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Sagittarius Dwarf Galaxy
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Title: The Chemistry of the Trailing arm of the Sagittarius Dwarf Galaxy
Authors: Stefan C. Keller, David Yong, Gary S. Da Costa

We present abundances of C, O, Ti, and Fe for eleven M-giant stars in the trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived by comparing synthetic spectra with high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. The targeted stars are drawn from two regions of the Sgr trailing arm separated by 66 degrees (5 stars) and 132 degrees (6 stars) from the main body of Sgr. The trailing arm provides a more direct diagnostic of the chemical evolution of Sgr compared to the extensively phase-mixed leading arm.
Within our restricted sample of ~2-3 Gyr old stars, we find that the stream material exhibits a significant metallicity gradient of -(2.4 0.3)x10^{-3} dex / degree (-(9.4 1.1)x10^{-4} dex / kpc) away from the main body of Sgr. The tidal disruption of Sgr is a relatively recently event. We therefore interpret the presence of a metallicity gradient in the debris as indicative of a similar gradient in the progenitor. The fact that such a metallicity gradient survived for almost a Hubble time indicates that the efficiency of radial mixing was very low in the Sgr progenitor.
No significant gradient is seen to exist in the [alpha/Fe] abundance ratio along the trailing arm. Our results may be accounted for by a radial decrease in star formation efficiency and/or radial increase in the efficiency of galactic wind-driven metal loss in the chemical evolution of the Sgr progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream are distinct from those of the Galactic halo. We conclude that the fraction of the intermediate to metal-rich halo population contributed by the recent dissolution (<3 Gyr) of Sgr-like dwarf galaxies can not be substantial.

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Posts: 131433
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RE: Milky Way Companions
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Cosmic rays help make small galaxies invisible

Cosmic rays can help sterilise small galaxies, preventing them from forming new stars, new simulations suggest. The effect could help explain why we see fewer dwarf galaxies than expected in orbit around the Milky Way.
Standard theories of dark matter and galactic evolution predict that small galaxies should merge over time to form large ones, and that hundreds or even thousands of these undersized, unmerged galaxies should be spiralling around the Milky Way today. But observations have turned up just 35, a gap known as the 'missing satellite problem'.

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Title: The Sagittarius Dwarf Galaxy: a Model for Evolution in a Triaxial Milky Way Halo
Authors: David R. Law, Steven R. Majewski

We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Phi = 0.72 and intermediate/major axis ratio (b/a)_Phi = 0.99 at radii 20 < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7, 0), (0, 90), and (97, 0) respectively, corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies.

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Posts: 131433
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Milky Way halo
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Title: Stealth Galaxies in the Halo of the Milky Way
Authors: James S. Bullock, Kyle R. Stewart, Manoj Kaplinghat, Erik J. Tollerud (UC Irvine)
(9 Dec 2009)

We predict that there is a population of low-luminosity dwarf galaxies orbiting within the halo of the Milky Way that have surface brightnesses low enough to have escaped detection in star-count surveys. The overall count of stealth galaxies is sensitive to the presence (or lack) of a low-mass threshold in galaxy formation. These systems have luminosities and stellar velocity dispersions that are similar to those of known ultrafaint dwarf galaxies but they have more extended stellar distributions (half light radii greater than about 100 pc) because they inhabit dark subhalos that are slightly less massive than their higher surface brightness counterparts. As a result, the typical peak surface brightness is fainter than 30 mag per square arcsec. One implication is that the inferred common mass scale for Milky Way dwarfs may be an artifact of selection bias. If there is no sharp threshold in galaxy formation at low halo mass, then ultrafaint galaxies like Segue 1 represent the high-mass, early forming tail of a much larger population of objects that could number in the hundreds and have typical peak circular velocities of about 8 km/s and masses within 300 pc of about 5 million solar masses. Alternatively, if we impose a low-mass threshold in galaxy formation in order to explain the unexpectedly high densities of the ultrafaint dwarfs, then we expect only a handful of stealth galaxies in the halo of the Milky Way. A complete census of these objects will require deeper sky surveys, 30m-class follow-up telescopes, and more refined methods to identify extended, self-bound groupings of stars in the halo.

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Posts: 131433
Date:
Sagittarius Stream
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Title: Exploring the Sagittarius Stream with SEKBO Survey RR Lyrae Stars
Authors: Sayuri L Prior, G S Da Costa, Stefan C Keller

A sample of RR Lyrae (RRL) variables from the Southern Edgeworth-Kuiper Belt Object survey in regions overlapping the expected position of debris from the interaction of the Sagittarius (Sgr) dwarf galaxy with the Milky Way (RA ~ 20 and 21.5 h; distance 16-21 kpc) has been followed up spectroscopically and photometrically. The 21 photometrically confirmed type ab RRLs have mean [Fe/H] = -1.79 0.08, consistent with the abundances found for RRLs in a different portion of the Sgr tidal debris stream. The distribution of velocities in the Galactic standard of rest frame (V_GSR) of the 26 RRLs in the region is not consistent with a smooth halo population. Upon comparison with the Sgr disruption models of Law et al (2005), a prominent group of five stars having highly negative radial velocities (V_GSR ~ -175 kms-1) is consistent with predictions for old trailing debris when the Galactic halo is modelled as oblate. The observations also require that the recent trailing debris stream has a broader spread perpendicular to the Sgr plane than predicted by the models. We have also investigated the possible association of the Virgo Stellar Stream (VSS) with Sgr debris by comparing radial velocities for RRLs in the region with the same models, finding similarities in the velocity-position trends. As suggested by our earlier work, the stars in the VSS region with large negative V_GSR values are likely to be old leading Sgr debris, but we find that while old trailing Sgr debris may well make a contribution at positive V_GSR values, it is unlikely to fully account for the VSS feature. Overall we find that further modelling is needed, as trailing arm data generally favours oblate models while leading arm data favours prolate models, with no single potential fitting all the observed data.

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Posts: 131433
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RE: Milky Way Companions
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A large satellite galaxy may be lurking, hidden from view, next door to our own.
Sukanya Chakrabarti and Leo Blitz of the University of California, Berkeley, suspected that the gravity of a nearby galaxy was causing perturbations that have been observed in gas on the fringes of the Milky Way.

"We did a large range of simulations where we varied the mass of the perturber and the distance of closest approach" - Sukanya Chakrabarti.

In the best-fitting simulation, the unseen galaxy has about 1 per cent of the Milky Way's mass, or 10 billion times the mass of the sun.
That's a lot. It means the object has roughly the same mass as the Milky Way's brightest satellite galaxy, the Large Magellanic Cloud (LMC).

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Posts: 131433
Date:
Milky Way satellites
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Title: On the nature of the Milky Way satellites
Authors: Yang-Shyang Li (1), Gabriella De Lucia (2), Amina Helmi (1) ((1) Kapteyn Astronomical Institute, Groningen, (2) INAF, Trieste)

We combine a series of high-resolution simulations with semi-analytic galaxy formation models to follow the evolution of a system resembling the Milky Way and its satellites. The semi-analytic model is based on that developed for the Millennium Simulation, and successfully reproduces the properties of galaxies on large scales, as well as those of the Milky Way. In this model, we are able to reproduce the luminosity function of the satellites around the Milky Way by preventing cooling in haloes with Vvir < 16.7 km/s (i.e. the atomic hydrogen cooling limit) and including the impact of the reionisation of the Universe. The physical properties of our model satellites (e.g. mean metallicities, ages, half-light radii and mass-to-light ratios) are in good agreement with the latest observational measurements. We do not find a strong dependence upon the particular implementation of supernova feedback, but a scheme which is more efficient in galaxies embedded in smaller haloes, i.e. shallower potential wells, gives better agreement with the properties of the ultra-faint satellites. Our model predicts that the brightest satellites are associated with the most massive subhaloes, are accreted later (z \lta 1), and have extended star formation histories, with only 1 per cent of their stars made by the end of the reionisation. On the other hand, the faintest satellites were accreted early, are dominated by stars with age > 10 Gyr, and a few of them formed most of their stars before the reionisation was complete. Objects with luminosities comparable to those of the classical MW satellites are associated with dark matter subhaloes with a peak circular velocity \gta 10 km/s, in agreement with the latest constraints.

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Posts: 131433
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RE: Milky Way Companions
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Is the Milky Way doomed to be destroyed by galactic bombardment? Probably not, study says
As scientists attempt to learn more about how galaxies evolve, an open question has been whether collisions with our dwarf galactic neighbours will one day tear apart the disk of the Milky Way.
That grisly fate is unlikely, a new study now suggests.
While astronomers know that such collisions have probably occurred in the past, the new computer simulations show that instead of destroying a galaxy, these collisions "puff up" a galactic disk, particularly around the edges, and produce structures called stellar rings.

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Posts: 131433
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Mystery of the missing mini-galaxies
Like moths about a flame, thousands of tiny satellite galaxies flutter about our Milky Way. For astronomers this is a dream scenario, fitting perfectly with the established models of how our galaxy's cosmic neighbourhood should be. Unfortunately, it's a dream in more ways than one and the reality could hardly be more different.
As far as we can tell, barely 25 straggly satellites loiter forlornly around the outskirts of the Milky Way.

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Posts: 131433
Date:
Ultra-faint Dwarf galaxies
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Title: In Pursuit of the Least Luminous Galaxies
Authors: Beth Willman (Haverford College)

The dwarf galaxy companions to the Milky Way are unique cosmological laboratories. With luminosities as low as 10^-7 L_MW, they inhabit the lowest mass dark matter halos known to host stars and are presently the most direct tracers of the distribution, mass spectrum, and clustering scale of dark matter. Their resolved stellar populations also facilitate detailed studies of their history and mass content. To fully exploit this potential requires a well-defined census of virtually invisible galaxies to the faintest possible limits and to the largest possible distances. I review the past and present impacts of survey astronomy on the census of Milky Way dwarf galaxy companions, and discuss the future of finding ultra-faint dwarf galaxies around the Milky Way and beyond in wide-field survey data.

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