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Posts: 131433
Date:
Andromeda XII
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A newly discovered dwarf galaxy in the Local Group has been found to have formed in a region of space far from our own and is falling into our system for the first time in its history, according to new data obtained at the W. M. Keck Observatory. An international team of astronomers report that the dwarf galaxy, Andromeda XII, marks the best piece of evidence for small galaxies which are just now arriving in our Local Group. The finding provides an important test for simulations of galaxy formation.
Dwarf galaxies and streams of stellar material mark the visible remnants of galactic merging events from which large galaxies are made. Cosmology models predict small galaxies form along a web of filamentary structures in the universe, and then gradually fall into dense groups and cluster environments. Small galaxies should still be falling into the Local Group, yet none have been found--until now.

"Other Local Group dwarf galaxies are thought to have extreme orbits, including Leo I, Andromeda XIV and Andromeda XI, but Andromeda XII really stands out as a contender for a new entrant into the Local Group. The others have likely already been seriously harassed by Andromeda and the Milky Way" - Dr. Scott C. Chapman,  lead author of the study,  University of Cambridge, Institute of Astronomy.

Nicknamed the Olympian Galaxy after the Twelve Olympians in the Greek Pantheon, Andromeda XII was first discovered in October 2006 during a wide-field survey taken with the Canada-France Hawaii Telescopes MegaCam instrument. It is the faintest dwarf galaxy ever discovered near to Andromeda (M31), and may have the lowest mass ever measured. Dwarf galaxies are the smallest stellar systems showing evidence for a substantial amount of dark matter.
Dr. Chapmans observations confirmed Andromeda XII is distinct from all other satellite galaxies in the Local Group. It is a fast-moving galaxy on a highly eccentric orbit, located at a great distance from the centre of M31, about 115 kiloparsecs (375,000 light years). Importantly, Andromeda XII lies significantly behind M31 as viewed from the Milky Way, almost certainly falling in for the first time. Because Andromeda XII has lived its life in a very different environment than the Local Group, it gives astronomers a pristine object for studying star formation histories, dark matter distribution, and other parameters that would be influenced by the Local Group gravity that has affected all the in other dwarf galaxies.

"Andromeda XII may be the first galaxy of the local group ever observed that has not yet been disrupted by the strong gravity of the Local Group" - Dr. Jorge Penarrubia of the University of Victoria, a co-author of the study.

The DEIMOS spectrograph at Keck II, one of two 10-meter telescopes the W. M. Keck Observatory operates on the summit of Mauna Kea, was key in making the discovery. It was used to observe 49 stars in the region of Andromeda XII, and confirmed that eight were members of the new dwarf galaxy. Follow-up observations were also conducted at the Green Bank Telescope in West Virginia to measure the amount of interstellar gas in the galaxy, and the Subaru telescope in Hawaii helped determine a more precise distance.

"Without the spectra we obtained with DEIMOS, it would have been impossible to make any useful claims about the orbit of Andromeda XII, its evolution, its speed or its dark matter content"  -  Dr. Chapman.

Andromeda XII is falling very quickly through the Local Group from behind Andromeda, the only one of Andromedas satellites which exceeds the apparent escape velocity for Andromeda. It is possible that Andromeda XII may be just a short-term visitor. It is such a low-mass galaxy that it may not slow down much as it passes through the Local Group.

"It is a pleasure to see the speed of this new, fascinating member of the Local Group clocked using Keck II and DEIMOS. The powerful combination of Keck and DEIMOS has added many contributions to our understanding of Local Group galaxies"  - Taft Armandroff, W. M. Keck Observatory Director.

The age of the Universe is not old enough for Andromeda XII to have started in the dense Local Group and be on its second trip through our system. Andromeda XII probably formed in a dense filament structure, toward the general direction of the M81 group. However, the distance is about three times too large for it to have actually come from the M81 group. A likely scenario is Andromeda XII formed in a filamentary region of space that connects the Local Group to the M81 group.

"The high speed of Andromeda XII really surprised me; I wasnt expecting to see any of our newly discovered dwarfs moving so fast. We will likely have to revise our mass estimates of Andromeda upward as a result" - Rodrigo Ibata.

A paper reporting the discovery, Strangers in the Night: The discovery of a Dwarf Spheroidal Galaxy on its First Local Group Infall, will appear in an upcoming issue of the Astrophysical Journal. Funding was provided by a fellowship from the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada. Additional support was provided by Adrian Jenkins who provided use of important computer simulations.
The study was co-authored by Jorge Penarrubia, Alan McConnachie, Aaron Ludlow of the University of Victoria; Rodrigo Ibata, Observatoire de Strasbourg; Nicolas F. Martin, Max-Planck Institut fur Astronomie; Andrew Blain and Bruno LeTarte, California Institute of Technology; Michael Irwin, University of Cambridge Institute of Astronomy; Geraint Lewis, University of Sydney Institute of Astronomy; Fred Lo and Karen ONeil, NRAO Green Bank Telescope.


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Posts: 131433
Date:
Ursa Minor dwarf galaxy
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 The discovery of thorium in the Ursa Minor Galaxy offers the first chance to measure another galaxy's age via radioactive dating, say astronomers in Japan. Furthermore, the abundance of thorium and other heavy elements hints that this galaxy evolved differently than the Milky Way.
Thorium is a heavy, radioactive element with an atomic number of 90. Thus, each thorium atom has ninety protons in its nucleus. Until now, no one had seen such a heavy element beyond our Galaxy.
Wako Aoki, Satoshi Honda, and Nobuo Arimoto of the National Astronomical Observatory in Tokyo and Kozo Sadakane of Osaka Kyoiku University observed COS 82, a seventeenth-magnitude orange giant star in the Ursa Minor dwarf spheroidal galaxy. Only 220,000 light-years from Earth, the Ursa Minor dwarf is a satellite of the Milky Way, orbiting our Galaxy just as the Moon orbits the Earth. Unlike the Milky Way, Ursa Minor has no gas from which to create new stars, and its stars are old.

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Posts: 131433
Date:
Leo A dwarf galaxy
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Title: Stellar Velocity Dispersion of the Leo A Dwarf Galaxy
Authors: Warren R. Brown, Margaret J. Geller, Scott J. Kenyon, Michael J. Kurtz (Smithsonian Astrophysical Observatory)

We measure the first stellar velocity dispersion of the Leo A dwarf galaxy, \sigma = 9.3 ± 1.3 km/s. We derive the velocity dispersion from the radial velocities of ten young B supergiants and two HII regions in the central region of Leo A. We estimate a projected mass of 8 ± 2.7 x10^7 solar masses within a radius of 2 arcmin, and a mass to light ratio of at least 20 ± 6 M_sun/L_sun. These results imply Leo A is at least ~80% dark matter by mass.

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Posts: 131433
Date:
Wolf-Lundmark-Melotte Galaxy
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Title: The Araucaria Project. The Distance to the Local Group Galaxy WLM from Cepheid Variables discovered in a Wide-Field Imaging Survey
Authors: G. Pietrzynski, W. Gieren, A. Udalski, I. Soszynski, F. Bresolin, R.P. Kudritzki, A. Garcia, D. Minniti, R. Mennickent, O. Szewczyk, M. Szymanski, M. Kubiak, L. Wyrzykowski

We have conducted an extensive wide-field imaging survey for Cepheid variables in the Local Group irregular galaxy WLM. From data obtained on 101 nights, we have discovered 60 Cepheids which include 14 of the 15 Cepheid variables previously detected by Sandage and Carlson. Down to a period of 3 days, our Cepheid survey in WLM should be practically complete. Importantly, we have found for the first time a long-period Cepheid (P=54.2 days) in this galaxy, alleviating the puzzle that WLM with its many blue, massive stars does not contain Cepheids with periods longer than about 10 days. Our data define tight period-luminosity relations in V, I and the reddening-free Wesenheit magnitude  W_ I which are all extremely well fit by the corresponding slopes of the LMC Cepheid PL relation, suggesting no change of the PL relation slope down to a Cepheid metal abundance of about -1.0 dex, in agreement with other recent studies. We derive a true distance modulus to WLM of 25.144 ±0.03 (r) ±0.07 (s) mag from our data, in good agreement with the earlier 24.92 ±0.21 mag determination of Lee, Freedman and Madore (1993a) from Cepheid variables. The quoted value of the systematic uncertainty does not include the contribution from the LMC distance which we have assumed to be 18.50 mag, as in the previous papers in our project.

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Posts: 131433
Date:
Nearby Galaxy Clusters
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Title: A Study of Catalogued Nearby Galaxy Clusters in the SDSS-DR4: I. Cluster Global Properties
Authors: J. A. L. Aguerri, R. Sanchez-Janssen, C. Munoz-Tunon

We have selected a sample of 88 nearby (z<0.1) galaxy clusters from the SDSS-DR4 with redshift information for the cluster members. We have derived global properties for each cluster, such as their mean recessional velocity, velocity dispersion, and virial radii. Cluster galaxies have been grouped in two families according to their u-r colours. The total sample consists of 10865 galaxies. As expected, the highest fraction of galaxies (62%) turned to be early-type (red) ones, being located at smaller distances from the cluster centre and showing lower velocity dispersions than late-type (blue) ones. The brightest cluster galaxies are located in the innermost regions and show the smallest velocity dispersions. Early-type galaxies also show constant velocity dispersion profiles inside the virial radius and a mild decline in the outermost regions. In contrast, late-type galaxies show always decreasing velocity dispersions profiles. No correlation has been found between the fraction of blue galaxies and cluster global properties, such as cluster velocity dispersion and galaxy concentration. In contrast, we found correlation between the X-ray luminosity and the fraction of blue galaxies. These results indicate that early- and late-type galaxies may have had different evolution. Thus, blue galaxies are located in more anisotropic and radial orbits than early-type ones. Their star formation seems to be independent of the cluster global properties in low mass clusters, but not for the most massive ones. These observational results suggest that the global environment could be important for driving the evolution of galaxies in the most massive cluster (\sigma > 800 km s^{-1}). However, the local environment could play a key role in galaxy evolution for low mass clusters.

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Posts: 131433
Date:
RE: The Local Volume
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Title: The Chemistry of the Local Group
Authors: Brad K. Gibson

Simulations of the chemical enrichment histories of ten Local Group (LG) dwarf galaxies are presented, employing empirically-derived star formation histories (SFHs), a rich network of isotopic and elemental nucleosynthetic yields, and a range of prescriptions for supernova (SN)-driven outflows. Our main conclusions are that (i) neutron-capture element patterns (particularly that of Ba/Y) suggest a strong contribution from low- and intermediate-mass stars, (ii) neutron star mergers may play a relatively larger role in the nucleosynthesis of dwarfs, (iii) SN feedback alone can explain the observed gas fraction in dwarf irregulars (dIrrs), but dwarf spheroidals (dSphs) require almost all their gas to be removed via ram pressure and/or tidal stripping, (iv) the predicted heavy Mg isotope enhancements in the interstellar medium of dwarfs may provide an alternate solution to claims of a varying fine structure (v) the gas lost from dwarfs have O,Si/C abundances in broad agreement with intergalactic medium abundances at redshifts 2<z<4, and (vi) the chemical properties of dSphs are well-matched by preventing galactic winds from re-accreting, whilst those of dIrrs are better-matched by incorporating metallicity-dependent cooling and re-accretion of hot winds. Finally, doubts are cast upon a claimed association between LG dSph UMaII and High-Velocity Cloud Complex A.

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Posts: 131433
Date:
Local Group Galaxies
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Title: A Survey of Local Group Galaxies Currently Forming Stars: \\II. UBVRI Photometry of Stars in Seven Dwarfs
Authors: Philip Massey, K. A. G. Olsen, Paul W. Hodge, George H. Jacoby, Reagin T. McNeill, R. C Smith, Shay B. Strong

We have obtained UBVRI images with the Kitt Peak and Cerro Tololo 4-m telescopes and Mosaic cameras of seven dwarfs in (or near) the Local Group, all of which have known evidence of recent star formation: IC10, NGC 6822, WLM, Sextans B, Sextans A, Pegasus,and Phoenix. We construct colour-magnitude diagrams (CMDs) of these systems, as well as neighbouring regions that can be used to evaluate the degree of foreground contamination by stars in the Milky Way. Inter-comparison of these CMDs with those of M31, M33, the LMC, and the SMC permits us to determine improved reddening values for a typical OB star found within these galaxies. All of the CMDs reveal a strong or modest number of blue supergiants. All but Pegasus and Phoenix also show the clear presence of red supergiants in the CMD, although IC10 appears to be deficient in these objects given its large WR population. The bright stars of intermediate colour in the CMD are badly contaminated by foreground stars (30-100%), and considerable spectroscopy is needed before statistics on the yellow supergiants in these systems will be known. This study is intended to serve both as the impetus and "finding charts" for further space-based imaging, and for many spectroscopic programs at large aperture.

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Posts: 131433
Date:
Phoenix Dwarf Galaxy
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Title: The Aptly Named Phoenix Dwarf Galaxy
Authors: Lisa M. Young (New Mexico Tech), Evan D. Skillman, Daniel R. Weisz (University of Minnesota), Andrew E. Dolphin (University of Arizona; Raytheon)

The Local Group galaxy Phoenix has the properties of a dwarf spheroidal galaxy, but an adjacent HI cloud has been found to be at the same radial velocity as the stars. The proximity suggests that this cloud is associated with the most recent (100 Myr) star formation in Phoenix. We have obtained relatively high sensitivity and high resolution HI imaging with the VLA with the goal of distinguishing between different processes for displacing the gas from the galaxy. Due to the outer curvature of the HI cloud, it appears that expulsion from the galaxy by winds from supernovae is more likely than ram-pressure stripping. The isolation of the galaxy makes tidal stripping highly unlikely. Using a star formation history constructed from HST imaging, we construct a simple kinematic model which implies that the HI cloud is still gravitationally bound to the galaxy. Gas which is expelled from the centres of dwarf galaxies but which remains gravitationally bound may explain the episodic star formation observed in several dwarfs. In the specific case of Phoenix, there may be future star formation in this currently dSph-like galaxy.

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Posts: 131433
Date:
Cetus dwarf spheroidal galaxy
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Title: Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal
Authors: G. F. Lewis, R. A. Ibata, S. C. Chapman, A. McConnachie, M. J. Irwin, E. Tolstoy, N. R. Tanvir

This paper presents a study of the Cetus dwarf, an isolated dwarf galaxy within the Local Group. A matched-filter analysis of the INT/WFC imaging of this system reveals no evidence for significant tidal debris that could have been torn from the galaxy, bolstering the hypothesis that Cetus has never significantly interacted with either the Milky Way or M31. Additionally, Keck/Deimos spectroscopic observations identify this galaxy as a distinct kinematic population possessing a systematic velocity of -87 ±2 km s^-1 and with a velocity dispersion of 17 ±2 km s^-1; while tentative, these data also suggest that Cetus possesses a moderate rotational velocity of ~8 km s^-1. The population is confirmed to be relatively metal-poor, consistent with (Fe/H) ~ -1.9}, and, assuming virial equilibrium, implies that the Cetus dwarf galaxy possesses a M/L ~ 70. It appears, therefore, that Cetus may represent a primordial dwarf galaxy, retaining the kinematic and structural properties lost by other members of the dwarf population of the Local Group in their interactions with the large galaxies. An analysis of Cetus's orbit through the Local Group indicates that it is at apocentre; taken in conjunction with the general dwarf population, this shows the mass of the Local Group to be > ~ 2 x 10^12 solar masses.

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Posts: 131433
Date:
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A dwarf galaxy in the constellation Cetus may be a pristine building block of large galaxies like our own, say astronomers. Furthermore, this galaxy is close enough that it belongs to the Local Group, the collection of nearby galaxies that includes our own.
Giant galaxies, like the Milky Way and Andromeda, formed when smaller galaxies smashed together. In the Local Group, these dwarf galaxies come in two main varieties: dwarf irregulars and dwarf spheroidals. Dwarf irregular galaxies, such as IC 10 in Cassiopeia and Barnard's Galaxy in Sagittarius, harbour young stars as well as the gas that gives birth to such stars. In the Local Group, most dwarf irregular galaxies lie far from both the Milky Way and Andromeda.
In contrast, dwarf spheroidal galaxies, such as Ursa Minor and Draco, lack the gas that gives birth to new stars. Furthermore, most dwarf spheroidals in the Local Group orbit Andromeda or the Milky Way, suggesting that the giant galaxies stripped the lesser ones of gas, ending their satellites' star-forming careers.
But there are exceptions to this rule. One of them is in Cetus: a galaxy that is a dwarf spheroidal but nevertheless lies far from both the Milky Way and Andromeda. Discovered in 1998, the Cetus dwarf spheroidal galaxy is 2.5 million light-years from the Milky Way and 2.2 million light-years from the Andromeda Galaxy.

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