Astronomical instrumentation

Abstract:

We describe the results of a very deep imaging survey of the Trapezium cluster in the IJH bands, using the UKIRT high-resolution camera UFTI. Approximately 32 per cent of the 515 point sources detected are brown dwarf candidates, including several free-floating objects with masses below the deuterium-burning (planetary) threshold at 0.013 M⊙, which are detectable because of their extreme youth. We have confidence that almost all the sources detected are cluster members, since foreground contamination is minimal in the 33-arcmin2 area surveyed, and the dense backdrop of OMC-1 obscures all background stars at these wavelengths. Extinction is calculated from the (J - H) colours, permitting accurate luminosity estimates, and temperatures are derived from the dereddened (I - J) colours. There is some evidence for a cut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the initial mass function. Since star formation is complete in the Trapezium, this limit could have wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a process for which the time-scale will vary between star formation regions.

Abstract:

We report on advances in the study of the cores of NGC 6302 and 6537 using infrared grating and echelle spectroscopy. In NGC 6302, emission lines from species spanning a large range of ionization potential, and in particular [Si IX] 3.934 μm, are interpreted using photoionization models (including CLOUDY), which allow us to re-estimate the temperature of the central star to be about 250 000 K. All of the detected lines are consistent with this value, except for [Al V] and [Al VI]. Aluminium is found to be depleted to one hundredth of the solar abundance, which provides further evidence for some dust being mixed with the highly ionized gas (with photons harder than 154 eV). A similar depletion pattern is observed in NGC 6537. Echelle spectroscopy of IR coronal ions in NGC 6302 reveals a stratified structure in ionization potential, which confirms photoionization to be the dominant ionization mechanism. The lines are narrow (<22km s-1 FWHM), with no evidence of the broad wings found in optical lines from species with similar ionization potentials, such as [Ne V] 3426 Å. We note the absence of a hot bubble, or a wind-blown bipolar cavity filled with a hot plasma, at least on 1 arcsec and 10 km s-1 scales. The systemic heliocentric velocities for NGC 6302 and 6537, measured from the echelle spectra of IR recombination lines, are found to be -34.8±1 km s-1 and -17.8±3 km s-1. We also provide accurate new wavelengths for several of the infrared coronal lines observed with the echelle.

Abstract:

We present CCD (charge-coupled device) photometry for galaxies around 204 bright (mZ < 15.5) Zwicky galaxies in the equatorial extension of the APM Galaxy Survey, sampling an area over 400 deg2, which extends 6 h in right ascension. We fit a best linear relation between the Zwicky magnitude system, mZ, and the CCD photometry, BCCD, by doing a likelihood analysis that corrects for Malmquist bias. This fit yields a mean scale error in Zwicky of 0.38 mag mag-1: i.e. ΔmZ ≃ (0.62 ± 0.05)ΔBCCD and a mean zero-point of 〈BCCD - mZ〉 = -0.35 ± 0.15 mag. The scatter around this fit is about 0.4 mag. Correcting the Zwicky magnitude system with the best-fitting model results in a 60 per cent lower normalization and 0.35-mag brighter M* in the luminosity function. This brings the CfA2 luminosity function closer to the other low-redshift estimations (e.g. Stromlo-APM or LCRS). We find a significant positive angular correlation of magnitudes and position in the sky at scales smaller than about 5 arcmin, which corresponds to a mean separation of 120h-1 kpc. We also present colours, sizes and ellipticities for galaxies in our fields, which provides a good local reference for the studies of galaxy evolution.