Optical and Infrared Galactic Surveys

(Drew, Lucas)

Galactic astronomy is being revolutionised by a new generation of sensitive high resolution surveys that are covering the entire Galactic Plane in visible and infrared light. These surveys provide information on all the different types of star in the Galaxy, from young to old and from brown dwarfs up to the most massive stars. The large scale of these surveys is allowing astronomers to construct samples of rare types of star which are poorly understood at present. They also make possible sensitive wide-area studies of common populations of stars and of the distribution of obscuring dust across the plane of our galaxy.

The optical surveys are called IPHAS and VPHAS+. The IPHAS survey, nearing its completion on the 2.5-m Isaac Newton Telescope covers the entire northern plane down to Dec=-5 degrees in the r, i and H-alpha filters, in a ten degree wide band. (IPHAS stands for "INT/WFC Photometric H-alpha Survey...") The H-alpha data is nearly a thousand times more sensitive than any previous large scale H-alpha survey. H-alpha is the strongest visible emission line emitted by excited hydrogen atoms: the detection of its presence in the spectrum of a star immediately tells us that it is surrounded by hot diffuse circumstellar matter in the form of a wind, an accretion disc, or a nebula.

These optical surveys are being led by Prof Janet Drew at Hertfordshire and involve a large international group of astronomers.

The initial release of IPHAS survey data is scheduled for end 2007 and is being made through ASTROGRID, the British national Virtual Observatory system. IPHAS will be followed in 2009 by a southern counterpart, VPHAS+, which will be conducted with the new 2.5-m ESO VLT Survey Telescope (VST), in Chile. VPHAS+ will use the r, i and H-alpha filters, much like IPHAS, but it will also include the u and g filters, which are sensitive to changes in the chemical abundances of stars. The u band (near ultraviolet) data can also be used to detect a range of hot stars including white dwarf stars, some of which are found in exotic highly evolved binary systems (e.g. cataclysmic variables and 'supersoft' systems). IPHAS is also already being followed up in the north by UVEX (PI Paul Groot, Nijmegen, The Netherlands) to fill in short wavelengths with U and g, using the INT in La Palma also.

IPHAS image of part of the Rosette Nebula

An IPHAS image of part of the Rosette Nebula. The centre of this HII region, where the exciting star cluster (NGC 2244) is located, lies at the middle-bottom of this image (N is to the left, and E down). The longer dimension in this image is approximately 30 arcminutes. (credits: Nick Wright, University College London)
IPHAS image of NGC 6888, otherwise known as the Crescent Nebula

IPHAS image of NGC 6888, otherwise known as the Crescent Nebula. It is the illuminated ejecta of the WR star, HD192163. This image combines data from all three IPHAS survey passbands (H-alpha, Sloan r' and i') in a false colour composite. Emission in the H-alpha narrow band is in red, of course. The orientation is with N to the left and E down, and the image scale is about 20 x 18 square arcminutes (credits: Jonathan Irwin, Institute of Astronomy, Cambridge)
IPHAS continuum subtracted H-alpha image of the supernova remnant S147

An IPHAS continuum subtracted H-alpha image of the supernova remnant S147, located on the sky toward the anti-galactic centre. A strength of IPHAS is that it offers the chance to construct images of parts of the Galactic Plane on any scale from arcseconds up to degrees. This is a very large-scale image, built up by mosaicking a large number of overlapping IPHAS fields together: the total imaged area is roughly 5 x 3.5 square degrees. The bright blob to the left of the picture is a more typically compact Galactic nebula. (credits: Albert Zijlstra, University of Manchester and Jonathan Irwin, IoA Cambridge)

The infrared surveys are the UKIDSS Galactic Plane Survey (GPS) and the VISTA VVV survey. The UKIDSS GPS is one of 5 deep infrared surveys (see www.ukidss.org) presently being conducted with the 3.8-m United Kingdom Infrared Telescope from 2005-2012. It covers the entire northern and equatorial Galactic Plane that is accessible to UKIRT, in a ten degree wide band (like IPHAS). The UKIDSS GPS uses three near infrared filters (J, H and K). Near infrared light penetrates the obscuring dust that fills the interstellar medium in the Galactic plane, thereby allowing us to detect stars that are on the far side of the Galaxy or enshrouded within a molecular cloud where new stars are being formed.

The UKIDSS GPS is led by Dr Phil Lucas at Hertfordshire.

The GPS is now well underway. The current DR2 data release is 12.5% complete, with much higher completeness near the inner part of the Galaxy. The data is available from the WFCAM Science Archive.

The VISTA VVV survey is a synoptic, or time series survey of the Galactic Bulge and the southern Galactic plane, also at near infrared wavelengths. It will run from 2008-2013, observing the Bulge and the southern plane 80 times in the 2.15 micron Ks filter. This is an unprecedented variability survey of approximately 1 billion stars. It will increase the number of known Galactic Cepheid and RR Lyrae variables by an order of magnitude. These "standard candles", or stars of known luminosity, will be used to map out the 3-D structure of the Galactic Bulge and the spiral arms of the Galaxy.

The variability data will also be used to identify numerous unstable stars in brief, rarely observed phases of stellar evolution. These include protostars undergoing unstable accretion (FU Ori types), Luminous Blue Variables (very massive stars near the Eddington limit), post-AGB stars undergoing unstable helium shell burning and X-ray sources associated with white dwarfs, neutron stars and black holes.

These infrared surveys also involve a large international group. The VVV survey is co-led by Chilean astronomer Dante Minniti at U.Catolica.

These surveys led at Hertfordshire are complemented by a whole suite of Galactic surveys at longer wavelengths, ranging from the mid-infrared through far-infrared to the submillimetre and radio wavebands. Hertfordshire is also leading the sub-millimetre SASSy survey with the James Clarke Maxwell Telescope.

Three colour image from the central regions of the Milky Way Galaxy

"My God, it's full of stars!". This three colour image from the central regions of the Milky Way Galaxy shows the incredible density of stars detected in the near infrared. In such regions the numbers of stars in the survey database exceeds 1 million per square degree on the sky. Red - K band, Green - H band, Blue - J band. (credits: Mike Read, WideField Astronomy Unit, Edinburgh)
Star formation region BDS121 Three colour image of S106, a large star formation region dominate by a massive star at the centre

Star formation region BDS121. This star formation region lies 9 kpc away on the far side of the Galaxy. It is almost entirely obscured from view in visible light, so it has not yet been studied in detail. This is typical of most distant star formation regions in the Galactic plane. Red - K band, Green - H band, Blue - J band. (credits: Phil Lucas, University of Hertfordshire)

This is a three colour image of S106, a large star formation region dominate by a massive star at the centre. Red - K band, Green - H band, Blue - J band. (credits: Mike Read, WideField Astronomy Unit, Edinburgh)