The INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS)

IPHAS is a major survey of the Northern Galactic Plane being carried out with the 2.5-metre Isaac Newton Telescope (INT) in La Palma (Drew et al. 2005). The IPHAS survey began taking data with the INT Wide Field Camera in 2003 with the goal of imaging the entire northern galactic plane in the latitude range -5° < b < +5 degrees. Imaging of this 1800 sq. degree area is now almost complete, and initial data products have been released (Gonzales-Solares et al. 2007)

The Hα spectral line is significant in that it both traces diffuse ionised nebulae and is commonly prominent in the spectra of pre- and post-main-sequence stars. These objects are in a minority in a galaxy like our own due to them representing relatively short-lived phases of evolution, yet the study of them is crucial to understanding the key stages of stellar evolution.

Pre-main-sequence stellar objects such as T Tauri stars, Herbig AeBe stars and Herbig-Haro objects help determine the growth of planetary systems. Post-main-sequence stars determine stellar end states and contribute to the recycling of energy and chemically enriched matter back into the ISM. These objects include supergiants, luminous blue variables, AGB stars, post-AGB objects, planetary nebulae and interacting binaries.

The data obtained will be mined both for spatially-resolved nebulae and for unresolved emission line stars. For point sources, the useful magnitude range is 13.5 < m(pv) < 20.5. Previous surveys of the area are more than 40 years old and begin to be incomplete even at m(V)=12.5. Preliminary results suggest that around 10,000 new Hα emission sources will be catalogued.

This programme will complement the Southern Hα Survey (SHS), an Hα survey of the southern galactic plane carried out photographically with the UK Schmidt Telescope. It is expected that IPHAS will exceed this in both sensitivity and resolution. ESO has recently selected VPHAS, an Hα survey of the Southern Galactic Plane that will be complementary to IPHAS and which will be carried out by the 2.5-metre VLT Survey Telescope (VST) at Paranal in Chile.

IPHAS Observations and Data Reduction

The IPHAS survey is being carried out using the Wide Field Camera (WFC) on the 2.5-metre Isaac Newton Telescope (INT) in La Palma. Observations are made using narrow band Hα, sloan r' and sloan i' filters on a mosaic of 4 CCDs, providing a field of view of 34×34 arcmins. Each pointing is accompanied by a second pointing at an offset position 5 arcmin W and 5 arcmin S. Data reduction is carried out by the Cambridge Astronomical Survey Unit (CASU), where it is pipeline processed. The pipeline calibrates the raw data, extracts sources from the Hα, r' and i' processed images, correlates them, and produces a final merged catalogue where astrometric and photometric information is stored. Also available for download are the background-subtracted images of the CCDs, available as FITS files (Flexible Image Transport System).

Useful links

• IPHAS website
• UCL press release, December 2007, about the initial data release
• Images produced from the IPHAS Survey are available on Nick Wright's website

UCL Contact

Professor Mike Barlow (mjb AT star.ucl.ac.uk)