Prof Gavin Dalton

Abstract:

We have measured the equivalent width of the Ha emission line for 11 006 galaxies brighter than Mb, = -19 (ΩΛ = 0.7, Ωm = 0.3, H0 = 70 km s^-1 Mpc^-1) at 0.05 < z < 0.1 in the 2dF Galaxy Redshift Survey (2dFGRS), in the fields of 17 known galaxy clusters. The limited redshift range ensures that our results are insensitive to aperture bias, and to residuals from night sky emission lines. We use these measurements to trace μ*, the star formation rate normalized to L*, as a function of distance from the cluster centre, and local projected galaxy density. We find that the distribution of μ* steadily skews toward larger values with increasing distance from the cluster centre, converging to the field distribution at distances greater than ∼3 times the virial radius. A correlation between star formation rate and local projected density is also found, which is independent of cluster velocity dispersion and disappears at projected densities below ∼1 galaxy Mpc^-2 (brighter than Mb, = -19). This characteristic scale corresponds approximately to the mean density at the cluster virial radius. The same correlation holds for galaxies more than two virial radii from the cluster centre. We conclude that environmental influences on galaxy properties are not restricted to cluster cores, but are effective in all groups where the density exceeds this critical value. The present-day abundance of such systems, and the strong evolution of this abundance, makes it likely that hierarchical growth of structure plays a significant role in decreasing the global average star formation rate. Finally, the low star formation rates well beyond the virialized cluster rule out severe physical processes, such as ram pressure stripping of disc gas, as being completely responsible for the variations in galaxy properties with environment.

Abstract:

We compare the amplitudes of fluctuations probed by the 2dF Galaxy Redshift Survey (2dFGRS) and by the latest measurements of the cosmic microwave background (CMB) anisotropies. By combining the 2dFGRS and CMB data, we find the linear-theory rms mass fluctuations in 8 h^-1 Mpc spheres to be σ8m = 0.73 ± 0.05 (after marginalization over the matter density parameter Ωm and three other free parameters). This normalization is lower than the COBE normalization and previous estimates from cluster abundance, but it is in agreement with some revised cluster abundance determinations. We also estimate the scale-independent bias parameter of present-epoch Ls = 1.9L* APM-selected galaxies to be b(Ls, z = 0) = 1.10 ± 0.08 on comoving scales of 0.02 < k < 0.15 h Mpc^-1. If luminosity segregation operates on these scales, L* galaxies would be almost unbiased, b(L*,z = O) ≈ 0.96. These results are derived by assuming a flat ACDM Universe, and by marginalizing over other free parameters and fixing the spectral index n = 1 and the optical depth due to reionization τ = 0. We also study the best-fitting pair (Ωm, b), and the robustness of the results to varying n and τ. Various modelling corrections can each change the resulting b by 5 - 15 per cent. The results are compared with other independent measurements from the 2dFGRS itself, and from the Sloan Digital Sky Survey (SDSS), cluster abundance and cosmic shear.

Abstract:

We use redshift determinations and spectral analysis of galaxies in the 2dF Galaxy Redshift Survey to study the properties of local radio sources with S ≥ 1 mJy. 557 objects (hereafter called the spectroscopic sample) drawn from the FIRST survey, corresponding to 2.3 per cent of the total radio sample, are found in the 2dFGRS catalogue within the area 9^h48^m ≲ RA(2000) ≲ 14^h32^m and -2^o.77 ≲ Dec.(2000) ≲ 2^o.25, down to a magnitude limit bJ = 19.45. The excellent quality of 2dF spectra allows us to divide these sources into classes, according to their optical spectra. Absorption-line systems make up 63 per cent of the spectroscopic sample. These may or may not show emission lines due to AGN activity, and correspond to 'classical' radio galaxies belonging mainly to the FRI class. They are characterized by relatively high radio-to-optical ratios, red colours, and high radio luminosities (10²¹ ≲ P1.4 GHz/W Hz^-1 sr^-1 ≲ 10²⁴). Actively star-forming galaxies contribute about 32 per cent of the sample. These objects are mainly found at low redshifts (z ≲ 0.1) and show low radio-to-optical ratios, blue colours and low radio luminosities. We also found 18 Seyfert 2 galaxies (3 per cent) and four Seyfert 1s (1 per cent). Analysis of the local radio luminosity function (LF) shows that radio galaxies are well described by models that assume pure luminosity evolution, at least down to radio powers P1.4 GHz ≲ 10^20.5 W Hz^-1 sr^-1. Late-type galaxies, whose relative contribution to the radio LF is found to be lower than was predicted by previous works, present an LF which is comparable with the IRAS galaxy LF. This class of sources therefore plausibly constitutes the radio counterpart of the dusty spirals and starbursts that dominate the counts at 60 μm.