* Astronomy

Title: Discovery of an Extended Halo of Metal-poor Stars in the Andromeda Spiral Galaxy
Authors: Puragra Guhathakurta, James C. Ostheimer, Karoline M. Gilbert, R. Michael Rich, Steven R. Majewski, Jasonjot S. Kalirai, David B. Reitzel, Michael C. Cooper, Richard J. Patterson

Researchers report on the discovery of an extended halo of metal-poor red giant branch (RGB) stars in the Andromeda spiral galaxy (M31).
Their ongoing survey of M31 includes wide-field deep optical images in the intermediate-width DDO51 band and Washington system M and T_2 bands obtained with the Kitt Peak National Observatory 4-m telescope and Mosaic camera. The DDO51 band allows them to screen M31 RGB star candidates from foreground Galactic dwarf star contaminants.
The imaging is followed up with spectroscopy using the Keck II 10-m telescope and DEIMOS. A combination of photometric and spectroscopic diagnostics is used to reliably isolate M31 RGB stars; these stars are seen in all of the fields out to a projected distance of 165 kpc from M31's centre.

These newly discovered RGB stars represent the hitherto elusive stellar halo of M31. The surface brightness of M31 beyond r > 30 kpc is characterised by a power law, r^{-2.6 ± 0.3}, distinct from the Sersic profile that characterises its extended bulge.
Their data show that M31 is 3-5 times larger than any of its previously mapped spheroidal/disk components. Together, the Galactic and M31 halos span > 1/3 of the distance between them, suggesting that stars occupy a substantial volume fraction of our Local Group, and possibly most galaxy groups.


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A new study suggests that the Milky Way and its nearest galactic neighbour, Andromeda had similar beginnings and evolved in similar ways, at least over their first several billion years.

Astronomers had previously thought that Andromeda’s stellar halo was metal-rich while the Milky Way’s halo was metal-poor but a new study lead by Scott Chapman of the California Institute of Technology has found that are similarly impoverished when it comes to metals suggests they share similar evolutionary histories

The bulk of the complex structure in Andromeda's outer region is rotating with the disk is a blessing for studying the true underlying stellar halo of the galaxy. All the inhomogeneous fuzz seen in the image above can be lifted off and removed from consideration, yielding a sample of about 1000 stars which appear as a metal-poor ([Fe/H] = -1.4) 'pure' stellar halo sample. Using this new information, careful measurements of the random motions of the stars in the stellar halo have been made, probing it's mass and the form of the elusive dark matter which surrounds it.
The new measurements of the stellar velocities in M31's halo show that M31's halo is remarkably similar to our own Milky Way.


300 kpc x 300 kpc box, showing different simulations of a stellar halo in a Milky Way or M31 type galaxy.

The results are made possible by technological advances in astrophysics. In this case, the Keck/DEIMOS multi-object spectrograph affixed to the Keck-II telescope possesses the mirror size and light-gathering capacity to image stars that are very faint, and the spectrographic sensitivity to obtain highly accurate radial velocities.

The study could lead to new insights on the nature of dark matter. This is the first time astronomers have been able to obtain a panoramic view of the motions of stars in the halo of a galaxy. These stars allow them to weigh the dark matter, and determine how it decreases with distance. While no one yet knows what dark matter is made of, its existence is well established because of the mass that must exist in galaxies for their stars to orbit the galactic centres the way they do.
Current theories of galactic evolution, in fact, assume that dark-matter wells acted as a sort of "seed" for today's galaxies, with the dark matter pulling in smaller groups of stars as they passed nearby.

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Astronomers from the Sloan Digital Sky Survey (SDSS) have discovered the faintest galaxy yet. Even better news is that it’s right in our cosmic background. The new galaxy is named Andromeda IX and is a dim bulb in the cosmological scheme of things. It is nearly twice as faint as the previous lowest luminosity record holder is: Ursa Minor dwarf spheroidal galaxy. Andromeda IX is situated near M31, the Andromeda galaxy, and is so diffuse that it appears 100 times dimmer than the night sky, and 100,000 times fainter than the Milky Way...

For the first time, an eclipsing binary has revealed the precise distance to the Andromeda Galaxy (M31), say astronomers in Spain, America, and Scotland. The distance is in excellent agreement with other, less direct determinations.

Andromeda is the largest and most luminous galaxy in the Local Group. Knowing its distance helps scientists determine the distances of more remote galaxies. In the past, astronomers have determined M31's distance by observing its stars, notably its Cepheids and RR Lyraes, but deducing these stars' intrinsic brightness has depended on observations in other galaxies.

Now, Ignasi Ribas of the Spanish Research Council (CSIC) and the Institute for Space Studies of Catalonia (IEEC) and his colleagues have determined Andromeda's distance directly, by observing a 19th-magnitude eclipsing binary in one of the galaxy's spiral arms. The binary, known as M31VJ00443799+4129236, has two hot blue stars of spectral types O and B. As the stars orbit each other every 3.54969 days; they pass in front of and behind each other.

Astronomers have long used eclipsing binaries in the Milky Way to measure distances. The longer an eclipse lasts, the bigger the eclipsing star must be. The bigger and hotter the star, the more light it emits. Knowing each star's size and temperature therefore yields the pair's luminosity. Comparing the luminosity with the apparent brightness then reveals the system's distance.


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For 21 nights spread over 4 years, Ribas and his colleagues used the 2.5-meter Isaac Newton Telescope in the Canary Islands to observe the Andromeda binary's varying brightness at blue and yellow wavelengths. The astronomers also used the 8-meter Gemini North telescope in Hawaii to measure the stars' velocities, which vary as they orbit each other.

From these observations, the team determined the stars' properties. The O star has a temperature of 33,900 kelvins, a mass 23 times the Sun's, and a diameter 13.1 times the Sun's. The B star's temperature is 27,700 kelvins, its mass is 15 solar masses, and its diameter is 11.3 solar diameters. The stars' centres are 16.5 solar diameters apart, and the B star is spilling material onto the O star. Most importantly, the stars' absolute magnitudes are –5.29 and –4.66.

By comparing the absolute and apparent magnitudes, Ribas's team concluded the Andromeda Galaxy is 2.52±0.14 million light-years from Earth. This agrees perfectly with the Cepheid-based distance to Andromeda: 2.5 million light-years. The newly determined distance, however, does not depend on assuming a distance to the Large Magellanic Cloud. The agreement means astronomers can probably trust Cepheid distances to more distant galaxies, such as those in the Virgo and Fornax clusters.

Ribas and his colleagues plan to use other eclipsing binaries in Andromeda to further refine the galaxy's distance. They will publish their work in a future issue of Astrophysical Journal Letters.

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