* Astronomy

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RE: Andromeda galaxy (M31)

Blobrana · L

Astronomers surveying the nearby Andromeda galaxy have discovered an association of stars in its outskirts, which they believe to be part of a separate galaxy that merged with Andromeda about 700 million years ago. The discovery corresponds to computer models created by University of Massachusetts Amherst astronomer Mark Fardal, which simulate a dwarf galaxy merging with Andromeda.
The findings suggest that Andromeda’s outer swathes of stars are from the same parent galaxy, and may help astronomers determine the Andromeda’s total mass. Both findings were presented Jan. 7 at the American Astronomical Society meeting in Seattle.

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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 neighbour. 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" - 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 centre. 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.
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. 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" - coauthor R. Michael Rich of UCLA.

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 centre, 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 centre of the galaxy, while the halo extends five times farther than that.

"We now believe that previous groups have been mistakenly identifying the outer parts of the Andromeda bulge as its halo" - Puragra (Raja) Guhathakurta.

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.

"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" - Puragra (Raja) Guhathakurta.

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. There are assuredly other kinds of stars in Andromeda's halo, but they are just too faint for us to get spectra of them" - Karoline Gilbert.

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.

"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" - Puragra (Raja) Guhathakurta.

Source UC Santa Cruz

Blobrana · L

The "Super-Halo" of M31 and M33

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RE: Andromeda galaxy (M31)

Blobrana · L

A two-decade-long riddle about the bizarre shape of the Milky Way's nearest spiral-shaped galaxy, Andromeda, has been solved, suggests a new study.
Instead of having the flat plane and outflung arms that are the hallmarks of a mature spiral galaxy, Andromeda has a warped plane and several rather chaotic, overlapping outer rings.
The reason, according to an international team of astronomers, is that Andromeda suffered a head-on collision with a smaller galaxy some 210 million years ago.

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Stellar 'generation gap' revealed in nearby galaxy

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RE: Andromeda galaxy (M31)

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Title: Eclipsing binaries suitable for distance determination in the Andromeda galaxy
Authors: F. Vilardell (1), I. Ribas (2 and 3), C. Jordi (1 and 3) ((1) Universitat de Barcelona, (2) Institut de Ciencies de l'Espai-CSIC, (3) Institut d'Estudis Espacials de Catalunya)

The Local Group galaxies constitute a fundamental step in the definition of cosmic distance scale. Therefore, obtaining accurate distance determinations to the galaxies in the Local Group, and notably to the Andromeda Galaxy (M31), is essential to determining the age and evolution of the Universe. With this ultimate goal in mind, we started a project to use eclipsing binaries as distance indicators to M31. Eclipsing binaries have been proved to yield direct and precise distances that are essentially assumption free. To do so, high-quality photometric and spectroscopic data are needed. As a first step in the project, broad band photometry (in Johnson B and V) has been obtained in a region (34'x34') at the North-Eastern quadrant of the galaxy over 5 years. The data, containing more than 250 observations per filter, have been reduced by means of the so-called difference image analysis technique and the DAOPHOT program. A catalogue with 236238 objects with photometry in both B and V passbands has been obtained. The catalogue is the deepest (V<25.5 mag) obtained so far in the studied region and contains 3964 identified variable stars, with 437 eclipsing binaries and 416 Cepheids. The most suitable eclipsing binary candidates for distance determination have been selected according to their brightness and from the modelling of the obtained light curves. The resulting sample includes 24 targets with photometric errors around 0.01 mag. Detailed analysis (including spectroscopy) of some 5-10 of these eclipsing systems should result in a distance determination to M31 with a relative uncertainty of 2-3% and essentially free from systematic errors, thus representing the most accurate and reliable determination to date.

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A new radio frequency map of the Andromeda galaxy has been made by a German-French research team of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn and the Institut de Radioastronomie Millimétrique (IRAM) in Grenoble. The map shows the first detailed distribution of cold gas in a neighbouring galaxy, revealing the sites where new stars are born. The motions of this gas were also obtained. With more than 800 hours of telescope time this study is one of the most extensive observational projects in millimetre radio astronomy.

How are stars formed?
This is one of the most important questions in astronomy. We know that star formation takes place in cold gas clouds with temperatures below -220 C (50 K). Only in these regions of dense gas can gravitation lead to a collapse and hence to star formation. Cold gas clouds in galaxies are composed preferentially of molecular hydrogen, H2 (two hydrogen atoms bound as one molecule). This molecule emits a weak spectral line in the infrared bandwidth of the spectrum that cannot be observed by Earth-based telescopes because the atmosphere absorbs this radiation. Therefore, astronomers study another molecule which is always found in the neighbourhood of H2, namely carbon monoxide, CO. The intense spectral line of CO at the wavelength of 2.6 mm can be observed with radio telescopes that are placed on atmospherically favourable sites: high and dry mountains, in the desert or at the South Pole. In cosmic space carbon monoxide is an indicator of conditions favourable for the formation of new stars and planets.

In our galaxy, the Milky Way, studies of the distribution of carbon monoxide have been carried out for a long time. Astronomers find enough cold gas for star formation during millions of years to come. But many questions are unanswered; for instance how this raw material of molecular gas comes to exist in the first place. Is it supplied by the early development stage of the Galaxy, or can it be formed from warmer atomic gas? Can a molecular cloud collapse spontaneously or does it need an action from outside to make it unstable and collapse? Since the Sun is located in the disk of the Milky Way it is very difficult to obtain an overview of the processes taking place in our Galaxy. Looking from "outside" would help and so too does a look at our cosmic neighbours.

The Andromeda galaxy, also known under its catalogue number M31, is a system of billions of stars, similar to our Milky Way. The distance of M31 is 'only' 2.5 million light years, making it the nearest spiral galaxy The galaxy extends over some 5 degrees in the sky and can be seen with the naked eye as a tiny diffuse cloud. Studies of this cosmic neighbour can help to understand processes in our own Galaxy. Unfortunately, we are seeing the disk of gas and stars in M31 nearly edge-on.
In 1995 a team of radio astronomers at the Institut de Radioastronomie Millimétrique (IRAM) in Grenoble (Michel Guélin, Hans Ungerechts, Robert Lucas) and at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn (Christoph Nieten, Nikolaus Neininger, Elly Berkhuijsen, Rainer Beck, Richard Wielebinski) started the ambitious project of mapping the entire Andromeda galaxie in the carbon monoxide spectral line. The instrument used for this project was the 30-meter radio telescope of IRAM which is situated on Pico Veleta (2970 metres) near Granada in Spain. With an angular resolution of 23 arcseconds (at the observing frequency of 115 GHz = wavelength of 2.6 mm) 1.5 million individual positions had to be measured. To speed up the observing process a new method of measurement was used. Rather than observing at each position, the radio telescope was driven in strips across the galaxy with continuous recording of the data. This observing method, called 'on the fly', was especially developed for the M31 project; it is now standard practice, not only at the Pico Veleta radio telescope but also at other telescopes observing at millimetre wavelengths.

For each observed position in M31 not just one value of CO intensity was recorded, but 256 values simultaneously across the spectrum with a bandwidth of 0.2% of the central wavelength of 2.6 mm. Thus the complete observational data set consists of some 400 million numbers! The exact position of the CO line in the spectrum gives us information about the velocity of the cold gas. If the gas is moving towards us, then the line is shifted to shorter wavelengths. When the source moves away from us, then we see a shift to longer wavelengths. This is the same effect (the Doppler effect) that we can hear when an ambulance’s siren moves towards us or away from us. In astronomy the Doppler effect allows the motions of gas clouds to be studied; even clouds with different velocities seen in the same line of sight can be distinguished. If the spectral line is broad, then the cloud may be expanding or else it consists of several clouds at different velocities.

The observations were finished in 2001. With more than 800 hours of telescope time this is one of the largest observing projects carried out with the telescopes of IRAM or MPIfR. After extensive processing and analysis of the huge quantities of data, the complete distribution of the cold gas in M31 has just been published.

The cold gas in M31 is concentrated in very filigree structures in the spiral arms. The CO line appears well suited to trace the spiral arm structure. The distinctive spiral arms are seen at distances between 25,000 and 40,000 light years from the centre of Andromeda, where most of the star formation occurs. In the central regions, where the bulk of older stars are located, the CO arms are much weaker. As a result of the high inclination of M31 relative to the line of sight (about 78 degrees) the spiral arms seem to form a large, elliptical ring with a major axis of 2 degrees. In fact, for a long time Andromeda was taken, mistakenly, to be a 'ring'-galaxy.

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The map of the gas velocities resembles a snap shot of a giant fire wheel. On the one side (in the south, left) the CO gas is moving with some 500 km/second towards us (blue), but on the other side (north, right) with 'only' 100 km/second (red). Since the Andromeda galaxy is moving towards us with a velocity of about 300 km/second, it will closely pass the Milky Way in about 2 billion years. In addition, M31 is rotating with about 200 km/second around its central axis. Since the inner CO clouds are moving on a shorter path than the outer clouds, they can overtake each other. This leads to a spiral structure.
The density of the cold molecular gas in the spiral arms is much larger than in the regions between the arms, whereas the atomic gas is more uniformly distributed. This suggests that molecular gas is formed from the atomic gas in the spiral arms, especially in the narrow ring of star formation. The origin of this ring is still unclear. It could be that the gas in this ring is just material not yet used for stars. Or perhaps the very regular magnetic field in M31 triggers the star formation in the spiral arms. Observations with the Effelsberg telescope showed that the magnetic field closely follows the spiral arms seen in CO.

The ring of star formation ('birth zone') in our own Milky Way, extending from 10,000 to 20,000 light years from the centre, is smaller than in M31. In spite of this, it contains nearly 10 times as much molecular gas. As all galaxies are about the same age, the Milky Way has been more economical with its raw material. On the other hand, the many old stars near the centre of M31 indicate that in the past the star formation rate was much higher than at present: here most of the gas has already been processed. The new CO map shows us that Andromeda was very effective in forming stars in the past. In some billions of years from now our Milky Way may look similar to Andromeda now.

Source: Max Planck Institute for Radio Astronomy

-- Edited by Blobrana at 04:25, 2006-06-30