The SALT info sheet highlights what data you can get from SALT and how you can get observing time (also available as a high resolution version).

SALT is owned by a consortium of international partners. See the partner leaflet for all the details (also available as a high resolution version).

salt newsletter 202004

salt newsletter 202001

When it comes to galaxies, how fast is fast? The Milky Way, an average spiral galaxy, spins at a speed of 210 kilometres per second in our Sun’s neighbourhood. New research has found that the most massive spiral galaxies spin faster than expected. These “super spirals,” the largest of which weigh about 20 times more than our Milky Way, spin at a rate of up to 570 kilometres per second.

Super spirals are exceptional in almost every way. In addition to being much more massive than the Milky Way, they’re also brighter and larger in physical size. The largest span 450,000 light-years compared to the Milky Way’s 100,000-light-year diameter. Only about 100 super spirals are known to date. Super spirals were discovered as an important new class of galaxies while studying data from the Sloan Digital Sky Survey as well as the NASA/IPAC Extragalactic Database (NED).

The top row of this mosaic features Hubble images of three spiral galaxies, each of which weighs several times as much as the Milky Way. The bottom row shows three even more massive spiral galaxies that qualify as “super spirals,” which were observed by the ground-based Sloan Digital Sky Survey. Super spirals typically have 10 to 20 times the mass of the Milky Way. The galaxy at lower right, 2MFGC 08638, is the most massive super spiral known to date, with a dark matter halo weighing at least 40 trillion Suns. Astronomers have measured the rotation rates in the outer reaches of these spirals to determine how much dark matter they contain. They found that the super spirals tend to rotate much faster than expected for their stellar masses, making them outliers. Their speed may be due to the influence of a surrounding dark matter halo, the largest of which contains the mass of at least 40 trillion suns. Credits: Top row: NASA, ESA, P. Ogle and J. DePasquale (STScI). Bottom row: SDSS, P. Ogle and J. DePasquale (STScI)

“Super spirals are extreme by many measures,” says Patrick Ogle of the Space Telescope Science Institute in Baltimore, Maryland. “They break the records for rotation speeds.”
Ogle is the first author of a paper that was published October 10, 2019, in the Astrophysical Journal Letters. The paper presents new data on the rotation rates of super spirals collected with the Southern African Large Telescope (SALT), the largest single optical telescope in the southern hemisphere. Additional data were obtained using the 5-meter Hale telescope of the Palomar Observatory, operated by the California Institute of Technology. Data from NASA’s Wide-field Infrared Survey Explorer (WISE) mission was crucial for measuring the galaxy masses in stars and star formation rates.

“This work beautifully illustrates the powerful synergy between optical and infrared observations of galaxies, revealing stellar motions with SDSS and SALT spectroscopy, and other stellar properties — notably the stellar mass or ‘backbone’ of the host galaxies — through the WISE mid-infrared imaging,” says Tom Jarrett of the University of Cape Town, South Africa.

Theory suggests that super spirals spin rapidly because they are located within incredibly large clouds, or halos, of dark matter. Dark matter has been linked to galaxy rotation for decades. Astronomer Vera Rubin pioneered work on galaxy rotation rates, showing that spiral galaxies rotate faster than if their gravity were solely due to the constituent stars and gas. An additional, invisible substance known as dark matter must influence galaxy rotation. A spiral galaxy of a given mass in stars is expected to rotate at a certain speed. Ogle’s team finds that super spirals significantly exceed the expected rotation rate.

Super spirals also reside in larger than average dark matter halos. The most massive halo that Ogle measured contains enough dark matter to weigh at least 40 trillion times as much as our Sun. That amount of dark matter would normally contain a group of galaxies rather than a single galaxy.

“It appears that the spin of a galaxy is set by the mass of its dark matter halo,” Ogle explains.
The fact that super spirals break the usual relationship between galaxy mass in stars and rotation rate is a new piece of evidence against an alternative theory of gravity known as Modified Newtonian Dynamics, or MOND. MOND proposes that on the largest scales like galaxies and galaxy clusters, gravity is slightly stronger than would be predicted by Newton or Einstein. This would cause the outer regions of a spiral galaxy, for example, to spin faster than otherwise expected based on its mass in stars. MOND is designed to reproduce the standard relationship in spiral rotation rates, therefore it cannot explain outliers like super spirals. The super spiral observations suggest no non-Newtonian dynamics is required.
Despite being the most massive spiral galaxies in the universe, super spirals are actually underweight in stars compared to what would be expected for the amount of dark matter they contain. This suggests that the sheer amount of dark matter inhibits star formation. There are two possible causes: 1) Any additional gas that is pulled into the galaxy crashes together and heats up, preventing it from cooling down and forming stars, or 2) The fast spin of the galaxy makes it harder for gas clouds to collapse against the influence of centrifugal force.

“This is the first time we’ve found spiral galaxies that are as big as they can ever get,” Ogle says.
Despite these disruptive influences, super spirals are still able to form stars. Although the largest elliptical galaxies formed all or most of their stars more than 10 billion years ago, super spirals are still forming stars today. They convert about 30 times the mass of the Sun into stars every year, which is normal for a galaxy of that size. By comparison, our Milky Way forms about one solar mass of stars per year.

Ogle and his team have proposed additional observations to help answer key questions about super spirals, including observations designed to better study the motion of gas and stars within their disks. After its 2021 launch, NASA’s James Webb Space Telescope could study super spirals at greater distances and correspondingly younger ages to learn how they evolve over time. And NASA’s WFIRST mission may help locate more super spirals, which are exceedingly rare, thanks to its large field of view.

The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the science operations centre of the Hubble Space Telescope, the science and operations centre for the James Webb Space Telescope, and the science operations centre for the future Wide Field Infrared Survey Telescope (WFIRST). STScI also houses the Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide to the astronomical community a variety of astronomical data archives and is the data repository for the Hubble, Webb, Kepler, K2, TESS missions and more.

salt newsletter 201909

The SALT science brochure (also available as a high resolution pdf) gives a brief overview of SALT and highlights its scientific focus areas.

salt newsletter 201904

The SALT Annual Report 2018 (also available as a high resolution pdf) includes science highlights, an introduction to the SALT partners, operations news and feedback on outreach & education.

Zoomed in image of the supernova in Figure 2 below, highlighting it in one of the host galaxy’s spiral arms. For a brief time the supernova can be as bright as the 100 billion stars of the host galaxy. Image credit: SALT/SAAO

The Southern African Large Telescope (SALT), the largest optical telescopes in the Southern Hemisphere, has played an important role as part of the international Dark Energy Survey’s (DES, https://www.darkenergysurvey.org/) quest to pin down dark energy, the mysterious force accelerating the expansion of the universe.

As part of the hunt, SALT conducted follow-up spectroscopy of supernovae – stars that explode at the end of their lives – discovered by DES. Supernovae are so bright that they can be seen on the other side of the Universe and astronomers can accurately calculate the distances to a small subclass of them – the so-called Type Ia supernovae. Once their distances are known, Type Ia supernovae can be used to measure the acceleration of the expansion of the universe. Sorting through the chaff of variable objects to find and classify the Type Ia jewels was the important role undertaken by SALT and several other of the world’s biggest telescopes.

“To measure the acceleration of the universe’s expansion by studying stars that died hundreds of millions of years ago takes the most powerful telescopes in the world, combined with meticulous analysis. SALT has provided a key contribution to the international Dark Energy Survey, the most sophisticated study of dark energy with supernovae yet” said Dr. Eli Kasai, former PhD student at SAAO, now lecturer at the University of Namibia, and Principal Investigator for the South African Astronomical Observatory DES program from 2014 to the end of the survey in 2018.

“We need to control systematic uncertainties to very high precision so that we have confidence in our conclusions. SALT, with its massive mirror and ability to rapidly target exciting new candidates, allows us to take a confirming spectrum when the supernova candidate is at its brightest. This translates into clean answers to exactly what kind of exploding star we are looking at.”, said Dr. Mathew Smith, who is based at Southampton University and was the PI of the SALT spectroscopic follow-up program of DES supernova candidates from 2013 to 2014.

An example image depicting one of the supernovae that SALT took spectra of. Left: Image of a galaxy before the supernova, Right: Image of the same galaxy after the supernova, with the explosion clearly visible in the left of the galaxy. Image credit: SALT/SAAO

DES began science observations in 2013, in Chile, South America, with an overall goal of measuring the expansion history of the Universe in order to place tight constraints on the quantity and properties of dark energy at an accuracy of about 1%. DES employs several methods of constraining dark energy, of which supernova observations, are a primary tool.

“20 years ago we discovered Dark Energy and the acceleration of the universe by carefully observing supernovae. Today, two decades later, dark energy is still one of the great mysteries of our time. These results, with the purest sample of supernovae to date, confirm yet again that dark energy is real, and will be a key target of investigation for the Square Kilometre Array (SKA), that will be built primarily in South Africa.” said Professor Bruce Bassett, a member of the SALT DES supernova follow-up program, astronomer at SAAO and head of the Data Science group at the South African Radio Astronomical Observatory (SARAO). SARAO has recently completed construction of the MeerKAT radio telescope that will form an important part of the Square Kilometre Array.

“Dark Energy is perceived to exist in the vast empty spaces between galaxies in the Universe known as voids and we believe that it is responsible for making galaxies move away from each other at ever-increasing speeds. In other words, it is responsible for the accelerated expansion of the Universe that we observe” said Prof. Roy Maartens, an SKA Chair in Astrophysics and Cosmology at the University of the Western Cape and a member of the SALT DES supernova follow-up program. He went on saying “observing more and more supernovae in many galaxies gives us a handle to quantify the properties of dark energy and also provides us insight into the true nature of supernovae”.

A spectrum of the Type Ia supernova “DES15S2ocv” taken by SALT on the 9th of January 2016. Over-plotted in red is a template spectrum used to classify the supernova. Image credit: SALT/SAAO

SALT consistently played a pivotal role of classifying into various types the discovered supernova candidates and successfully determining how far they were from Earth, two important parameters that were key to the success of the DES experiment, which came to an end at the end of February 2018. Spectral observations of the discovered SN candidates by SALT and other spectroscopic capable telescopes in DES played a crucial role in helping algorithms that could classify the discovered supernova candidates and determine their redshift using only the images in which such candidates were discovered. This type of classification and redshift determination is less accurate in comparison to that performed with spectroscopic data from SALT and other spectroscopic capable telescopes.

“The DES team has independently confirmed the existence of dark energy by combining four different cosmic probes: (1) supernova observations, (2) baryonic acoustic oscillations, (3) weak gravitational lensing and (4) galaxy clustering”, said Dr Eli Kasai. He continued by saying “The conclusions from DES from combining these four probes mean that for the first time we have been able to find strong evidence for cosmic acceleration and dark energy from a single experiment, instead of combining results from many different telescopes and different analyses.”

The past three weeks have seen the release of 8 DES papers to Arxiv.org, reporting the findings of the analyses of DES supernova data observed over the first three years of the survey. The papers made use of the survey’s spectroscopic data including that taken with SALT.

https://arxiv.org/search/astro-ph?query=Bassett+kasai&searchtype=author&abstracts=show&order=-announced_date_first&size=50