HIPASS Data Release Help Page

Content

Introduction

The `HI Parkes All Sky Survey' (HIPASS) covers the whole southern sky as well as northern declinations up to +25 degrees. HIPASS commenced in Feb 1997, and the last northern scans were taken in 2002. HIPASS was carried out with the Australia Telescope National Facility's Parkes telescope Parkes 64-m telescope equipped with a novel 21-cm multibeam system. The latter comprises a cooled, 13 beam receiver and digital correlator (Staveley-Smith et al. 1996, Wilson et al. 1997 and Sinclair et al. 1997). The 64 MHz observing band, centred on 1394.5 MHz, was divided into 1024 channels, providing a velocity range of -1280 < cz < 12700 km/s with a channel width of approx. 13.2 km/s and a velocity resolution of 18 km/s. The HIPASS integration time per beam is 450s. After gridding an r.m.s. noise of about 13.3 mJy was achieved in the first set of HIPASS cubes. The observations and techniques used to calibrate and image the data are described in Barnes et al. (2001). The processing algorithms are successfully designed to be statistically robust to the presence of interference signals and are particular to imaging point (or nearly point) sources, not extended emission. Specifically, a major improvement in image quality is obtained by designing a median-gridding algorithm which uses the median estimator in place of the mean estimator. While the average Parkes beam FWHM is 14.3 arcmin, the average gridded HIPASS beam is about 15.5 arcmin.

The full data release contains data from 538 data cubes (size: 8o x 8o, 388 over the entire southern sky (DEC < +2o), and 150 over the northern sky, (DEC < +25o). The one-dimensional spectral data for a given position is available for downloading in a variety of different formats.

The multibeam receiver is described by Staveley-Smith et al. (1996) (PASA, 13, 243). The observations and data reduction are described in Barnes et al (2001) (MNRAS, 322, 486).

Catalogs

Published Catalogs:
More Information

The catalogs obtained from various Parkes 21-cm multibeam HI surveys will also soon be available here. We hope to develop interactive on-line tools for selection and display of properties from multiple catalogs.

Products

The angular resolution of the gridded HIPASS data is 15.5 arcmin. The channel spacing is 13.2 km s-1 and the velocity resolution is 18.0 km s-1 (or 27.0 km s-1 if Hanning smoothing is applied).

Browsers

This form has tested using Netscape 4.6 and Internet Explorer 5.0. The download buttons suggest long filenames with embedded coordinates (e.g. HIPASS_132157-3637.fits). The length (or the presence of a `+' sign) may give older operating systems some problems, so you may have to force a new filename (e.g. ctrl-right-mouse button in Netscape).

Search for Object Co-ordinates

Object Name

This field takes the name of the object which you are searching for. When you enter the name, spaces are not important with the search accepting both:
	NGC 5253  and NGC5253 

Catalog

Galaxy positions can be obtained from (a) the Lyon/Meudon Extragalactic Database (LEDA) (a locally-stored 1996 version) or (b) the NASA Extragalactic Database (NED) (on-line search).

Search

The SEARCH button initiates the catalogue search. If successful, co-ordinates are entered in the RA and DEC box below. You can then proceed to set the plot parameters, and press the PLOT button.

Plot HIPASS Spectra

RA,DEC

The RA and DEC field refers to the Right Ascension (RA) and Declination (DEC) co-ordinates of the position whose HI spectrum you want to plot. There are a two different ways you can enter these co-ordinates: The nearest available spectrum will be plotted. The current pixel size is 8 arcmin, and the post-gridding resolution of the data is 15.5 arcmin.

Co-ordinate System

Currently, J2000 is supported.

x-axis

The x-axis can either be velocity (cz) in units of km s-1 or frequency in units of MHz. In either case the reference frame is the solar system barycentre (heliocentric).

Range

The lower and upper limits to the x-axis range in the plotted units.

Plot Style

The plot style can either be "Curve" (data points joined by straight lines) or "Histogram".

Hanning Smoothing

The data can be plotted in its original form (velocity resolution 18.0 km s-1), or can be Hanning-smoothed (velocity resolution 26.0 km s-1). In both cases, the velocity spacing between channels is 13.2 km s-1.

Plot

The PLOT button initiates the plotting of the HI spectrum at the requested position, with the requested plot parameters. If the spectrum is unavailable or not yet released, a message is printed below the RA,DEC entry box. Otherwise, the HI spectrum is displayed, and various download options are listed.

Downloading HIPASS Spectra

Once a satisfactory HI spectrum has been plotted, it may be downloaded in a variety of formats. The download name suggested to your browser has got the source coordinates embedded: e.g. HIPASS_132157-3637.ext , where ext is one of gif, ps, ps.gz, txt or fits. This does NOT reflect the official HIPASS name of the source.

GIF

GIF format (typical size 7k).

Postscript

Postscript format (typical size 26k).

Compressed Postscript

Compressed (gzip) postscript format (typical size 6k).

Ascii

Ascii format (typical size 44k). The data is in 3 columns: channel number, velocity (km s-1 in optical cz convention) or frequency (MHz), and flux density in Jy beam-1. The reference frame for the spectral axis is the solar system barycentre.

FITS

One-dimensional image FITS format (typical size 12k). The first axis contains the spectral data. The second and third axes are dummy axes (i.e. of length one pixel) containing the requested RA and Decl., respectively (not the actual RA and Decl. of the nearest pixel).

A quick guide to your HI spectrum.

Local HI

Around zero velocity, or 1420.4 MHz, there are strong positive and negative spectral lines. These are real signals produced by Galactic hydrogen. Because the bandpass calibration of HIPASS data involves spatial filtering, these data are not useful for measuring column densities for Galactic gas. The positive and negative signals represent the deviation, at the current position, from the median signal level calculated in a declination strip of length 4o, centred at the current position. So, a negative signal usually does not imply absorption.

Interference

Although interference is highly suppressed in HIPASS data, the final data product still contains traces of interference, which you need to be wary of. The prime interfering line is the 11th harmonic of the 128 MHz sampler clock at 1408 MHz, or cz=2640 km s-1. This is a narrow line, although Doppler corrections may of up to 0.15 MHz or 30 km s-1 may broaden and shift this line. Interference from the GPSL3 beacon at 1380 MHz, or cz=8800 km s-1 is only sporadic, but may bias the noise level over a substantial frequency range (+/- 1 MHz). Other residual narrow-band signals may be present in HIPASS cubes, notably near 1400 MHz, or 4400 km s-1. Cubes in the Sculptor region and the equatorial strip are known to contain more interference lines. If you suspect an interference signal, the best way to check is to examine a few nearby spectrum (say about a degree away in position).

Continuum Sources

Radio continuum sources are suppressed in the data reduction procedure, so that the median level of any spectrum should be close to zero flux density. However, there are residual features which appear at the position of very strong sources (e.g. Cen A itself). These features are commonly a baseline curvature, a quasi-periodic 5.8 MHz ripple (arising from the standing wave between the focus cabin and the vertex of the Parkes telescope), and an increase of rms noise due to the increase of system temperature. The PMN catalogue, although at the higher frequency of 5 GHz gives a good indication of the presence of discrete continuum sources. This catalogue is available through NED.

Flux Scale

For an unresolved source located at the centre of a pixel, the flux scale is accurate to about 5%. If your source is not at the centre of a pixel, there will be some flux loss due to the nearest-neighbour sampling algorithm. The pixel size is 8 arcmin for data released on the web.

For resolved sources, the TOTAL flux density, spatially integrated over the source, may greatly exceed the flux density per beam (which is a surface brightness). For such cases, spatially integration of the cube is the only way of obtaining an accurate flux. This functionality is not yet provided.

On very bright radio continuum sources (flux densities greater than about 100 Jy), there is saturation in the total power measurement system, leading to an inaccurate and unpredictable flux density scale.

Velocity Scale

The velocity scale of the HIPASS data plotted here is with respect to the solar barycentre (commonly, heliocentric) and in the usual optical (cz) convention. Note that the spacing, in km s-1, between adjacent channels changes with velocity.

Frequency Scale

The frequency scale of the HIPASS data plotted here is with respect to the solar barycentre (commonly, heliocentric).

Noise

The canonical RMS noise in HIPASS cubes is 13-14 mJy beam-1, but higher noise may be measured for cubes near the Galactic Plane and in the north. The following figure is a guide to the RMS (in Jy) for each cube. The HIPASS cube numbers appear under "Object" in the public data release spectrum, though can also be found here.

HIPASS rms

Acknowledging HIPASS Data

This data is provided under the auspices of the Multibeam Survey Working Group. The data version number is provided with FITS and ascii data files. Users of this facility are requested to (1) acknowledge the ATNF in any publications as follows:
The Parkes telescope is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.
and (2) cite an appropriate HIPASS observations and data reduction paper, e.g., Barnes et al. (2001) [MNRAS 322, 486]. Further HIPASS papers are listed here. Where possible, authors are requested to include one of the terms `ATNF', or `Parkes Telescope', in the ABSTRACT of their papers. This is to facilitate electronic searches for publications that include ATNF data.

Please inform Julie Tesoriero ( Julie.Tesoriero -at- csiro.au ) of any publications which include ATNF data.


Data provided by the Multibeam Survey Working Group.
Pages created by M. Marquarding and L. Staveley-Smith.
Last Modified: 29-March-2010 by B. Koribalski