Astronomical instrumentation

Abstract:

We present absorption-line strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained as part of the SAURON (Spectrographic Areal Unit for Research on Optical Nebulae) survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise ratio, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick Observatory Image Dissector Scanner (Lick/IDS) system over a contiguous two-dimensional field including bulge and inner disc. We present maps of Hβ, Fe 5015 and Mg b for each galaxy. We find that Sa galaxies on the average have slightly smaller Mg b and Fe 5015 line strengths than ellipticals and S0s, and higher Hβ values, but with a much larger scatter. The absorption-line maps show that many galaxies contain some younger populations (≤1 Gyr), distributed in small or large inner discs, or in circumnuclear star-forming rings. In many cases these young stars are formed in circumnuclear ministarbursts, which are dominating the light in the centres of some of the early-type spirals. These ministarburst cause a considerable scatter in index-index diagrams such as Mg b-Hβ and Mg b-Fe 5015, more than is measured for early-type galaxies. We find that the central regions of Sa galaxies display a wide range in ages, even within the galaxies. We find that the central regions of early-type spirals are often dusty, with a good correlation between the presence of young central stellar populations and a significant amount of dust extinction. 50 per cent of the sample show velocity dispersion drops in their centres. All of the galaxies of our sample lie on or below the Mg b -σ relation for elliptical galaxies in the Coma cluster, and above the Hβ absorption line-σ relation for elliptical galaxies. If those relations are considered to be relations for the oldest local galaxies we see that our sample of spirals has a considerable scatter in age, with the largest scatter at the lowest σ. This is in disagreement with highly inclined samples, in which generally only old stellar populations are found in the central regions. The discrepancy between our sample and highly inclined samples, and the presence of so many stellar velocity dispersion dips, i.e. so-called σ drops, in these spiral galaxies with large bulges (type Sa) can be understood if the central regions of Sa galaxies contain at least two components: a thin, disc-like component, often containing recent star formation, and another, elliptical-like component, consisting of old stars and rotating more slowly, dominating the light above the plane. These components together form the photometrically defined bulge, in the same way as the thin and the thick disc co-exist in the solar neighbourhood. In this picture, consistent with the current literature, part of the bulge, the thicker component, formed a very long time ago. Later, stars continued to form in the central regions of the disc, rejuvenating in this way the bulge through dynamical processes. This picture is able to explain in a natural way the heterogeneous stellar populations and star formation characteristics that we are seeing in detailed observations of early-type spiral galaxies. © 2007 RAS.

Abstract:

Stellar kinematics show no evidence of hidden mass concentrations at the centre of M83. We show the clearest evidence yet of an age gradient along the starburst arc and interpret the arc to have formed from orbital motion away from a starforming region in the dust lane. © 2008 Copyright International Astronomical Union 2008.

Abstract:

We analyse the first publicly released deep field of the UK Infrared Deep Sky Survey (UKIDSS) Deep eXtragalactic Survey to identify candidate galaxy overdensities at z ∼ 1 across ∼1 deg2 in the ELAIS-N1 field. Using I - K, J - K and K - 3.6 μm colours, we identify and spectroscopically follow up five candidate structures with Gemini/Gemini Multi-Object Spectrograph and confirm that they are all true overdensities with between five and 19 members each. Surprisingly, all five structures lie in a narrow redshift range at z = 0.89 ± 0.01, although they are spread across 30 Mpc on the sky. We also find a more distant overdensity at z = 1.09 in one of the spectroscopic survey regions. These five overdense regions lying in a narrow redshift range indicate the presence of a supercluster in this field and by comparing with mock cluster catalogues from N-body simulations we discuss the likely properties of this structure. Overall, we show that the properties of this supercluster are similar to the well-studied Shapley and Hercules superclusters at lower redshift. © 2007 RAS.

Abstract:

In order to enable high quality lens designs using N-BK7, BaLKN3, SF15, and E-SF03 at cryogenic temperatures, we have measured the absolute refractive index of prisms of these four materials using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, as a function of both wavelength and temperature. For N-BK7, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 50 to 300 K at wavelengths from 0.45 to 2.7 mu m; for BaLKN3 we cover temperatures ranging from 40 to 300 K and wavelengths from 0.4 to 2.6 mu m; for SF15 we cover temperatures ranging from 50 to 300 K and wavelengths from 0.45 to 2.6 mu m; for E-SF03 we cover temperatures ranging from 30 to 300 K and wavelengths from 0.45 to 2.8 mu m. We compare our measurements with others in the literature and provide temperature-dependent Sellmeier coefficients based on our data to allow accurate interpolation of index to other wavelengths and temperatures. While we generally find good agreement (+/-2 x 10(-4) for N-BK7, +/-4 x 10(-4) for E-SF03, <1X10(-4) for the other materials) at room temperature between our measured values and those provided by the vendor, there is some variation between the datasheets provided with the prisms we measured and the catalog values published by the vendor. This underlines the importance of measuring the absolute refractive index of the material when precise knowledge of the refractive index is required.