Astronomical instrumentation

Abstract:

We estimate the cluster-galaxy cross-correlation function (ξcg), from the APM galaxy and galaxy cluster surveys. We obtain estimates both in real space from the inversion of projected statistics and in redshift space using the galaxy and cluster redshift samples. The amplitude of ξcg is found to be almost independent of cluster richness. At large separations, r ≳ 5 h-1 Mpc (h = H0/100 km s-1 Mpc-1, where H0 is the Hubble constant), ξcg has a similar shape to the galaxy-galaxy and cluster-cluster autocorrelation functions. ξcg in redshift space can be related to the real-space ξcg by convolution with an appropriate velocity field model. Here we apply a spherical collapse model, which we have tested against N-body simulations, finding that it provides an accurate description of the averaged infall velocity of matter into galaxy clusters. We use this model to estimate β(β = Ω0.6/b, where b is the linear bias parameter), and find that it tends to overestimate the true result in simulations by only ∼10-30 per cent. Application to the APM results yields β = 0.46 with β < 0.73 at 95 per cent confidence. This measure is complementary to the estimates made of the density parameter from larger scale bulk flows and from the virialized regions of clusters on smaller scales. We also compare the APM ξcg and galaxy autocorrelations directly with the mass correlation and cluster-mass correlations in COBE-normalized simulations of popular cosmological models, and derive two independent estimates of the galaxy biasing expected as a function of scale. This analysis reveals that both low-density and critical-density COBE-normalized cold dark matter (CDM) models require anti-biasing by a factor ∼2 on scales r ≤ 2 h-1 Mpc, and that the mixed dark matter (MDM) model is consistent with a constant biasing factor on all scales. The critical-density CDM model also suffers from the usual deficit of power on large scales (r ≳ 20 h-1 Mpc). We use the velocity fields predicted from the different models to distort the APM real-space cross-correlation function. Comparison with the APM redshift-space ξcg yields an estimate of the value of Ω0.6 needed in each model. We find that only the low-Ω model is fully consistent with observations, with MDM marginally excluded at the ∼2σ level.

Abstract:

We report 0.5"x0.9" resolution, interferometric observations of the 1.3 mm CO J=2-1 line in the infrared luminous galactic merger NGC 6240. About half of the CO flux is concentrated in a rotating but highly turbulent, thick disk structure centered between the two radio and near-infrared nuclei. A number of gas features connect this ~500 pc diameter central disk to larger scales. Throughout this region the molecular gas has local velocity widths which exceed 300 km/s FWHM and even reach FWZP line widths of 1000 km/s in a number of directions. The mass of the central gas concentration constitutes a significant fraction of the dynamical mass, M_gas(R<470 pc) ~ 2-4x10^9 M_o ~ 0.3-0.7 M_dyn. We conclude that NGC 6240 is in an earlier merging stage than the prototypical ultraluminous galaxy, Arp 220. The interstellar gas in NGC 6240 is in the process of settling between the two progenitor stellar nuclei, is dissipating rapidly and will likely form a central thin disk. In the next merger stage, NGC 6240 may well experience a major starburst like that observed in Arp 220.

Abstract:

We present mid-infrared images and an 8-13 μm spectrum of the southern H II region RCW 38. We determine the dust colour temperature from both our spectrum and images at 10 and 20 μm, and deduce the gas excitation from an image in the [S IV] fine-structure line, as well as spectra of the [Ar III], [S IV] and [Ne II] fine-structure lines. Our observations are consistent with a complex of sources associated with the RCW 38 IRS1 region, which represent knots of material in a shell, or ridge, surrounding a cavity of about 0.1 pc in radius, which is itself created by the stellar wind of the hot young source IRS2. The dust temperature does not peak closest to IRS2, but rather along the centre of the ridge, and is remarkably uniform over the extent of our image. From photoionization models for the observed line ratios at IRS1 we deduce a stellar effective temperature and gas density of about 43 000-48 000 K and 104 cm-3 respectively. Whilst the star, or star cluster, IRS2 is ultimately responsible for the observed thermal and ionic emission, the relatively uniform dust temperature implies that the bulk of the dust heating in the region is provided by resonantly trapped Lyman α photons, rather than direct stellar photons. This then also implies that the dust is depleted with respect to the gas by a factor of at least 100 from its normal interstellar value. The small-scale spatial variations in the continuum emission and temperature can be explained by changes in the density and/or gas-to-dust mass ratio.

Abstract:

We report first results from the Streaming Motions of Abell Clusters (SMAC) project, an all-sky Fundamental Plane survey of 699 early-type galaxies in 56 clusters between ∼3000 and ∼14,000 km s-1. For this sample, with a median distance of ∼8000 km s-1, we find a bulk flow of amplitude 630 ± 200 km s-1 toward l = 260 ± 15°, b = -1 ± 12° with respect to the cosmic microwave background. The flow is robust against the effects of individual clusters and data subsets, the choice of Galactic extinction maps, Malmquist bias, and stellar population effects. The direction of the SMAC flow is ∼90° away from the flow found by Lauer & Postman, but it is in good agreement with the gravity dipole predicted from the distribution of X-ray-luminous clusters. Our detection of a high-amplitude coherent flow on such a large scale argues for excess mass density fluctuation power at wavelengths λ ≳ 60 h-1 Mpc, relative to the predictions of currently popular cosmological models.