The SkyMapper Southern Survey (SMSS) and its Data Release 4
Published in the RSAA Lunations
Vol1 Issue48 1–29 February 2024
Almost ten years ago, in March 2014, the ANU SkyMapper Telescope started routinely taking observations for its eponymous Southern Survey. Brian Paul Schmidt had initiated the project already over 20 years ago. Using type-Ia supernovae as probes, both Brian’s research group and the competitors led by a Berkeley particle physicist had found in the late 1990s that the expansion of the Universe seemed to be accelerating. But the precision of the solution was limited by shortcomings of the sample of low-redshift supernovae, which would need to be searched with a widefield approach. In addition, the Sloan Digital Sky Survey had started to create a scientifically enormously productive public data set of the Northern extragalactic sky.
Brian had moved from Harvard to Stromlo, where a strong group of Galactic archaeologists were not helped much by Sloan. Somehow the idea arose to start a new survey in the South, that would digitise the Southern sky with three differences over Sloan: it would include the Galactic Plane and, being in the South, the Galactic Centre and the Magellanic Clouds; it would use filters adapted to searching for metal-poor stars – gems for Galactic archaeologists; and it would include repeat observations of 10% of the extragalactic sky in bad seeing to find the low-redshift type-Ia’s needed by the cosmologists to make progress on dark energy.
Getting from this plan to where we are today took more time getting ready for observing than for observing itself. But that long story needs to be told elsewhere. In 2014 SkyMapper started observing, and in 2018 Data Release 1 was released with shallow observations of most of the Southern hemisphere. A Data Release 4 was never planned. But both the needs of the project and technology raced away in sobering fashion from the initial plans. By March 2019 funding had finished for the project that was supposed to take a mere five years. On a shoestring it was nursed into the future that is now in the past.
In 2014, Australian astronomers had grand plans to use SkyMapper as a universal optical data source for the sky, as a complement for ASKAP, as a complement for eROSITA, as a treasure map of the sky to find attractive targets for 8m-class telescopes, and possibly for several thousand papers. Alas, these projects were delayed themselves and are now being realised in a world that has several other new survey datasets available, such as DES and Gaia.
Back to SkyMapper: its best data release yet, DR4 was released this week, to both ANU and the world public simultaneously. The weakest side of DR4 is that it is not as deep and sharp as some other large surveys and has fewer visits in its time domain axis. The supernova component of the survey was discontinued when a competing project overtook it and raced ahead. The strongest side of SMSS DR4 is the full hemispheric coverage and the actual availability of images, things that Gaia, e.g., does not offer; also unique are the six passbands, including the two ultraviolet and violet bands that are useful for stellar classification. And the SQL database is already cross-matched with a large range of recent survey catalogues.
Altogether, SMSS DR4 includes over 400,000 science images with over 260 Mega-pixels each. It covers 26,000 square degrees of sky, and its photometry table contains 13 billion detections that are distilled into a master catalogue of around 700 million unique objects. The catalogue has been used to create a synthetic image of the hemisphere by adding the light from all the sources (see figure).
The earlier data releases had their strongest science impact in the areas of metal-poor stars, quasars at all redshifts, and supernovae and variable phenomena. Some highlights include new records of the most metal-poor star known in the Galaxy (Stromlo keeps beating itself), new records for the most luminous quasar in the Universe (even there, spoiler alert, Stromlo keeps beating itself), and the discovery of an eclipsing tight binary that shows first evidence of an ejected common-envelope. Several of these discoveries have been subject of Research Bytes in earlier Lunations and ANU press releases. The most-luminous quasar of 2018 was published in many outlets from the New York Times to social media in Arabic language. The eclipsing tight binary saw a press conference in Beijing organised by our Chinese collaborators on the project and was all over Chinese TV channels. And the metal-poor stars are still shining above our heads.
The data can be accessed with TopCat or via the SkyMapper website (see below) and are there for everyone’s benefit. If anyone wants to do the small surviving team a favour, the best option is to ruthlessly exploit the resource for scientific or educational purposes. Most people who ever worked on SkyMapper in one capacity or another, are retired, or long gone from Stromlo to other shores. On Friday 16 February, some of them are returning for a celebration of the data release and the project’s history and anniversary.
Christian Wolf and Chris Onken
Figure: the Southern Celestial Hemisphere as seen by SkyMapper – a synthetic image created from Data Release 4. Credit: Chris Onken
SkyMapper DR4 is now accessible at SkyMapper Website (with documentation, data access description, cone search, image cutout service, and object viewer).
The data release paper is submitted to Publications of the Astronomical Society of Australia: Christopher A. Onken, Christian Wolf, Michael S. Bessell, Seo-Won Chang, Lance C. Luvaul, John L. Tonry, Marc C. White, & Gary S. Da Costa, see also preprint on arXiv
