The following is a synopsis of the new overview paper describing the APOGEE survey. You can find the full paper here.

The first thing to show you about this paper is its authors list. Check out how many contributors there are to this survey:

This, together with its length (50 pages) and number of figures (38!!) may give you some idea of the huge scope of the project. To help us sift through it, here is the executive summary, from the APOGEE Principal Investigator, Steven Majewski:

“This paper gives a broad overview of the motivations, design and execution of the APOGEE survey — a synopsis of how the survey was structured and why.”

This isn’t just a technical overview of the survey, however. Says Majewski:

“The most fun part of this new overview paper — a part for which the entire APOGEE team can be justly proud — is the summary of what the project managed to accomplish in such a relatively short amount of time.”

Here are some examples of what he means:

• 13 technical papers on subjects like spectrograph design, target selection, data reduction, stellar parameter determination, and collaborations with other surveys
• Uses of time series spectral data for projects as diverse as spectroscopic binary characterization and circumstellar variations around hot stars
• Detailed maps of stellar radial velocities and metallicities across the Milky Way

It should be mentioned that APOGEE was made possible by a unique 300-fiber-fed, high resolution spectrograph working in the infrared H-band. Hence, thirteen technical papers, while a seemingly large number, were all necessary because APOGEE truly broke ground in a number of ways, both scientific and technical. The actual construction was accomplished by the APOGEE instrument team led by John Wilson (Instrument Scientist) and Fred Hearty (Project Manager).

The detailed maps mentioned in the last bullet deserve a couple of images — they’re too cool not to show. In Figure 24 from the paper, the velocities of stars relative to the Sun are shown against an artist’s impression of the Milky Way Galaxy. Notice how there are clear areas where stars are approaching us, co-moving with us, and moving away from us:

Figure 25 is a similar plot, but showing instead the chemical composition (metal content with respect to hydrogen) in stars across the same survey area. Check out the clear gradient from low-metallicity stars near the edge of the Milky Way’s disk to high-metallicity stars near the Galactic Center:

The general trends shown here are not new; instead, it is APOGEE’s unprecedented detail that makes it the biggest kid on the Galactic evolution block. To put APOGEE into context for us, here is Ricardo Schiavon, the APOGEE Survey Scientist, to sum it up:

“APOGEE has, for the first time, provided a homogeneous database of high quality, high resolution infrared spectra for 150,000 stars, which together offer a rigorously systematic spectroscopic census of our home galaxy. Because of it’s unique infrared sensitivity and ability to punch through the blankets of obscuring Galactic dust in its most crowded regions, APOGEE is unveiling the chemical and kinematical properties of stars in parts of the Milky Way never before probed in such exquisite detail. It is a groundbreaking experiment.”

To put it more bluntly: no one has ever done an infrared survey of Milky Way stars in this way.

Two undergraduates at San José State University have used public SDSS data to discover two galaxies that are the densest known. Similar to ordinary globular star clusters but a hundred to a thousand times brighter, the new systems have properties intermediate in size and luminosity between galaxies and star clusters.

The first system discovered by the investigators, M59-UCD3, has a width two hundred times smaller than our own Milky Way Galaxy and a stellar density 10,000 times larger than that in the neighborhood of the Sun. For an observer in the core of M59-UCD3, the night sky would be a dazzling display, lit up by a million stars. The stellar density of the second system, M85-HCC1, is higher still: about a million times that of the Solar neighborhood. Both systems belong to the new class of galaxies known as ultracompact dwarfs (UCDs).

The study, led by undergraduates Michael Sandoval and Richard Vo, used imaging data from the Sloan Digital Sky Survey, the Subaru Telescope, and Hubble Space Telescope, as well as spectroscopy from the Goodman Spectrograph on the Southern Astrophysical Research Telescope (SOAR), located on the Cerro Tololo Inter-American Observatory site. The National Optical Astronomy Observatory (NOAO) is a SOAR partner. The SOAR spectrum was used to show that M59-UCD3 is associated with a larger host galaxy, M59, and to measure the age and elemental abundances of the galaxy’s stars.

Two ultra-dense galaxies (insets) have been discovered orbiting larger host galaxies. The compact systems are thought to be the remnants of once normal galaxies that were swallowed by the host, a process that removed the fluffy outer parts of the systems, leaving the dense centers behind. Image credit: A. Romanowsky (SJSU), Subaru, Hubble Legacy Archive

“Ultracompact stellar systems like these are easy to find once you know what to look for. However, they were overlooked for decades because no one imagined such objects existed: they were hiding in plain sight”, said Richard Vo. “When we discovered one UCD serendipitously, we realized there must be others, and we set out to find them.”

The students were motivated by the idea that all it takes to initiate a discovery is a good idea, archival data, and dedication. The last element was critical, because the students worked on the project on their own time. Aaron Romanowsky, the faculty mentor and coauthor on the study, explained, “The combination of these elements and the use of national facilities for follow up spectroscopy is a great way to engage undergraduates in frontline astronomical research, especially for teaching universities like San José State that lack large research budgets and their own astronomical facilities.”

The nature and origins of UCDs are mysterious – are they the remnant nuclei of tidally stripped dwarf galaxies, merged stellar super-clusters, or genuine compact dwarf galaxies formed in the smallest peaks of primordial dark matter fluctuations?

Michael Sandoval favors the tidally stripped hypothesis. “One of the best clues is that some UCDs host overweight supermassive black holes. This suggests that UCDs were originally much bigger galaxies with normal supermassive black holes, whose fluffy outer parts were stripped away, leaving their dense centers behind. This is plausible because the known UCDs are found near massive galaxies that could have done the stripping.”

An additional line of evidence is the high abundance of heavy elements such as iron in UCDs. Because large galaxies are more efficient factories to make these metals, a high metal content may indicate that the galaxy used to be much larger.

To test this hypothesis, the team will investigate the motions of stars in the center of M59-UCD3 to look for a supermassive black hole. They are also on the hunt for more UCDs, to understand how commonly they occur and how diverse they are.

Reference:

“Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey,” Michael A. Sandoval, Richard P. Vo, Aaron J. Romanowsky et al. 2015, Astrophysical Journal Letters, 808, L32. (Preprint: http://arxiv.org/abs/1506.08828)

This post is copied from a press release from the National Optical Astronomy Observatory.

NOAO is operated by Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.