Dark Energy Camera Plane 2 (DECaPS2) survey has been released. This is the second data release from DECaPS, and the survey contains more than 3 billion objects in the Milky Way. As the main image shows, there are many stars that appear to have no space between them.
Throughout most of human history, the nature of the Milky Way has confused us. Aristotle thought it was caused by the Earth’s upper atmosphere flaring up, similar to the way comets produce their tails. The famous Persian scientist Al-Biruni believed that the Milky Way consisted of countless fragments of obscure stars. There are countless other ideas about what the Milky Way could be like.
Our modern understanding of the Milky Way is exquisitely detailed. Large-scale surveys of the galaxy have played a huge role in our growing understanding of the Milky Way. ESA’s Gaia mission, an ongoing survey of the Milky Way, has collected detailed data on more than a billion stars, including their ages, masses, chemical compositions, colors, temperatures and metal content.
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Will DECaPS2 make a similar contribution? May be based on raw data alone.
A new paper in the Astrophysical Journal Supplement describes the new data release. It’s titled “The Dark Energy Camera Plane Survey 2 (DECaPS2): More Sky, Less Bias, Better Uncertainty.” The lead author is Andrew K. Saidgari, a Harvard graduate student and researcher at the Center for Astrophysics | Harvard and Smithsonian.
DECaPS2 contains 3.32 billion objects built from 34 billion detections. 21,400 exposures were detected adding up to 260 hours of shutter open time using the Dark Energy Camera (DECam) at Cerro Tololo Observatory. Of the 3.32 billion objects, about 2 billion are stars. It took two years and generated more than 10 terabytes of data. The first dataset was released in 2017, and included 2 billion objects, most of which were also stars.
The survey focuses on the galactic disk, where most of the galaxy’s stars and dust reside. But the high density makes it difficult to monitor the area. The DECam Plane scan overcomes this by performing “deep scans spanning a wide wavelength range (optical to NIR).”
“One of the main reasons for DECaPS2’s success is that we simply pointed to a region with a very high density of stars and were careful to identify the sources that appear almost on top of each other,” Sedjari said. “Doing this allowed us to produce the largest catalog ever from a single camera, in terms of the number of items observed.”
Combined, the two versions of the data cover 6.5 percent of the night sky and span 130 degrees. This is 13,000 times larger than the full moon. While 6.5 percent may not sound like much, it is. It’s even more impressive when combined with other sky surveys.
“When combined with images from Pan-STARRS 1, DECaPS2 completes a panoramic 360-degree view of the Milky Way’s disk and also reaches out to fainter stars,” says Edward Schlafly, a researcher at the AURA-run Space Telescope Science Institute and colleagues. The author of the newspaper. “With this new survey, we are able to map the 3D structure of the Milky Way’s stars and dust in unprecedented detail.”
It is difficult to observe the galactic plane. We are embedded in it, and when we look toward the center, we are looking through our arm of the Milky Way, across the central disk and beyond, to the spiral arms on the other side. Not only are there hundreds of millions of stars – perhaps many more – in this view. This is also where most of the dust is.
The photo shows how beautiful and interesting the dark lanes of dust are, but they are also problematic. It absorbs light from stars and can even completely block out faint stars. There are also many scattered nebulae, and their light overlaps with measurements of light from individual stars. The sheer number of stars is also a challenge because they can interfere with each other.
But understanding the central disk is crucial to understanding the Milky Way. Infrared monitoring helps overcome some of the challenges of disk monitoring. Innovative data processing also helps. The team responsible for the survey came up with a way to predict the background of each star. This made it easier to reduce the effects of interfering stars and dotted nebulae in the images.
“This is a very technical feat. Imagine a group photo of more than three billion people, and everyone can be identified!” says Debra Fisher, director of the Astronomy Division at NSF, one of the agencies that operates DECam. “Astronomers will look at this detailed picture of more than three billion stars in the Milky Way for decades to come. This is a great example of what partnerships across federal agencies can achieve.”
“Since my work on the Sloan Digital Sky Survey two decades ago, I have been looking for a way to make better measurements over complex backgrounds,” said Douglas Finkbeiner, professor at the Center for Astrophysics, co-author of the paper, and the principal investigator behind the project. “This business accomplished that and more!”
A century ago, we didn’t even know other galaxies existed. When astronomers saw the Andromeda galaxy and other spiral galaxies, they thought it was part of the Milky Way. They called them spiral nebula. Now we know better. We also know that the Milky Way is 90% dark matter and that the galaxy is not flat; It is distorted by the pull of the Large and Small Magellanic Clouds on it. We know that there is a supermassive black hole lurking in the center of the galaxy called Sagittarius A-star. We also know that galaxies are growing very large by consuming and merging with other galaxies.
Large-scale astronomical surveys of the sky have helped astronomers make some of these discoveries, and DECaPS promises to propel us into similar developments. The first data release helped lead to some very interesting results. One such example is a 2018 paper that identified an ancient, mineral-poor globular cluster in the galactic bulge. This was unusual because most globular clusters are in the galactic halo. Now, thanks to DECaPS and others, we know of many spheroids in the bulge.
There is no doubt that the second release of the data will lead to more discoveries and fill our knowledge at the galactic level. “In combination with PS1, this completes a similar imaging of the entire galactic plane necessary for examining stars, gas, and dust in our galaxy,” the authors wrote. They explained that all data products from the survey “…should provide a rich and adaptable resource for society, facilitating a variety of studies of the Milky Way”.
We are fortunate that our galaxy, the Milky Way, is a good model for understanding galaxies in general. About half of the stars in the universe are found in galaxies just like our own. While nothing can be assumed, it is a reasonable assertion that much of what DECaPS tells us about the Milky Way will have some effect on other galaxies.
Astronomers have striven to understand galaxies and how they formed and evolved. The most comprehensive way to study them is to do big star surveys like this one. Who knows what researchers will find in all this data?