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Post Info TOPIC: Segue 1
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Title: Segue 1: An Unevolved Fossil Galaxy from the Early Universe
Author: Anna Frebel (MIT), Joshua D. Simon (Carnegie), Evan N. Kirby (Irvine)

We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star observed by Norris et al. (2010), high-resolution spectra have now been obtained for every red giant in Segue 1. Remarkably, three of these seven stars have metallicities below [Fe/H] = -3.5, suggesting that Segue 1 is the least chemically evolved galaxy known. We confirm previous medium-resolution analyses demonstrating that Segue 1 stars span a metallicity range of more than 2 dex, from [Fe/H] = -1.4 to [Fe/H] = -3.8. All of the Segue 1 stars are alpha-enhanced, with [alpha/Fe] ~ 0.5. High alpha-element abundances are typical for metal-poor stars, but in every previously studied galaxy [alpha/Fe] declines for more metal-rich stars, which is typically interpreted as iron enrichment from supernova Ia. The absence of this signature in Segue 1 indicates that it was enriched exclusively by massive stars. Other light element abundance ratios in Segue 1, including carbon-enhancement in the three most metal-poor stars, closely resemble those of metal-poor halo stars. Finally, we classify the most metal-rich star as a CH star given its large overabundances of carbon and s-process elements. The other six stars show remarkably low neutron-capture element abundances of [Sr/H] < -4.9 and [Ba/H] < -4.2, which are comparable to the lowest levels ever detected in halo stars. This suggests minimal neutron-capture enrichment, perhaps limited to a single r-process or weak s-process synthesizing event. Altogether, the chemical abundances of Segue 1 indicate no substantial chemical evolution, supporting the idea that it may be a surviving first galaxy that experienced only one burst of star formation.

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Title: VERITAS Deep Observations of the Dwarf Spheroidal Galaxy Segue 1
Authors: E. Aliu, S. Archambault, T. Arlen, T. Aune, M. Beilicke, W. Benbow, A. Bouvier, S. M. Bradbury, J. H. Buckley, V. Bugaev, K. Byrum, A. Cannon, A. Cesarini, J. L. Christiansen, L. Ciupik, E. Collins-Hughes, M. P. Connolly, W. Cui, G. Decerprit, R. Dickherber, J. Dumm, M. Errando, A. Falcone, Q. Feng, F. Ferrer, J. P. Finley, G. Finnegan, L. Fortson, A. Furniss, N. Galante, D. Gall, S. Godambe, S. Griffin, J. Grube, G. Gyuk, D. Hanna, J. Holder, H. Huan, G. Hughes, T. B. Humensky, P. Kaaret, N. Karlsson, M. Kertzman, Y. Khassen, D. Kieda, H. Krawczynski, F. Krennrich, K. Lee, A. S Madhavan, G. Maier, P. Majumdar, S. McArthur, A. McCann, P. Moriarty, R. Mukherjee, R. A. Ong, M. Orr, A. N. Otte, N. Park, J. S. Perkins, M. Pohl, H. Prokoph, J. Quinn, K. Ragan, L. C. Reyes, P. T. Reynolds, E. Roache, et al. (27 additional authors not shown)

The VERITAS array of Cherenkov telescopes has carried out a deep observational program on the nearby dwarf spheroidal galaxy Segue 1. We report on the results of nearly 48 hours of good quality selected data, taken between January 2010 and May 2011. No significant \gamma-ray emission is detected at the nominal position of Segue 1, and upper limits on the integrated flux are derived. According to recent studies, Segue 1 is the most dark matter-dominated dwarf spheroidal galaxy currently known. We derive stringent bounds on various annihilating and decaying dark matter particle models. The upper limits on the velocity-weighted annihilation cross-section are \mathrm{^{95% CL} \lesssim 10^{-23} cm^{3} s^{-1}}, improving our limits from previous observations of dwarf spheroidal galaxies by at least a factor of two for dark matter particle masses \mathrm{m_{\chi}\gtrsim 300 GeV}. The lower limits on the decay lifetime are at the level of \mathrm{\tau^{95% CL} \gtrsim 10^{24} s}. Finally, we address the interpretation of the cosmic ray lepton anomalies measured by ATIC and PAMELA in terms of dark matter annihilation, and show that the VERITAS observations of Segue 1 disfavour such a scenario.

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Blobrana


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Title: Segue 1: the best dark matter candidate dwarf galaxy surveyed by MAGIC
Authors: J. Aleksi, M. Doro, S. Lombardi, D. Nieto, for the MAGIC Collaboration, M. Fornasa

Despite the interest in Dark Matter (DM) searches is currently more focused on underground experiments, a signature of DM annihilation/decay in gamma-rays from space would constitute a smoking gun for its identification. In this contribution, we start with a brief review of the efforts of the ground-based MAGIC Cherenkov telescopes system to detect DM signatures from dwarf satellite galaxies orbiting the Milky Way halo. We then present the recent survey of Segue 1, considered by many as possibly the most DM dominated satellite galaxy known in our galaxy. No significant gamma-ray emission was found above the background in around 30 hours of observation. This is the largest survey ever made on a single dwarf by Cherenkov telescopes. We present a novel analysis that fully takes into account the spectral features of the gamma-ray spectrum of specific DM models in a Supersymmetric scenario. We also discuss the prospects of detection after the Fermi observation of similar objects at lower energies.

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Galaxy loaded with dark matter

A faint galaxy that is orbiting the Milky Way has proved to be a plentiful lode of mysterious dark matter, observations from Mauna Kea confirm.
Astronomers using the Keck II telescope say that the group of 1,000 stars known as Segue 1 make up the dimmest known galaxy.
Like an iceberg with most of its body hidden, the dwarf galaxy appears to have 3,400 times more mass than its visible stars would account for, researchers reported recently in the Astrophysical Journal.
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Blobrana


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Found: Heart of Darkness

Astronomers using the 10-meter Keck II telescope in Hawaii have confirmed in a new paper that a troupe of about 1,000 small, dim stars just outside the Milky Way comprise the darkest known galaxy, as well as something else: a treasure trove of ancient stars.
By "dark" astronomers are not referring to how much light the galaxy, called Segue 1, puts out, but the fact that the dwarf galaxy appears to have 3,400 times more mass than can be accounted for by its visible stars. In other words, Segue 1 is mostly an enormous cloud of dark matter decorated with a sprinkling of stars.
The initial announcement of the "Darkest Galaxy" was made two years ago by Marla Geha, a Yale University astronomer, Joshua Simon from the Carnegie Institution of Washington, and their colleagues. This original claim was based on data from the Sloan Digital Sky Survey and the Keck II telescope. Those observations indicated the stars were all moving together and were a diverse group, rather than simply a cluster of similar stars that had been ripped out of the nearby and more star-rich Sagittarius dwarf galaxy. A competing group of astronomers at Cambridge University were, however, not convinced.

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Title: Swift observation of Segue 1: constraints on sterile neutrino parameters in the darkest galaxy
Authors: N. Mirabal (Universidad Complutense de Madrid)

Some extensions of standard particle physics postulate that dark matter may be partially composed of weakly interacting sterile neutrino particles that have so far eluded detection. We use a short (~5 ks) archival X-ray observation of Segue 1 obtained with the X-ray Telescope (XRT) onboard the Swift satellite to exclude the presence of sterile neutrinos in the 1.6 - 14 keV mass range down to a flux limit of 6 x 10^{-12} erg cm-2 s-1 within 67 pc of its centre. With an estimated mass-to-light ratio of ~3400 Msun/Lsun, Segue 1 is the darkest ultrafaint dwarf galaxy currently measured. Spectral analysis of the Swift XRT data fails to find any non-instrumental spectral feature possibly connected with the radiative decay of a dark matter particle. Accordingly, we establish upper bounds on the sterile neutrino parameter space based on the non-detection of emission lines in the spectrum. The present work provides the most sensitive X-ray search for sterile neutrinos in a region with the highest dark matter density yet measured.

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Seg 1-7
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Title: An Extremely Carbon-rich, Extremely Metal-poor Star in the Segue 1 System
Authors: John E. Norris, Gerard Gilmore, Rosemary F. G. Wyse, David Yong, Anna Frebel

We report the analysis of high-resolution, high-S/N spectra of an extremely metal-poor, extremely C-rich red giant, Seg 1-7, in the Segue 1 system - described in the literature alternatively as an unusually extended globular cluster or an ultra-faint dwarf galaxy. The radial velocity of Seg 1-7 coincides precisely with the systemic velocity of Segue 1, and its chemical abundance signature of [Fe/H] = -3.52, [C/Fe] = +2.3, [N/Fe] = +0.8, [Na/Fe] = +0.53, [Mg/Fe] = +0.94, [Al/Fe] = +0.23 and [Ba/Fe] < -1.0 is similar to that of the rare and enigmatic class of Galactic halo objects designated CEMP-no (Carbon-rich, Extremely Metal-Poor and with no enhancement (over solar ratios) of heavy neutron-capture elements). This is the first star in a Milky Way "satellite" that unambiguously lies on the metal-poor, C-rich branch of the Aoki et al. (2007) bimodal distribution defined by field halo stars in the ([C/Fe], [Fe/H])-plane. Available data permit us only to identify Seg 1-7 as a member of an ultra-faint dwarf galaxy or as debris from the Sgr dwarf spheroidal galaxy. In either case, this demonstrates that at extremely low abundance, [Fe/H ] < -3.0, star formation and associated chemical evolution proceeded similarly in the progenitors of both the field halo and satellite systems. By extension, this is consistent with other recent suggestions the most metal-poor dwarf spheroidal and ultra-faint dwarf satellites were the building blocks of the Milky Way's outer halo.

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Title: A Complete Spectroscopic Survey of the Milky Way Satellite Segue 1: The Darkest Galaxy
Authors: Joshua D. Simon (Carnegie), Marla Geha (Yale), Quinn E. Minor, Gregory D. Martinez (UC Irvine), Evan N. Kirby (Caltech), James S. Bullock, Manoj Kaplinghat (UC Irvine), Louis E. Strigari (Stanford), Beth Willman (Haverford), Philip I. Choi (Pomona), Erik J. Tollerud, Joe Wolf (UC Irvine)

We present the results of a comprehensive Keck/DEIMOS spectroscopic survey of the ultra-faint Milky Way satellite galaxy Segue 1. We have obtained velocity measurements for 99.1% of the stars within 67 pc (2.3 half-light radii) of the center of Segue 1 that have colours and magnitudes consistent with membership, down to a magnitude limit of r=21.7. Based on photometric, kinematic, and metallicity information, we identify 71 stars as probable Segue 1 members, including some as far out as 87 pc. After correcting for the influence of binary stars using repeated velocity measurements, we determine a velocity dispersion of 3.7^{+1.4}_{-1.1} km/s, with a corresponding mass within the half-light radius of 5.8^{+8.2}_{-3.1} x 10^5 Msun. The stellar kinematics of Segue 1 require very high mass-to-light ratios unless it is far from dynamical equilibrium, even if the period distribution of unresolved binary stars is skewed toward implausibly short periods. With a total luminosity less than that of a single bright red giant and a V-band mass-to-light ratio of 3400 Msun/Lsun, Segue 1 is the darkest galaxy currently known. We critically re-examine recent claims that Segue 1 is a tidally disrupting star cluster and that kinematic samples are contaminated by the Sagittarius stream. The extremely low metallicities ([Fe/H] < -3) of two Segue 1 stars and the large metallicity spread among the members demonstrate conclusively that Segue 1 is a dwarf galaxy, and we find no evidence in favour of tidal effects. We also show that contamination by the Sagittarius stream has been overestimated. Segue 1 has the highest measured dark matter density of any known galaxy and will therefore be a prime testing ground for dark matter physics and galaxy formation on small scales.

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Wednesday, September 24, 2008
The dark matter galaxy that could be orbiting our Milky Way

A satellite galaxy made almost entirely of dark matter could be orbiting the Milky Way, a new image has revealed.
It is one of 24 neighbouring galaxies spotted by the Sloan Digital Sky Survey, which has recorded the night sky in greater detail than ever before.
The results have doubled the number of known dwarf galaxies orbiting the Milky Way.
The latest finding is particularly significant as it appears to have a very low light-to-mass ratio - a billion times less bright than the Milky Way itself.
Yale Professor of Astronomy Marla Geha, who led the team that made the discovery, believes that the galaxy, named 'Segue 1' is mainly composed of dark matter.
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Title: The Origin of Segue 1
Authors: M. Niederste-Ostholt (1), V. Belokurov (1), N.W. Evans (1), G. Gilmore (1), R.F.G. Wyse (2), J.E. Norris (3) ((1) Cambridge, (2), JHU, (3) ANU)

We apply the optimal filter technique to Sloan Digital Sky Survey photometry around Segue 1 and find that the outer parts of the cluster are distorted. There is strong evidence for ~ 1 degree elongations of extra-tidal stars, extending both eastwards and southwestwards of the cluster. The extensions have similar differential Hess diagrams to Segue 1. A Kolmogorov-Smirnov test suggests a high probability that both come from the same parent distribution. The location of Segue 1 is close to crossings of the tidal wraps of the Sagittarius stream. By extracting blue horizontal branch stars from Sloan's spectral database, two kinematic features are isolated and identified with different wraps of the Sagittarius stream. We show that Segue 1 is moving with a velocity that is close to one of the wraps. At this location, we estimate that there are enough Sagittarius stars, indistinguishable from Segue 1 stars, to inflate the velocity dispersion and hence the mass-to-light ratio. All the available evidence is consistent with the interpretation that Segue 1 is a star cluster, originally from the Sagittarius galaxy, and now dissolving in the Milky Way.

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