How do you weigh galaxies for the greatest sky surveys?

NCBJ coordinates Polish participation in the largest observational astronomy project in history. The field of view of a telescope built in Chile will cover an area 40 times larger than the Moon’s disc. The observations planned for 10 years will provide, among others data about variable objects. Scientists from NCBJ working as part of the ASTROdust team are already preparing algorithms that will enrich the set of information obtained from the observations.

Large surveys of the sky play a very important role in modern astrophysics and observational cosmology. Currently, the largest survey of this type in the optical field is the SDSS (Sloan Digital Sky Survey), which covers approx. 35% of the entire celestial sphere and covers mainly the local universe. Further on – or deeper, as astronomers say – with a coverage of ~ 1/3 of the area covered by the SDSS, the currently emerging DES (Dark Energy Survey) extends. However, these reviews have a fundamental disadvantage – they are static, show objects at the same time of observation. They do not capture much information about the variable objects that the Universe is full of – quasars, stars, asteroids. The gap in observing the changing universe is to be filled by the new international project Legacy Survey of Space and Time (LSST), realized in the currently under construction Very Rubin Observatory (Vera Rubin Observatory) in Chile. The survey, with observations scheduled to begin as early as 2024, will scan 18,000 square degrees of the southern sky every three days for 10 years. Thanks to this, it will not only become the deepest existing catalog, but also create a unique film showing how the sky will change during this period.

No overview provides all the information we need to fully understand the observed objects. LSST will be called photometric survey providing an image of the sky in six optical filters. If we need additional information – for example spectroscopic spectra or infrared data – we will have to look elsewhere or make additional observations. However, it is worth knowing in advance in which situations such additional data will be necessary. Researchers from the National Center for Nuclear Research, under the supervision of PhD student Gabriele Riccio and his supervisor, Prof. Katarzyna Małek, in cooperation with scientists from other international centers, asked themselves the question: how well can we measure the physical properties of galaxies, using only LSST data, and how can we improve this measurement? To this end, scientists created a simulated catalog of the most typical galaxies in the Universe – star-forming active galaxies observed in the 0 < z < 2.5 redshift range (up to 11 billion light years away) as LSST will see them. The simulations were based on real data of 50,000 galaxies observed as part of the HELP (Herschel Extragalactic Legacy Project) review. HELP data, which measures galaxies across a wide spectrum from ultraviolet, through optics, to the far infrared, allows the physical properties of galaxies to be measured very accurately. The question is: how much is this possible if we only have LSST data at our disposal?

Researchers focused on parameters such as total stellar mass, dust mass, the galaxy’s total infrared brightness, and the galaxy’s star formation rate. The parameters determined from the simulated observations were compared with the parameters determined on the basis of the observation data from the HELP catalog. It turned out that the basic parameters that characterize the stellar part of a galaxy, such as stellar mass, will be measured very accurately. On the other hand, the values ​​of parameters related to galaxy dust, such as dust suppression or the rate of new star formation in the vicinity of dust clouds, determined solely on the basis of LSST data, will be overestimated. Worse, the degree of overestimation depends on the galaxy’s distance from us. This is not entirely surprising, because both the star-forming processes in galaxies, as well as the associated dust emission, it is best to observe infrared, which range will not be available for the LSST. However, knowing what error to expect, the scientists were able to suggest corrections to be applied to working with the real LSST data. The ASTROdust team, led by Prof. Katarzyna Małek, together with international collaborators from France and Chile, began work on the implementation of these amendments in publicly available tools enabling galaxy modeling. This work, on the use of stellar mass as a guide necessary to determine the value of dust attenuation in the ultraviolet range of the galaxy’s spectrum, will help to correctly describe the basic physical parameters of the analyzed galaxies.

The research of the ASTROdust team is just one of the many activities of scientists from Polish institutions that are planned as part of the Polish participation in the Vera Rubin Observatory and the LSST project. Currently, the Polish LSST consortium includes: NCBJ as a coordinating unit, UJ, UMK, UW, CAMK PAN and CFT PAN. „As part of the Polish own contribution, it is planned, inter alia, construction of a local data center in Poland"– says Prof. Agnieszka Pollo, head of the NCBJ Astrophysics Department and at the same time the project of the Polish LSST consortium. „The group of interested parties in Poland is constantly growing, and the list of affiliated scientists amounts to several dozen people. We’re all excited about the project, and we’re excited to see these petabytes of data and research and numerous publications based on it. This data has not been available yet, so it is also a chance for completely new, unexpected discoveries. But there will also be logistical challenges: how to deal with huge data sets? How to adapt machine learning methods to them? And finally – as shown in the work by Riccio et al. (2021) – LSST data alone is not always enough."

„LSST will be a key element in the puzzle of many data sets” – explains Prof. Katarzyna Małek from the NCBJ Astrophysics Department. „Although the data obtained under the LSST will be very accurate and very detailed, it will still be only optical photometric data. We will have to supplement them with data from other observatories – for example, obtained with the European Southern Observatory (ESO) telescopes, the James Webb Telescope (JWST) launched on December 25, 2021 or the SALT (Southern African Large Telescope), on which Polish astronomers have the right to 10% of the observation time. That is why we are now trying to plan our place well in this puzzle."

Astronomical and astrophysical research belongs to the group of basic research that, above all, broadens our knowledge about the world and the laws that govern it. „Thanks to LSST research, we expect to better understand the nature of matter and energy in the universe and verify the basic laws of physics”, explains Professor Pollo. „The observations will also concern our immediate space around – as part of the review, monitoring of near-Earth asteroids will be carried out, which will significantly increase the chances of an early detection of a potentially dangerous asteroid. On an even more practical side – the data collected by LSST, unprecedentedly large and complex, will require the development of sophisticated methods and machine learning algorithms, which will then probably also be used in tools used in our daily lives."

Information about the LSST project:

The Legacy Survey of Space and Time (LSST) is an international observation project that will be carried out with the 8.4 m telescope at the Vera C. Rubin Observatory, located 2,682 m above sea level on Mount Cerro Pachón in Chile. A telescope with a field of view of over 9 square degrees (observing at a time about 40 times larger area of ​​the sky than the full Moon), within 10 years of mapping the entire southern sky, it will provide about 500 petabytes of data in the form of photos and numerical data – the values ​​of photometric fluxes. It is estimated that in a week it will collect as much data as is covered by the entire SDSS overview! The main projects implemented under the LSST will focus on: researching dark matter and dark energy, searching for nearby asteroids, potentially threatening the Earth (the so-called Near Earth Objects, NEO),

Detailed information about the project: https://www.lsst.org/

Link to the publication: https://www.aanda.org/articles/aa/abs/2021/09/aa40854-21/aa40854-21.html