Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Astronomers discover an enormous halo of red giant stars around Andromeda

09.01.2007
Astronomers have found an enormous halo of stars bound to the Andromeda galaxy and extending far beyond the swirling disk seen in images of the famous galaxy, our nearest large galactic neighbor. The discovery, reported at the American Astronomical Society meeting in Seattle, suggests that Andromeda is as much as five times larger than astronomers had previously thought.

"I am absolutely astounded by how big this halo is. As we looked farther and farther out, we kept finding stars that look like halo stars," said Puragra (Raja) Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz, who will present the findings at the meeting.

Guhathakurta and his collaborators at UCSC, UCLA, and the University of Virginia are conducting an ongoing study of Andromeda's stellar halo, using observations at the Kitt Peak National Observatory in Arizona and the W. M. Keck Observatory in Hawaii. Their new findings are based on data gathered using the 4-meter Mayall Telescope at Kitt Peak and the DEIMOS spectrograph on the 10-meter Keck II Telescope in Hawaii.

The researchers detected a sparse population of red giant stars--bright, bloated stars in a late stage of stellar evolution--that appear to be smoothly distributed around the galaxy out to a distance of at least 500,000 light-years from the center. Even at that great distance, the stars are bound to the galaxy by gravity. These stars probably represent Andromeda's stellar halo, a distinct structural component of the galaxy that has eluded astronomers for over 20 years, Guhathakurta said.

Following up on their discovery of Andromeda's halo, the researchers have found evidence that stars in the halo are chemically anemic compared with stars in the inner parts of the galaxy, said Jasonjot Kalirai, a postdoctoral fellow at UCSC. The halo stars are "metal-poor," meaning they contain smaller amounts of the heavier elements, a finding that is consistent with theoretical models of galaxy formation, Kalirai said.

Andromeda (also known as M31) is a large spiral galaxy very similar to our own Milky Way. While it is difficult for astronomers to study the overall structure of the Milky Way from Earth's vantage point within it, Andromeda offers a global view of a classic spiral galaxy that is close enough for astronomers to observe individual stars within it. Andromeda is about 2.5 million light-years from Earth and is the largest galaxy in the "Local Group," which also includes the Milky Way and about 30 smaller galaxies.

"The physical size of this galaxy is really striking," said coauthor R. Michael Rich of UCLA. "The suburbs of M31 and the Milky Way are so extended that they nearly overlap in space, despite the great distance between these two galaxies. If the whole of M31 were bright enough to be visible to the naked eye, it would appear to be huge, larger in apparent size than the Big Dipper."

Spiral galaxies typically have three main components: a flattened disk, a bright central bulge with a dense concentration of stars, and an extended spherical halo of sparsely distributed stars. The concentration of stars in the central bulge decreases exponentially with increasing distance from the center, whereas the density of the halo stars falls off more gradually (as an inverse power of the radius).

In Andromeda, the disk has a radius of about 100,000 light-years. Outside the plane of the disk, stars plausibly belonging to the central bulge can be found as far out as 100,000 light-years from the center of the galaxy, while the halo extends five times farther than that, according to Guhathakurta.

"We now believe that previous groups have been mistakenly identifying the outer parts of the Andromeda bulge as its halo," he said.

Guhathakurta's group was able to detect the halo by developing a sophisticated technique for clearly distinguishing halo stars in Andromeda from the more numerous foreground stars in the Milky Way. A foreground star with low luminosity and a luminous star that is much farther away can be hard to tell apart because they appear to be equally bright from our perspective, Guhathakurta said.

"A firefly 10 feet away and a powerful beacon in the distance can have the same apparent brightness. In this case, the fireflies are dwarf stars in our own galaxy and the beacons are red giant stars in Andromeda," he said.

Karoline Gilbert, a UCSC graduate student, developed the technique for separating the fireflies from the beacons. Her technique provided a clear separation between the two populations of stars by combining five diagnostic criteria based on photometry (brightness measurements) and spectroscopy (which separates starlight into a spectrum of different wavelengths). The diagnostic criteria include radial velocity and parameters based on differences in surface gravity between red giants and dwarf stars.

"We focused on detecting red giant stars in the halo because they are bright enough for us to obtain spectra," Gilbert said. "There are assuredly other kinds of stars in Andromeda's halo, but they are just too faint for us to get spectra of them."

In addition to Gilbert, Guhathakurta, Kalirai, and Rich, the other collaborators include Steven Majewski, James Ostheimer, and Richard Patterson at the University of Virginia and David Reitzel at UCLA.

The group's ongoing investigation of Andromeda's halo promises to shed new light on the question of how large galaxies formed, Guhathakurta said.

"Galaxy formation theories tell us that halos are pristine--the oldest component of the galaxy--but this is based almost entirely on studies of our own galaxy. A detailed study of this newly discovered Andromeda halo will allow us to test whether these theories apply more generally to galaxies other than the Milky Way," he said.

Tim Stephens | EurekAlert!
Further information:
http://www.ucsc.edu

More articles from Physics and Astronomy:

nachricht A tale of two pulsars' tails: Plumes offer geometry lessons to astronomers
18.01.2017 | Penn State

nachricht Studying fundamental particles in materials
17.01.2017 | Max-Planck-Institut für Struktur und Dynamik der Materie

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

Im Focus: How to inflate a hardened concrete shell with a weight of 80 t

At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).

Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

Nothing will happen without batteries making it happen!

05.01.2017 | Event News

 
Latest News

A big nano boost for solar cells

18.01.2017 | Power and Electrical Engineering

Glass's off-kilter harmonies

18.01.2017 | Materials Sciences

Toward a 'smart' patch that automatically delivers insulin when needed

18.01.2017 | Life Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>