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TOPIC: Milky Way
Blobrana


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RE: Milky Way
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Title: The distribution of the ISM in the Milky Way A three-dimensional large-scale model
Authors: A. Misiriotis, E. M. Xilouris, J. Papamastorakis, P. Boumis, C. D. Goudis

Researchers use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 µm) maps and the COBE/FIRAS spectra (for the wavelength range 100 - 1000 µm) to constrain a model for the spatial distribution of the dust, the stars, and the gas in the Milky Way. By assuming exponential axisymmetric distributions for the dust and the stars and by performing the corresponding radiative transfer calculations they closely (given the simple geometry of the model) reproduce the FIR and NIR maps of the Milky Way. Similar distributions for the atomic and molecular hydrogen in the disk are used (with an inner cut-off radius for the atomic hydrogen) to fit the gas data.
The star formation rate as a function of the Galactic radius is derived from the FIR emission and is well in agreement with existing estimates from various star formation tracers. The gas surface density is plotted against the star formation rate density and an ''intrinsic'' Galactic Schmidt law is derived with excellent agreement with the ''external'' Schmidt law found for spiral galaxies.
The Milky Way is found to consume ~ 1% and ~ 10% of its gas in the outer and inner regions respectively (for a period of 0.1 billion years) to make stars. The dust-induced B-V colour excess observed in various directions and distances (up to ~ 6.5 kpc) with well-studied Cepheid stars is compared with the model predictions showing a good agreement. The simple assumption of exponential distributions of stars and dust in the Galaxy is found to be quite instructive and adequate in modelling all the available data sets from 0.45 µm (B-band) to 1000 µm.

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Blobrana


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Posts: 131433
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The Centre
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The centre of the Milky Way is a crowded neighbourhood and not always a calm one, according to the latest image from NASA's Chandra X-ray Observatory. In addition to the supermassive black hole at the centre, the area is filled with all sorts of different inhabitants that affect and influence one another.


Position(2000): RA 17h 45m 50.30s Dec -28° 49' 20.00"
Credit: NASA/CXC/UMass Amherst/Q.D.Wang et al.

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-- Edited by Blobrana at 23:41, 2006-07-19

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Blobrana


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Posts: 131433
Date:
Central 200 Parsecs
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Title: The H.E.S.S. View of the Central 200 Parsecs
Authors: Jim Hinton (for the H.E.S.S. Collaboration)

The inner few hundred parsecs of our galaxy provide a laboratory for the study of the production and propagation of energetic particles. Very-high-energy gamma-rays provide an effective probe of these processes and, especially when combined with data from other wave-bands, gamma-rays observations are a powerful diagnostic tool. Within this central region, data from the H.E.S.S. instrument have revealed three discrete sources of very-high-energy gamma-rays and diffuse emission correlated with the distribution of molecular material. Here Jim Hinton provides an overview of these recent results from H.E.S.S.

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Blobrana


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Posts: 131433
Date:
RE: Milky Way
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Title: The Spiral Structure of the Outer Milky Way in Hydrogen
Authors: E.S. Levine, Leo Blitz, Carl Heiles

Researchers have produced a detailed map of the perturbed surface density of neutral hydrogen in the outer Milky Way disk demonstrating that the Galaxy is a non-axisymmetric multi-armed spiral.
Spiral structure in the southern half of the Galaxy can be traced out to at least 25 kpc, implying a minimum radius for the gas disk. Overdensities in the surface density are coincident with regions of reduced gas thickness.
The ratio of the surface density to the local median surface density is relatively constant along an arm. Logarithmic spirals can be fit to the arms with pitch angles of 20-25 degrees.

milky way hydrogen map

A: Contour plot of the surface density Sigma(R, 0). The location of the Sun is marked at (0, 8.5 kpc) by the solar symbol. The regions excluded due to unreliable distances are the large blank wedges near the Sun-Galactic centre line. B: Contour plot of Pi(R, 0), as defined in the text. Coloured regions are overdense compared to the local median, whereas greyscale regions are underdense. The solid contour marks the line Pi = 1.1. The values of Pi for the different contour levels are given by the colourbar.

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Blobrana


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Posts: 131433
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Orphan Stream
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orphan Stream1

Left: A false colour RGB composite of density of stars with 20 < r < 22. Blue corresponds to 0 < g - r = 0.2, green 0.2 < g - r = 0.4 and red 0.4 < g - r = 0.6. Right: The Sagittarius and Monoceros structures are marked, together with the on-stream and off-stream fields along the Orphan Stream

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Blobrana


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Posts: 131433
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Title: An Orphan in the "Field of Streams"
Authors: V. Belokurov , N. W. Evans , M. J. Irwin , D. Lynden-Bell , B. Yanny , S. Vidrih , G. Gilmore , G. Seabroke, D. B. Zucker , M. I. Wilkinson, P. C. Hewett , D. M. Bramich , M. Fellhauer, H. J. Newberg , R. F. G. Wyse, T. C. Beers, E. F. Bell , J. C. Barentine, J. Brinkmann, N. Cole, K. Pan, D. G. York

Researchers used the Sloan Digital Sky Survey Data Release 5 u,g,r,i,z photometry to study a tidal stream that extends over 50 degrees in the "Field of Streams'' map of the North Galactic Cap. From the analysis of the path of the stream and the colours and magnitudes of its stars, the stream is about 15 kpc away at its nearest detection (the celestial equator).
There is evidence for a distance gradient towards larger heliocentric distances at higher declination. The stream lies on the same great circle as Complex A, a roughly linear association of HI high velocity clouds, stretching over 30 degrees on the sky. Lying close to the same great circle are a number of anomalous, young and metal-poor globular clusters, including Palomar 1 and Ruprecht 106. The agglomeration of features in this great circle plane suggests that all may be the remnants of the disruption of a much larger dwarf galaxy. As the progenitor remains unknown, this is truly the "[/url=http://www.activeboard.com/forum.spark?forumID=58381&subForumID=150692&action=viewTopic&commentID=3318385]Orphan Stream[/url]''.

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Blobrana


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Posts: 131433
Date:
Star Cluster Candidates
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Title: Discovery of new Milky Way star cluster candidates in the 2MASS point source catalog. V. Follow-up observations of the young stellar cluster candidates RCW87, (BDSB2003)164 and (DBSB2003)172
Authors: J.Borissova, V.D.Ivanov, D.Minniti, D.Geisler

Massive clusters are more often found in actively star forming galaxies than in quiescent ones, similar to the Milky Way. We have carried out an extensive survey of obscured Milky Way clusters to determine whether our Galaxy is still forming such objects. Near-infrared spectral classification of some probable cluster members was used to derive the distances to the cluster candidates. Isochrone analysis of deep near-infrared colour-magnitude diagrams allowed us to obtain age and mass estimates. We report the physical parameters of three cluster candidates:
RCW87 is approx. 25 Million years old, located at a distance of D=7.6 Kpc in the general direction of the Galactic centre. Adding the mass of the suspected cluster members we obtain an estimated total cluster mass of approx. 10300 solar masses. The mid-infrared photometry indicates that some next-generation star formation is on-going in the associated HII region, probably triggered by supernovae or stellar wind from the older stars in RCW87.
The brightest member of (BDSB2003)164 is an O5V type star, based on our spectroscopy. This limits the cluster age to less than a few million years. The estimated total mass is approx. 1760 solar masses and the distance is D=3.2 Kpc.
(DBSB2003)172 lacks central concentration and most likely this is not a cluster but an extended star forming region.
The high mass of RCW87 -- if confirmed -- puts this object in line with Arches and Quintuplet, among the most young massive clusters in the Galaxy. Further study is necessary to confirm this important result.

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Blobrana


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Posts: 131433
Date:
Canis Major Warp
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Title: Galactic Warp in the overdensity of the Canis Major Region
Authors: M. Lopez-Corredoira


Bellazzini et al. (2006b) claim that Lopez-Corredoira et al.'s (2002) warp model is totally unable to reproduce the Canis Major structure in the red clump stars.
However, slight variations in the azimuth of the Lopez-Corredoira et al. (2002) warp model, justified by the uncertainties in the parameter as well as the local irregularities with respect to the average model, derive a result much closer to the observations of the overdensity south/north.
The bump of red clump stars with mk=13-13.5 around l=241 deg., b=-8.5 deg. and the depth of the Canis Major structure are also explainable in terms of the warp with an appropriate extrapolation of constant height between galactocentric radii of 13 and 16 kpc, as observed roughly in the southern warp, instead of a monotonically increasing height like the northern warp; and the observed velocity distribution of stars cannot exclude the warp possibility.
A warp model is therefore still a possible explanation of the Canis Major overdensity, and the hypothesis of the existence of a dwarf galaxy is unnecessary, although still a possibility too.

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Blobrana


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Posts: 131433
Date:
W3(OH)
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Astronomers based at Jodrell Bank Observatory have discovered a giant bridge of methyl alcohol, spanning approximately 288 billion miles, wrapped around a stellar nursery. The gas cloud could help our understanding of how the most massive stars in our galaxy are formed.

The new observations were taken with the UK's MERLIN radio telescopes, which have recently been upgraded. The team studied an area called W3(OH), a region in our galaxy where stars are being formed by the gravitational collapse of a cloud of gas and dust. The observations have revealed giant filaments of gas that are emitting as `masers' (molecules in the gas are amplifying and emitting beams of microwave radiation in much the same way as a laser emits beams of light).

The filaments of masing gas form giant bridges between maser `spots' in W3(OH) that had been observed previously. The largest of these maser filaments is 463 billion km long. Observations show that the entire gas cloud appears to be rotating as a disc around a central star, in a similar manner to the accretion discs in which planets form around young stars. The maser filaments occur at shock boundaries where large regions of gas are colliding.

"Our discovery is very interesting because it challenges some long-accepted views held in astronomical maser research. Until we found these filaments, we thought of masers as point-like objects or very small bright hotspots surrounded by halos of fainter emission" - Dr Lisa Harvey-Smith, Principal Investigator for the study and is presenting results at the Royal Astronomical Society’s National Astronomy Meeting on 4th April.


Continuum emission in W3(OH) from Guilloteau et al. (1983) (left) and Wilner et al. (1999) (right) superimposed with the new studies methanol maser spots (white dots), OH maser filament (red contours) and extended methanol filaments (green contours).

Since the upgrade of the UK's MERLIN telescope network, astronomers have been able to image methanol masers with a much higher sensitivity and, for the first time, get a complete picture of all the radiation surrounding maser sources. In the new study, the Jodrell Bank team looked at the motion of the W3(OH) star forming region in 3-dimensions and also measured physical properties of the gas such as temperature, pressure and the strength and direction of the magnetic fields. This information is vital when testing theories about how stars are born from the primordial gas in stellar nurseries.

"There are still many unanswered questions about the birth of massive stars because the formation centres are shrouded by dust. The only radiation that can escape is at radio wavelengths and the upgraded MERLIN network is now giving us the first opportunity to look deep into these star forming regions and see what’s really going on" - Dr Harvey-Smith

The many different types of interactions between molecules in star forming regions lead to emissions in many different wavelengths. Future observations of masers at other frequencies are planned to complete the complex jigsaw puzzle that has now been revealed.

"Although it is exciting to discover a cloud of alcohol almost 300 billion miles across, unfortunately methanol, unlike its chemical cousin ethanol, is not suitable for human consumption!" - Dr Harvey-Smith

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Blobrana


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Posts: 131433
Date:
RE: Milky Way
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“Data from the Galactic Ring Survey have shown that these clouds are the counterparts to active, bright star-forming clouds, but because they have not yet been heated by the embedded stars, they are much colder and quieter. Follow-up studies of these clouds will provide additional important clues about the origin of stars since we’ll be able to examine them at an earlier point in their life” - James Jackson, astronomy professor at Boston University and lead investigator of the study.

Source

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