1,125 research outputs found
MAGELLAN/M2FS SPECTROSCOPY of the RETICULUM 2 DWARF SPHEROIDAL GALAXY
We present results from spectroscopic observations with the Michigan/Magellan
Fiber System (M2FS) of 182 stellar targets along the line of sight to the
newly-discovered `ultrafaint' object Reticulum 2 (Ret 2). For 38 of these
targets, the spectra are sufficient to provide simultaneous estimates of
line-of-sight velocity (, median random error km s), effective temperature (, K), surface gravity (, dex) and
iron abundance ([Fe/H], dex). We use these
results to confirm 18 stars as members of Ret 2. From the member sample we
estimate a velocity dispersion of km
s about a mean of km
s in the solar rest frame ( km s in the Galactic rest
frame), and a metallicity dispersion of dex about a mean of . These estimates marginalize over possible
velocity and metallicity gradients, which are consistent with zero. Our results
place Ret 2 on chemodynamical scaling relations followed by the Milky Way's
dwarf-galactic satellites. Under assumptions of dynamic equilibrium and
negligible contamination from binary stars---both of which must be checked with
deeper imaging and repeat spectroscopic observations---the estimated velocity
dispersion suggests a dynamical mass of
enclosed within projected halflight radius pc, with
mass-to-light ratio in
solar units
MAGELLAN/M2FS SPECTROSCOPY of the RETICULUM 2 DWARF SPHEROIDAL GALAXY
© 2015. The American Astronomical Society. All rights reserved.We present results from spectroscopic observations with the Michigan/Magellan Fiber System (M2FS) of 182 stellar targets along the line of sight (LOS) to the newly discovered "ultrafaint" object Reticulum 2 (Ret 2). For 37 of these targets, the spectra are sufficient to provide simultaneous estimates of LOS velocity (Îœlos, median random error ÎŽÎœlos= 1.4 km s-1), effective temperature (Teff, Î Teff K), surface gravity (loh g, ÎŽlog g = 0.63 dex), and iron abundance ([Fe/H], ÎŽ[Fe/H] = 0.47 dex). We use these results to confirm 17 stars as members of Ret 2. From the member sample we estimate a velocity dispersion of ÏÎœlos = 3.60.7+1.0km s-1 about a mean of ăÎœlosă = 64.3-1.2+1.2 km s-1 in the solar rest frame ( ⌠-90.9 km s-1 in the Galactic rest frame), and a metallicity dispersion of Ï [Fe/H] = 0.49-0.140.19 dex about a mean of ă [Fe/H]ă= -2.58-0.33+0.34. These estimates marginalize over possible velocity and metallicity gradients, which are consistent with zero. Our results place Ret 2 on chemodynamical scaling relations followed by the Milky Way's dwarf-galactic satellites. Under assumptions of dynamic equilibrium and negligible contamination from binary stars - both of which must be checked with deeper imaging and repeat spectroscopic observations - the estimated velocity dispersion suggests a dynamical mass of M(Rh)â5RhÏÎœlos;2/(2G) = 2.4-0.8+1.4 Ă 105 Mâenclosed within projected halflight radius Rh âŒ32 pc, with mass-to-light ratio â2M (Rh)/LV = 467-168+286in solar units
The Chemical-Composition of Distant Globular-Clusters - Are There Any Metal-Poor Clusters
The authors report echelle spectroscopy of giants in the distant globular clusters N4147, M53, N5466, and N6229. Abundances are determined from model atmosphere analyses of the equivalent widths of the metal lines. The analyses yield [Fe/H] values in the range of -1.3 to -1.9. These values are moderately high for clusters at these galactocentric distances (R ⥠20 kpc). The data argue against the existence of a strong metallicity gradient in the halo. They may also suggest that the cluster system of the Galaxy predates the formation of the bulge and halo system
Blue diffuse dwarf galaxies: a clearer picture
The search for chemically unevolved galaxies remains prevalent in the nearby Universe, mostly because these systems provide excellent proxies for exploring in detail the physics of high-z systems. The most promising candidates are extremely metal-poor galaxies (XMPs), i.e., galaxies with <1/10 solar metallicity. However, due to the bright emission line based search criteria traditionally used to find XMPs, we may not be sampling the full XMP population. In 2014 we reoriented this search using only morphological properties and uncovered a population of ~150 `blue diffuse dwarf (BDD) galaxies', and published a sub-sample of 12 BDD spectra. Here we present optical spectroscopic observations of a larger sample of 51 BDDs, along with their SDSS photometric properties. With our improved statistics, we use direct-method abundances to confirm that BDDs are chemically unevolved (7.43<12+log(O/H)<8.01), with ~20% of our sample classified as being XMP galaxies, and find they are actively forming stars at rates of 1-33x10^-2 M_sol/yr in HII regions randomly embedded in a blue, low-surface brightness continuum. Stellar masses are calculated from population synthesis models and estimated to be in the range log(M_star/M_sol) ~5-9. Unlike other low-metallicity star-forming galaxies, BDDs are in agreement with the mass-metallicity relation at low masses, suggesting they are not accreting large amounts of pristine gas relative to their stellar mass. BDD galaxies appear to be a population of actively star-forming dwarf irregular (dIrr) galaxies who fall within the class of low-surface brightness dIrr galaxies. Their ongoing star-formation and irregular morphology make them excellent analogues for galaxies in the early Universe.BLJ thanks support from the Science & Technology Facilities Council (STFC). The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 308024. SK acknowledges financial support from the ERC. DPS acknowledges support from the National Science Foundation through the grant AST-1410155. EWO is partially supported by NSF grant AST1313006
The Distances of the Magellanic Clouds
The present status of our knowledge of the distances to the Magellanic Clouds
is evaluated from a post-Hipparcos perspective. After a brief summary of the
effects of structure, reddening, age and metallicity, the primary distance
indicators for the Large Magellanic Cloud are reviewed: The SN 1987A ring,
Cepheids, RR Lyraes, Mira variables, and Eclipsing Binaries. Distances derived
via these methods are weighted and combined to produce final "best" estimates
for the Magellanic Clouds distance moduli.Comment: Invited review article to appear in ``Post Hipparcos Cosmic
Candles'', F. Caputo & A. Heck (Eds.), Kluwer Academic Publ., Dordrecht, in
pres
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
Jet energy measurement with the ATLAS detector in proton-proton collisions at root s=7 TeV
The jet energy scale and its systematic uncertainty are determined for jets measured with the ATLAS detector at the LHC in proton-proton collision data at a centre-of-mass energy of âs = 7TeV corresponding to an integrated luminosity of 38 pb-1. Jets are reconstructed with the anti-kt algorithm with distance parameters R=0. 4 or R=0. 6. Jet energy and angle corrections are determined from Monte Carlo simulations to calibrate jets with transverse momenta pTâ„20 GeV and pseudorapidities {pipe}η{pipe}<4. 5. The jet energy systematic uncertainty is estimated using the single isolated hadron response measured in situ and in test-beams, exploiting the transverse momentum balance between central and forward jets in events with dijet topologies and studying systematic variations in Monte Carlo simulations. The jet energy uncertainty is less than 2. 5 % in the central calorimeter region ({pipe}η{pipe}<0. 8) for jets with 60â€pT<800 GeV, and is maximally 14 % for pT<30 GeV in the most forward region 3. 2â€{pipe}η{pipe}<4. 5. The jet energy is validated for jet transverse momenta up to 1 TeV to the level of a few percent using several in situ techniques by comparing a well-known reference such as the recoiling photon pT, the sum of the transverse momenta of tracks associated to the jet, or a system of low-pT jets recoiling against a high-pT jet. More sophisticated jet calibration schemes are presented based on calorimeter cell energy density weighting or hadronic properties of jets, aiming for an improved jet energy resolution and a reduced flavour dependence of the jet response. The systematic uncertainty of the jet energy determined from a combination of in situ techniques is consistent with the one derived from single hadron response measurements over a wide kinematic range. The nominal corrections and uncertainties are derived for isolated jets in an inclusive sample of high-pT jets. Special cases such as event topologies with close-by jets, or selections of samples with an enhanced content of jets originating from light quarks, heavy quarks or gluons are also discussed and the corresponding uncertainties are determined. © 2013 CERN for the benefit of the ATLAS collaboration
Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector
The inclusive and dijet production cross-sections have been measured for jets
containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass
energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The
measurements use data corresponding to an integrated luminosity of 34 pb^-1.
The b-jets are identified using either a lifetime-based method, where secondary
decay vertices of b-hadrons in jets are reconstructed using information from
the tracking detectors, or a muon-based method where the presence of a muon is
used to identify semileptonic decays of b-hadrons inside jets. The inclusive
b-jet cross-section is measured as a function of transverse momentum in the
range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet
cross-section is measured as a function of the dijet invariant mass in the
range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets
and the angular variable chi in two dijet mass regions. The results are
compared with next-to-leading-order QCD predictions. Good agreement is observed
between the measured cross-sections and the predictions obtained using POWHEG +
Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet
cross-section. However, it does not reproduce the measured inclusive
cross-section well, particularly for central b-jets with large transverse
momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final
version published in European Physical Journal
Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at â s = 8 TeV with the ATLAS detector
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fbâ1 of â s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente
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