ABSTRACT We show that in the anticenter region, between Galactic longitudes of 110° < l < 229°, there is an oscillating asymmetry in the main-sequence star counts on either side of the Galactic plane ...using data from the Sloan Digital Sky Survey. This asymmetry oscillates from more stars in the north at distances of about 2 kpc from the Sun to more stars in the south at 4-6 kpc from the Sun to more stars in the north at distances of 8-10 kpc from the Sun. We also see evidence that there are more stars in the south at distances of 12-16 kpc from the Sun. The three more distant asymmetries form roughly concentric rings around the Galactic center, opening in the direction of the Milky Way's spiral arms. The northern ring, 9 kpc from the Sun, is easily identified with the previously discovered Monoceros Ring. Parts of the southern ring at 14 kpc from the Sun (which we call the TriAnd Ring) have previously been identified as related to the Monoceros Ring, and others have been called the Triangulum Andromeda Overdensity. The two nearer oscillations are approximated by a toy model in which the disk plane is offset by the order of 100 pc up and then down at different radii. We also show that the disk is not azimuthally symmetric around the Galactic anticenter and that there could be a correspondence between our observed oscillations and the spiral structure of the Galaxy. Our observations suggest that the TriAnd and Monoceros Rings (which extend to at least 25 kpc from the Galactic center) are primarily the result of disk oscillations.
Abstract
We present deep
Magellan
/Megacam stellar photometry of four recently discovered faint Milky Way satellites: Sagittarius II (Sgr II), Reticulum II (Ret II), Phoenix II (Phe II), and ...Tucana III (Tuc III). Our photometry reaches ∼2–3 magnitudes deeper than the discovery data, allowing us to revisit the properties of these new objects (e.g., distance, structural properties, luminosity measurements, and signs of tidal disturbance). The satellite color-magnitude diagrams show that they are all old (∼13.5 Gyr) and metal poor (Fe/H ≲ −2.2). Sgr II is particularly interesting, as it sits in an intermediate position between the loci of dwarf galaxies and globular clusters in the size–luminosity plane. The ensemble of its structural parameters is more consistent with a globular cluster classification, indicating that Sgr II is the most extended globular cluster in its luminosity range. The other three satellites land directly on the locus defined by Milky Way ultra-faint dwarf galaxies of similar luminosity. Ret II is the most elongated nearby dwarf galaxy currently known for its luminosity range. Our structural parameters for Phe II and Tuc III suggest that they are both dwarf galaxies. Tuc III is known to be associated with a stellar stream, which is clearly visible in our matched-filter stellar density map. The other satellites do not show any clear evidence of tidal stripping in the form of extensions or distortions. Finally, we also use archival H
i
data to place limits on the gas content of each object.
Abstract
Recent discovery of many dwarf satellite galaxies in the direction of the Small and Large Magellanic Clouds (SMC and LMC) provokes questions of their origins, and what they can reveal about ...galaxy evolution theory. Here, we predict the satellite stellar mass function of Magellanic Cloud-mass host galaxies using abundance matching and reionization models applied to the Caterpillar simulations. Specifically focusing on the volume within 50 kpc of the LMC, we predict a mean of four to eight satellites with stellar mass M
* > 104 M⊙, and three to four satellites with 80 < M
* ≤ 3000 M⊙. Surprisingly, all 12 currently known satellite candidates have stellar masses of 80 < M
* ≤ 3000 M⊙. Reconciling the dearth of large satellites and profusion of small satellites is challenging and may require a combination of a major modification of the M
*–M
halo relationship (steep, but with an abrupt flattening at 103 M⊙), late reionization for the Local Group (z
reion ≲ 9 preferred) and/or strong tidal stripping. We can more robustly predict that ∼53 per cent of satellites within this volume were accreted together with the LMC and SMC and ∼47 per cent were only ever Milky Way satellites. Observing satellites of isolated LMC-sized field galaxies is essential to place the LMC in context, and to better constrain the M
*–M
halo relationship. Modelling known LMC-sized galaxies within 8 Mpc, we predict 1–6 (2–12) satellites with M
* > 105 M⊙ (M
* > 104 M⊙) within the virial volume of each, and 1–3 (1–7) within a single 1
$_{.}^{\circ}$
5 diameter field of view, making their discovery likely.
Abstract
We present a three dimensional velocity analysis of Milky Way disc kinematics using LAMOST K giant stars and the GPS1 proper motion catalogue. We find that Galactic disc stars near the ...anticentre direction (in the range of Galactocentric distance between R = 8 and 13 kpc and vertical position between Z = −2 and 2 kpc) exhibit asymmetrical motions in the Galactocentric radial, azimuthal, and vertical components. Radial motions are not zero, thus departing from circularity in the orbits; they increase outwards within R ≲ 12 kpc, show some oscillation in the northern (0 < Z < 2 kpc) stars, and have north–south asymmetry in the region corresponding to a well-known nearby northern structure in the velocity field. There is a clear vertical gradient in azimuthal velocity, and also an asymmetry that shifts from a larger azimuthal velocity above the plane near the solar radius to faster rotation below the plane at radii of 11–12 kpc. Stars both above and below the plane at R ≳ 9 kpc exhibit net upward vertical motions. We discuss some possible mechanisms that might create the asymmetrical motions, such as external perturbations due to dwarf galaxy minor mergers or dark matter sub-haloes, warp dynamics, internal processes due to spiral arms or the Galactic bar, and (most likely) a combination of some or all of these components.
ABSTRACT We use about 200,000 FGK-type main-sequence stars from the LAMOST DR1 data to map the local stellar kinematics. With the velocity deprojection technique, we are able to derive the averaged ...three-dimensional velocity and velocity ellipsoids using only the line-of-sight velocity for the stars with various effective temperatures within pc. Using the mean velocities of the cool stars, we derive the solar motion of ( , , ) = (9.58 2.39, 10.52 1.96, 7.01 1.67) with respect to the local standard of rest. Moreover, we find that the stars with K show a net asymmetric motion of ∼3 in compared to the stars with K. And their azimuthal velocity increases when increases. This peculiar motion in the warmer stars is likely because they are young and not completely relaxed, although other reasons, such as the resonance induced by the central rotating bar or the spiral structures and the perturbation of the merging dwarf galaxies, cannot be ruled out. The derived velocity dispersions and cross-terms for the data are approximately consistent with previous studies. We also find that the vertical gradients of and are larger than that of . And the vertical gradient of shows a clear correlation with , while the other two do not. Finally, our sample shows a vertex deviation of about at pc, but roughly zero at pc.
We present proper motion (PM) measurements of Boötes III, an enigmatic stellar satellite of the Milky Way (MW), utilizing data from the second data release of the Gaia mission. By selecting 15 radial ...velocity confirmed members of Boötes III, along with a likely RR Lyrae member in the vicinity, we measure an error-weighted mean PM of ( , δ) = (−1.14, −0.98) (0.18, 0.20) mas yr−1. We select and present further stars that may be Boötes III members based on their combined PM and position in the color-magnitude diagram. We caution against assigning membership to stars that are not confirmed spectroscopically, as we demonstrate that there are contaminating stars from the disrupting globular cluster NGC 5466 in the vicinity of the main body of Boötes III, but we note that our results are consistent with previous Boötes III PM estimates that did not include spectroscopic members. Based on the measured PM and other known properties of Boötes III, we derive its Galactocentric velocity and compute its orbit given canonical MW potentials with halo masses of both 0.8 × 1012 M and 1.6 × 1012 M . These orbits robustly show that Boötes III passed within ∼12 kpc of the Galactic center on an eccentric orbit roughly ∼140 Myr ago. Additionally, the PM of Boötes III is in excellent agreement with predictions for the retrograde motion of the coincident Styx stellar stream. Given this, along with the small pericenter and metallicity spread of Boötes III itself, we suggest that it is a disrupting dwarf galaxy giving rise to the Styx stellar stream.
We characterize the kinematic and chemical properties of ∼3000 Sagittarius (Sgr) stream stars, including K-giants, M-giants, and blue horizontal branch stars (BHBs), selected from SEGUE-2, Large Sky ...Area Multi-Object Fibre Spectroscopic Telescope, and Sloan Digital Sky Survey separately in Integrals-of-Motion space. The orbit of the Sgr stream is quite clear from the velocity vector in the X-Z plane. Stars traced by K-giants and M-giants show that the apogalacticon of the trailing steam is ∼100 kpc. The metallicity distributions of Sgr K-giants, M-giants, and BHBs indicate that the M-giants are on average the most metal-rich population, followed by K-giants and BHBs. All of the K-giants, M-giants, and BHBs indicate that the trailing arm is on average more metal-rich than the leading arm, and the K-giants show that the Sgr debris is the most metal-poor part. The -abundance of Sgr stars exhibits a similar trend with the Galactic halo stars at lower metallicity (Fe/H <∼ −1.0 dex), and then evolve down to lower /Fe than disk stars at higher metallicity, which is close to the evolution pattern of the -element of Milky Way dwarf galaxies. We find that VY and metallicity of K-giants have gradients along the direction of the line of sight from the Galactic center in the X-Z plane, and the K-giants show that VY increases with metallicity at Fe/H >∼ −1.5 dex. After dividing the Sgr stream into bright and faint streams according to their locations in equatorial coordinates, the K-giants and BHBs show that the bright and faint streams present different VY and metallicities, the bright stream is on average higher in VY and metallicity than the faint stream.
We analyze chemical abundances of stars in the Sagittarius (Sgr) tidal stream using high-resolution Gemini+GRACES spectra of 42 members of the highest surface-brightness portions of both the trailing ...and leading arms. Targets were chosen using a 2MASS+WISE color-color selection, combined with the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) radial velocities. In this Letter, we analyze Fe/H and -elements produced by both hydrostatic (O, Mg) and explosive (Si, Ca, Ti) nucleosynthetic processes. The average Fe/H for our Sgr stream stars is lower than that for stars in the Sgr core, and stars in the trailing and leading arms show systematic differences in Fe/H. Both hydrostatic and explosive elements are depleted relative to Milky Way (MW) disk and halo stars, with a larger gap between the MW trend and Sgr stars for the hydrostatic elements. Chemical abundances of Sgr stream stars show similar patterns to those measured in the core of the Sgr dSph. We explore the ratio of hydrostatic to explosive -elements h/ex (which we refer to as the "HEx ratio"). Our observed HEx ratio trends for Sgr debris are deficient relative to MW stars. Via simple chemical evolution modeling, we show that these HEx ratio patterns are consistent with a Sgr IMF that lacks the most massive stars. This study provides a link between the chemical properties in the intact Sgr core and the significant portion of the Sgr system's luminosity that is estimated to currently reside in the streams.
We use LAMOST DR4 M giants combined with Gaia DR2 proper motions and ALLWISE photometry to obtain an extremely pure sample of Sagittarius (Sgr) stream stars. Using TiO5 and CaH spectral indices as ...indicators, we selected a large sample of M-giant stars from M-dwarf stars in LAMOST DR4 spectra. Considering the position, distance, proper motion, and angular momentum distribution, we obtained 164 pure Sgr stream stars. We find that the trailing arm has higher energy than the leading arm in the same angular momentum. The trailing arm we detected extends to a heliocentric distance of ∼130 kpc at Λ ˜ ∼ 170 ° , which is consistent with the feature found in RR Lyrae in Sesar et al. Both of these detections of Sgr, in M-giants and in RR Lyrae, imply that the Sgr stream may contain multiple stellar populations with a broad metallicity range.
Abstract
We present a deep Hubble Space Telescope (HST) imaging study of two dwarf galaxies in the halos of Local Volume Large Magellanic Cloud (LMC) analogs. These dwarfs were discovered as part of ...our Subaru+Hyper Suprime-Cam MADCASH survey: MADCASH-1 is a satellite of NGC 2403 (
D
∼ 3.2 Mpc), and MADCASH-2 is a previously unknown dwarf galaxy near NGC 4214 (
D
∼ 3 Mpc). Our HST data reach >3.5 mag below the tip of the red giant branch (TRGB) of each dwarf, allowing us to derive their structural parameters and assess their stellar populations. We measure TRGB distances (
D
MADCASH
−
1
=
3.41
−
0.23
+
0.24
Mpc,
D
MADCASH
−
2
=
3.00
−
0.15
+
0.13
Mpc), and confirm the dwarfs’ associations with their host galaxies. MADCASH-1 is a predominantly old, metal-poor stellar system (age ∼13.5 Gyr, M/H ∼ −2.0), similar to many Local Group dwarfs. Modelling of MADCASH-2's color–magnitude diagram suggests that it contains mostly ancient, metal-poor stars (age ∼13.5 Gyr, M/H ∼ −2.0), but that ∼10% of its stellar mass was formed 1.1–1.5 Gyr ago and ∼1% was formed 400–500 Myr ago. Given its recent star formation, we search MADCASH-2 for neutral hydrogen using the Green Bank Telescope, but find no emission and estimate an upper limit on the H
i
mass of <4.8 × 10
4
M
⊙
. These are the faintest dwarf satellites known around host galaxies of LMC mass outside the Local Group (
M
V
,MADCASH−1
= −7.81 ± 0.18,
M
V
,MADCASH−2
= −9.15 ± 0.12), and one of them shows signs of recent environmental quenching by its host. Once the MADCASH survey for faint dwarf satellites is complete, our census will enable us to test predictions from cold dark matter models for hierarchical structure formation and discover the physical mechanisms by which low-mass hosts influence the evolution of their satellites.
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