law, it is possible to estimate the extinction by dust in the LBGs. The median extinction at rest-frame 1500Å is found to be 1.7 mag, and integrated over the whole population the UV extinction is about a factor of 7. The smaller spectroscopic sample of LBGs selected at z4 does not show any significant change from the z3 population: the LFs of both samples are consistent in shape and normalisation. The LBGs are very strongly clustered, with co-moving correlation function amplitude at least as great as that of z0 galaxies. Given the weaker mass clustering at early epochs, this implies much stronger biasing. However this picture is consistent with the predictions of simple analytic models (e.g. Mo, Mau & White 1999) for the evolution of galaxy clustering based on the measured power spectrum for low-redshift galaxies.

Francis and Illingworth reported studies of high-redshift galaxy clusters. [Illingworth] showed HST results relating to the nature of galaxies in rich environments at z0.3-1. The mild evolution in the Fundamental Plane for the E and S0 galaxies in the cluster CL1358+62, relative to the nearby Coma cluster, implies that these galaxies are a mature, homogeneous population whose stars formed at redshifts z>1 (Kelson et al. 1999). The absorption linestrengths are also consistent with old, single-burst populations in which metallicity increases with velocity dispersion. For early-type spirals, however, there is a positive correlation between luminosity-weighted age and velocity dispersion. In contrast to this evidence for a high redshift of star formation in early-type galaxies, [Illingworth] also reported results from a morphological study of the cluster MS1054-03 at z=0.83 which show a high merger rate in this luminous X-ray cluster. If this rate is typical for clusters at this redshift, it implies that up to 50% of the massive ellipticals in clusters may be formed in mergers at redshifts z<1 (van Dokkum et al. 1999). Hence, the stars in cluster ellipticals form early, but the galaxies' final morphological forms may be more lately acquired.

[Francis] described a detailed investigation of a candidate galaxy cluster at z=2.38. Narrow-band imaging of this cluster shows three bright Lyman- sources, one of which is more than 50h^-1kpc in extent. Two of these sources have very red colours, and near-infrared photometry shows that their spectral energy distributions are poorly-fit by dust-obscured star-forming regions, but well-fit by a stellar populations with ages 0.5Gyr and masses of a few times 10¹¹M. A number of other objects with similar infrared colours are seen in the cluster, and have luminosity profiles more consistent with an R^1/4 law than an exponential disk. The cluster lies in front of three QSOs, and in all three sight-lines there are neutral hydrogen absorption lines (including damped Lyman- systems) at the cluster redshift. This suggests the cluster contains a large amount of neutral hydrogen, perhaps more than 10¹²M, which may be in the form of large numbers of short-lived 10⁶M clouds.

[Madau] reviewed the constraints on the history of star formation and the mass density in stars that can be derived from the extragalactic background light (EBL). He notes that the logarithmic slope of the faint galaxy counts drops below 0.4 at wavelengths from the ultraviolet to the near-infrared at the limits achieved in the Hubble Deep Fields (Figure 6a; Madau & Pozzetti 2000). This implies the EBL is converging, with the bulk of the light already observed (Figure 6b), and gives a lower limit to the EBL intensity over the range 0.2-2.2m of 15 nWm^-2sr^-1. Combined with measurements of the far-infrared background light from COBE, and making approximate corrections for unresolved sources and the wings of the galaxy light profiles, Madau estimates that the total EBL intensity is 5520 nWm^-2sr^-1. Using stellar population models and assumptions about the stellar initial mass function, he finds that the EBL requires that the present-day mass density in processed gas and stars has a lower limit of

>0.0013, corresponding to

>3.5. Plausible models for the star-formation history give twice as large. Another constraint arising from the EBL relates to the possibility raised by the MACHO gravitational lensing survey that galaxies' dark halos may be composed of faint white dwarfs. Madau finds that in fact no more than 5% of the baryons can be in form of stellar remnants without over-producing the EBL.

Figure 6: (a) The optical and near-infrared galaxy counts in various passbands as a function of AB magnitude. (b) The extragalactic background light per magnitude bin in each passband (arbitrarily scaled), showing the turnover and decreasing contributions at the faintest magnitudes (see Madau & Pozzetti 2000).

Cowie and Rowan-Robinson discussed what can be learnt about galaxy evolution by combining submillimetre observations with optical and infrared data. [Rowan-Robinson] presented models of the star-formation history of the universe based on results from imaging surveys of galaxies at infrared and submillimetre wavelengths. The European Large-Area ISO Survey (ELAIS; Rowan-Robinson et al. 1999) has employed the ISO satellite to image 4-12 sq.deg at 6.7m, 15m, 90m and 175m, reaching down to 3 mJy at 15m. This survey is supported by optical and near-infrared imaging, 21cm observations, and a SCUBA submillimetre survey of 0.02 sq.deg down to 8 mJy at 850m. Rowan-Robinson uses simple parametrised star-formation models to fit the infrared and submillimetre galaxy counts (Rowan-Robinson 1999). He finds that the star-formation rate increases to z1 and then remains high out to at least z5. Within this framework, the submillimetre background intensity can be used to discriminate between cosmological models by comparing their predictions with the observed far-infrared and submillimetre background spectrum. A low-density (0.3, =0) model is preferred; a high-density (=1, =0) model requires a spike in the star-formation rate at z>5, while a flat (0.3, 0.7) model requires truncation of star-formation at z5.

[Cowie] noted that the submillimetre background flux indicates that much of the energy produced by star-formation and AGN is re-processed by dust to far-infrared wavelengths. SCUBA observations at 850m show that the background is due to ultra-luminous infrared galaxies at redshifts z>1. Submillimetre imaging has poor spatial resolution, but the fact that both submillimetre and centimetre flux are related to star-formation rate has meant that centimetre radio observations can be used to locate the submillimetre sources (Barger, Cowie & Richards 1999). However these objects turn out to be extremely faint in the optical and near-infrared, and inaccessible to spectroscopic follow-up. Nonetheless it is possible to estimate their redshifts from the shape of their spectral energy distribution at submillimetre and radio wavelengths. This procedure suggests they are at redshifts z=1-3; if so, the submillimetre sources are a major star-forming population at high redshift (Cowie & Barger 1999).

[Calzetti] outlined models for the evolution of the star and gas content of galaxies, giving special consideration to the effects of dust (Calzetti & Heckman 1999). She estimates that the presence of dust suppresses the UV flux by a factor of 2.5-5 at z=1-3, with a significant fraction of the radiation from star-formation emerging in the far-infrared. Star formation histories with either low star-formation rates and low dust content at high redshift, or with high star-formation rates and high dust content, are both compatible with the observed ultraviolet flux, but can differ by an order of magnitude in the numbers of stars produced at z>3. Both types of model predict that the far-infrared background is mostly produced by high-redshift sources, but the model with a constant star-formation rate for 1<z<4 predicts the dust will be hotter at high redshift.

© Copyright Astronomical Society of Australia 1997