72 research outputs found
SMASH 1 : A VERY FAINT GLOBULAR CLUSTER DISRUPTING in the OUTER REACHES of the LMC?
We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the Magellanic Stellar History (SMASH), SMASH 1 is a compact (rh=9.1-3.4+5.9pc) and very low luminosity (Mv=-1.0±0.9,Lv=102.3±0.4L⊙ ) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of ∼18.8, i.e., a distance of . Situated at 11.°3 from the LMC in projection, its three-dimensional distance from the Cloud is 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least . Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only ≲19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (ϵ=0.62-0.21+0.17) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.Peer reviewe
AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY
We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe
Transform-domain analysis of packet delay in network nodes with QoS-aware scheduling
In order to differentiate the perceived QoS between traffic classes in heterogeneous packet networks, equipment discriminates incoming packets based on their class, particularly in the way queued packets are scheduled for further transmission. We review a common stochastic modelling framework in which scheduling mechanisms can be evaluated, especially with regard to the resulting per-class delay distribution. For this, a discrete-time single-server queue is considered with two classes of packet arrivals, either delay-sensitive (1) or delay-tolerant (2). The steady-state analysis relies on the use of well-chosen supplementary variables and is mainly done in the transform domain. Secondly, we propose and analyse a new type of scheduling mechanism that allows precise control over the amount of delay differentiation between the classes. The idea is to introduce N reserved places in the queue, intended for future arrivals of class 1
Magellan/M2FS Spectroscopy of Tucana 2 and Grus 1
We present results from spectroscopic observations with the Michigan/Magellan
Fiber System (M2FS) of stellar targets along the line of sight to the
newly-discovered `ultrafaint' stellar systems Tucana 2 (Tuc 2) and Grus 1 (Gru
1). Based on simultaneous estimates of line-of-sight velocity and
stellar-atmospheric parameters, we identify 8 and 7 stars as probable members
of Tuc 2 and and Gru 1, respectively. Our sample for Tuc 2 is sufficient to
resolve an internal velocity dispersion of km s
about a mean of km s (solar rest frame), and to
estimate a mean metallicity of [Fe/H]= . These results
place Tuc 2 on chemodynamical scaling relations followed by dwarf galaxies,
suggesting a dominant dark matter component with dynamical mass
enclosed within the central
pc, and dynamical mass-to-light ratio
. For Gru 1 we estimate a mean velocity of
km s and a mean metallicity of
[Fe/H]=, but our sample does not resolve Gru 1's
velocity dispersion. The radial coordinates of Tuc 2 and Gru 1 in Galactic
phase space suggest that their orbits are among the most energetic within
distance kpc. Moreover, their proximity to each other in this space
arises naturally if both objects are trailing the Large Magellanic Cloud.Comment: replaced with ApJ-accepted version, all spectra and data products
(including samples from posterior PDFs) are available at
http://www.andrew.cmu.edu/user/mgwalker/tuc2gru1_dataproducts.tar.g
Triangulum II: A Very Metal-poor and Dynamically Hot Stellar System
We present a study of the recently discovered compact stellar system
Triangulum II. From observations conducted with the DEIMOS spectrograph on Keck
II, we obtained spectra for 13 member stars that follow the CMD features of
this very faint stellar system and include two bright red giant branch stars.
Tri II has a very negative radial velocity (=-383.7^{+3.0}_{-3.3} km/s)
that translates to ~ -264 km/s and confirms it is a Milky Way
satellite. We show that, despite the small data set, there is evidence that Tri
II has complex internal kinematics. Its radial velocity dispersion increases
from 4.4^{+2.8}_{-2.0} km/s in the central 2' to 14.1^{+5.8}_{-4.2} km/s
outwards. The velocity dispersion of the full sample is inferred to be
\sigma_{vr}=9.9^{+3.2}_{-2.2} km/s. From the two bright RGB member stars we
measure an average metallicity =-2.6+/-0.2, placing Tri II among the
most metal-poor Milky Way dwarf galaxies. In addition, the spectra of the
fainter member stars exhibit differences in their line widths that could be the
indication of a metallicity dispersion in the system. All these properties
paint a complex picture for Tri II, whose nature and current state are largely
speculative. The inferred metallicity properties of the system however lead us
to favor a scenario in which Tri II is a dwarf galaxy that is either disrupting
or embedded in a stellar stream.Comment: 8 pages, 6 figures, 2 tables. ApJ, in press. v2: only minor changes
to the tex
BEASTS OF THE SOUTHERN WILD: DISCOVERY OF NINE ULTRA FAINT SATELLITES IN THE VICINITY OF THE MAGELLANIC CLOUDS
We have used the publicly released Dark Energy Survey data to hunt for new
satellites of the Milky Way in the Southern hemisphere. Our search yielded a
large number of promising candidates. In this paper, we announce the discovery
of 9 new unambiguous ultra-faint objects, whose authenticity can be established
with the DES data alone. Based on the morphological properties, three of the
new satellites are dwarf galaxies, one of which is located at the very
outskirts of the Milky Way, at a distance of 380 kpc. The remaining 6 objects
have sizes and luminosities comparable to the Segue~1 satellite and can not be
classified straightforwardly without follow-up spectroscopic observations. The
satellites we have discovered cluster around the LMC and the SMC. We show that
such spatial distribution is unlikely under the assumption of isotropy, and,
therefore, conclude that at least some of the new satellites must have been
associated with the Magellanic Clouds in the past.Comment: Accepted to ApJ; See also 1503.02584; Changes w.r.t. v2: the
discussion section updated slightly; small changes in the table with
satellite parameter
Nearest neighbor: the low-mass milky way satellite Tucana III*
We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Tucana III (Tuc III). We identify 26 member stars in Tuc III from which we measure a mean radial velocity of v hel = −102.3 ± 0.4 (stat.) ± 2.0 (sys.) km s-1, a velocity dispersion of 0.1 -0.1+0.7 km s-1, and a mean metallicity of [Fe/H]=-2.42 -0.08+0.07. The upper limit on the velocity dispersion is σ < 1.5 km s-1 at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 104 M ⊙. We cannot rule out mass-to-light ratios as large as 240 M ⊙/L ⊙ for Tuc III, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in Tuc III of 0.19 dex at 95.5% confidence. Tuc III has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that Tuc III is the tidally stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a definitive classification. If Tuc III is indeed a dwarf galaxy, it is one of the closest external galaxies to the Sun. Because of its proximity, the most luminous stars in Tuc III are quite bright, including one star at V = 15.7 that is the brightest known member star of an ultra-faint satellite
Farthest neighbor: the distant Milky Way satellite Eridanus II*
We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Eridanus II (Eri II). We identify 28 member stars in Eri II, from which we measure a systemic radial velocity of vhel=75.6 ±1.3(stat.) ±2.0(sys.) km s-1 and a velocity dispersion of 6.9 -0.9+1.2 km s-1. Assuming that Eri II is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of 1.2 -0.3+0.4 times 10^7 Mo, indicating a mass-to-light ratio of 420 -140+210 Mo Lo and confirming that it is a dark matter-dominated dwarf galaxy. From the equivalent width measurements of the Ca triplet lines of 16 red giant member stars, we derive a mean metallicity of [Fe/H] = −2.38 ± 0.13 and a metallicity dispersion of sigma[Fe/H]=0.47 -0.09+0.12. The velocity of Eri II in the Galactic standard of rest frame is v GSR = −66.6 km s-1, indicating that either Eri II is falling into the Milky Way potential for the first time or that it has passed the apocenter of its orbit on a subsequent passage. At a Galactocentric distance of ~370 kpc, Eri II is one of the Milky Way's most distant satellites known. Additionally, we show that the bright blue stars previously suggested to be a young stellar population are not associated with Eri II. The lack of gas and recent star formation in Eri II is surprising given its mass and distance from the Milky Way, and may place constraints on models of quenching in dwarf galaxies and on the distribution of hot gas in the Milky Way halo. Furthermore, the large velocity dispersion of Eri II can be combined with the existence of a central star cluster to constrain massive compact halo object dark matter with mass >~Mo
Kinematics and chemistry of recently discovered Reticulum 2 and Horologium 1 dwarf galaxies
Photometry alone is not sufficient to unambiguously distinguish between
ultra-faint star clusters and dwarf galaxies because of their overlap in
morphological properties. Here we report on VLT/GIRAFFE spectra of candidate
member stars in two recently discovered ultra-faint satellites Reticulum 2 and
Horologium 1, obtained as part of the ongoing Gaia-ESO Survey. We identify 18
members in Reticulum 2 and 5 in Horologium 1. We find Reticulum 2 to have a
velocity dispersion of ~3.22 km/s, implying a M/L ratio of ~ 500. We have
inferred stellar parameters for all candidates and we find Reticulum 2 to have
a mean metallicity of [Fe/H] = -2.46+/-0.1, with an intrinsic dispersion of ~
0.29, and is alpha-enhanced to the level of [alpha/Fe]~0.4. We conclude that
Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of
four stars in a previously unknown stellar substructure near Reticulum 2 with
[Fe/H] ~ -2 and V_hel ~ 220 km/s, far from the systemic velocity of Reticulum
2. For Horologium 1 we infer a velocity dispersion of 4.9^{+2.8}_{-0.9} km/s
and a consequent M/L ratio of ~ 600, leading us to conclude that Horologium 1
is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum
2 [Fe/H] = -2.76 +/- 0.1 and is similarly alpha-enhanced: [alpha/Fe] ~ 0.3.
Despite a large error-bar, we also measure a significant spread of
metallicities of 0.17 dex which strengthen the evidence that Horologium 1 is
indeed a dwarf galaxy. The line-of-sight velocity of Reticulum 2 is offset by
some 100 km/s from the prediction of the orbital velocity of the LMC, thus
making its association with the Cloud uncertain. However, at the location of
Horologium 1, both the backward integrated LMC's orbit and the LMC's halo are
predicted to have radial velocities similar to that of the dwarf. Therefore, it
is likely that Horologium 1 is or once was a member of the Magellanic Family.Comment: submitted to ApJ, abstract abridge
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