Annual Meeting of the Deep Sky Section, 2007 March 3

The Northern Sky Hα Survey

Prof. Drew explained that IPHAS⁴ was using the Wide Field Camera (WFC) of the INT to survey a 10° strip of sky following the northern galactic plane – an area totalling 1,800 deg². Images were taken using three filters: Hα, r' and i'. The latter two, pioneered by the Sloan Digital Sky Survey (SDSS), were similar to R- and I-band filters, but were preferred on account of their having more rectangular transmission profiles.

The survey collaboration consisted of 30-40 people, based in the UK, Spain and the Netherlands. Image reduction was been undertaken by the Cambridge Astronomical Survey Unit (CASU) of the Institute of Astronomy in Cambridge.

Prof. Drew explained that the Hα line (656.3 nm) was indicative of a range of interesting astrophysical environments. It was a transition line of atomic hydrogen, but was only excited where that hydrogen had become ionised; emission in it resulted only when free electrons and ionised hydrogen nuclei recombined. Astrophysically, exposure to ultraviolet photons was the principal mechanism for the ionisation of gas, and so Hα was a tracer of environments such as those around young massive stars, post-AGB stars, and accreting systems such as cataclysmic variable stars, which produced appreciable numbers of ultraviolet photons. In extragalactic astronomy, Hα was known as the tracer of star formation. Its position in the R-band was of practical use: this was the least dust-obscured part of the visible spectrum, allowing greater penetration of dusty environments, including the Milky Way's disc.

Until recently, a deep survey of Galactic emission had been lacking; the deepest catalogue of point sources had dated from the 1970s and stopped at mag 13. The Anglo-Australian Observatory (AAO) had rectified this in the southern hemisphere, conducting a survey using the UK Schmidt Telescope (UKST) which had gone 1000-times deeper, picking up all point sources to mag 20.5 (R). This had reached completion in 2003.

From the surveys of the 1970s, it was clear that the population of Hα sources seen in the northern galactic plane was markedly different from that seen in the south; it was much sparser – presumably due to the geometry of the galaxy. Given the difference, it seemed worthwhile to complement the AAO survey with a northern counterpart, which IPHAS now sought to provide. It would detect all northern Hα-emitters between mag 13–20 (r').

To cover the 1,800 deg² survey area required 7,635 pointings of the INT; each field was exposed for 120 s in Hα, 30 s in r' and 10 s in i'. Each field was observed twice, both to provide confirmation images, and to catch objects which fell between the four CCD chips of the WFC. The speaker remarked that an advantage of working so late in the INT's lifetime was that the camera's response had now been very well characterised. As the filters used by IPHAS all lay at the red end of the spectrum, the survey was comparatively tolerant of Moon interference, and tended to be scheduled during Moon bright time, when there was a reduced demand for telescope time.

To date, 72% of the survey had been completed, with an average seeing of 1.7". It was hoped that the observations would be completed in late 2007. CASU was presently working to calibrate the data to ensure photometric uniformity throughout; to date, some weather-induced variability in the photometry remained.

The speaker explained that whilst line emission was most commonly associated with nebulae, much of the science from IPHAS was actually stemming from unresolved point sources. The r'–Hα and r'–i' colours of main sequence stars provided superb diagnostics both of their spectral types and of the dust extinction along our lines of sight to them. This facilitated mappings of both stellar masses and dust extinction across the Galactic plane. Stars with strong line emission – Ae and Be stars, indicative of young systems, still enshrouded in their natal gas clouds, and accreting cataclysmic variable stars – stood out on account of their bright Hα emission. Conversely, white dwarfs stood out as Hα-faint objects on account of significant absorption by circumstellar hydrogen.

As an example of the power of IPHAS, the speaker showed work done on the Cygnus OB2 association. Though its O- and B-type stars were the most prominent, one would also expect a much larger population of fainter A-type stars to accompany them. IPHAS colour selection had allowed 1,500 such stars to be identified for the first time. Moreover it had allowed their distances to be accurately estimated: because their spectral types were so well constrained, so too were their luminosities, allowing their brightnesses to be converted into distance estimates. The resulting three-dimensional map of this cluster clearly showed two distinct stellar populations; this was apparently not a single cluster at all, but rather two, one in front of the other.

IPHAS was also expected to detect many hundreds of new planetary nebulae (PNs), which appeared as extended Hα-bright objects. The first to be reported, IPHAS PN-1, had proven an especially interesting object, situated in the Milky Way's extremities, 1.5 times further out than the Sun, at 13 kpc from the Galactic Centre. In the outer Galaxy, chemical abundances were expected to be radically different from those seen in the solar neighbourhood, and so there were likely to be many follow-up studies of this object. Aside from its chemistry, its morphology was also strikingly unusual. An Hα-bright point source in the centre – rarely seen in PNs – suggested that the parent star was in an accreting binary system. Moreover, the nebula itself showed two distinct rotation axes, a phenomenon which had only before been seen in a handful of PNs; the formation of such nebulae was yet to be understood.

Prof. Drew closed by adding that plans were already being made for a deeper survey of the southern Galactic plane after IPHAS. The Very Large Telescope (VLT) Survey Telescope (VST) Photometric Hα Survey (VPHAS) was scheduled to commence in 2008, and would image in Hα, u', g', r' and i'.

Following the applause, the Director introduced two short talks to conclude the meeting. The first was by Mr Gary Poyner of the Variable Star Section.