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Title: Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?

Abstract

Thanks to modern sky surveys, over twenty stellar streams and overdensity structures have been discovered in the halo of the Milky Way. Here, in this paper, we present an analysis of spectroscopic observations of individual stars from one such structure, "A13", first identified as an overdensity using the M giant catalog from the Two Micron All-Sky Survey. Our spectroscopic observations show that stars identified with A13 have a velocity dispersion of $$\lesssim$$ 40 $$\mathrm{km~s^{-1}}$$, implying that it is a genuine coherent structure rather than a chance super-position of random halo stars. From its position on the sky, distance ($$\sim$$15 kpc heliocentric), and kinematical properties, A13 is likely to be an extension of another low Galactic latitude substructure -- the Galactic Anticenter Stellar Structure (also known as the Monoceros Ring) -- towards smaller Galactic longitude and farther distance. Furthermore, the kinematics of A13 also connect it with another structure in the southern Galactic hemisphere -- the Triangulum-Andromeda overdensity. Finally, we discuss these three connected structures within the context of a previously proposed scenario that one or all of these features originate from the disk of the Milky Way.

Authors:
 [1];  [2];  [3]; ORCiD logo [4]; ORCiD logo [5]; ORCiD logo [6];  [7]; ORCiD logo [8]; ORCiD logo [9];  [5]; ORCiD logo [10]; ORCiD logo [11]
  1. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Texas A & M Univ., College Station, TX (United States). Department of Physics & Astronomy; George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, College Station, TX (United States)
  2. City University of New York, LaGuardia Community College, Long Island City, NY (United States). Department of Natural Sciences
  3. Columbia Univ., New York, NY (United States). Department of Astronomy
  4. Texas A & M Univ., College Station, TX (United States). Department of Physics & Astronomy; George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, College Station, TX (United States)
  5. Univ. of Virginia, Charlottesville, VA (United States). Department of Astronomy
  6. Princeton Univ., NJ (United States). Department of Astrophysical Sciences
  7. Univ. of Virginia, Charlottesville, VA (United States). Department of Astronomy; Universidad de La Serena (Chile). Departamento de Fisica y Astronomia, Facultad de Ciencias
  8. The Carnegie Observatories, Pasadena, CA (United States)
  9. Universite Cote d Azur, OCA, CNRS, Lagrange (France)
  10. Univ. of Sydney, NSW (Australia). Sydney Institute for Astronomy, School of Physics
  11. Max Planck Institute for Astronomy, Heidelberg (Germany)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1395476
Report Number(s):
arXiv:1703.05384; FERMILAB-PUB-17-389-AE
Journal ID: ISSN 1538-4357; 1624631
Grant/Contract Number:
AC02-07CH11359
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 844; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; galaxies: interactions; Galaxy: disk; Galaxy: formation; Galaxy: halo; Galaxy: structure

Citation Formats

Li, Ting S., Sheffield, Allyson A., Johnston, Kathryn V., Marshall, Jennifer L., Majewski, Steven R., Price-Whelan, Adrian M., Damke, Guillermo J., Beaton, Rachael L., Bernard, Edouard J., Richardson, Whitney, Sharma, Sanjib, and Sesar, Branimir. Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?. United States: N. p., 2017. Web. doi:10.3847/1538-4357/aa7a0d.
Li, Ting S., Sheffield, Allyson A., Johnston, Kathryn V., Marshall, Jennifer L., Majewski, Steven R., Price-Whelan, Adrian M., Damke, Guillermo J., Beaton, Rachael L., Bernard, Edouard J., Richardson, Whitney, Sharma, Sanjib, & Sesar, Branimir. Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?. United States. doi:10.3847/1538-4357/aa7a0d.
Li, Ting S., Sheffield, Allyson A., Johnston, Kathryn V., Marshall, Jennifer L., Majewski, Steven R., Price-Whelan, Adrian M., Damke, Guillermo J., Beaton, Rachael L., Bernard, Edouard J., Richardson, Whitney, Sharma, Sanjib, and Sesar, Branimir. 2017. "Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?". United States. doi:10.3847/1538-4357/aa7a0d.
@article{osti_1395476,
title = {Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?},
author = {Li, Ting S. and Sheffield, Allyson A. and Johnston, Kathryn V. and Marshall, Jennifer L. and Majewski, Steven R. and Price-Whelan, Adrian M. and Damke, Guillermo J. and Beaton, Rachael L. and Bernard, Edouard J. and Richardson, Whitney and Sharma, Sanjib and Sesar, Branimir},
abstractNote = {Thanks to modern sky surveys, over twenty stellar streams and overdensity structures have been discovered in the halo of the Milky Way. Here, in this paper, we present an analysis of spectroscopic observations of individual stars from one such structure, "A13", first identified as an overdensity using the M giant catalog from the Two Micron All-Sky Survey. Our spectroscopic observations show that stars identified with A13 have a velocity dispersion of $\lesssim$ 40 $\mathrm{km~s^{-1}}$, implying that it is a genuine coherent structure rather than a chance super-position of random halo stars. From its position on the sky, distance ($\sim$15 kpc heliocentric), and kinematical properties, A13 is likely to be an extension of another low Galactic latitude substructure -- the Galactic Anticenter Stellar Structure (also known as the Monoceros Ring) -- towards smaller Galactic longitude and farther distance. Furthermore, the kinematics of A13 also connect it with another structure in the southern Galactic hemisphere -- the Triangulum-Andromeda overdensity. Finally, we discuss these three connected structures within the context of a previously proposed scenario that one or all of these features originate from the disk of the Milky Way.},
doi = {10.3847/1538-4357/aa7a0d},
journal = {The Astrophysical Journal (Online)},
number = 1,
volume = 844,
place = {United States},
year = 2017,
month = 7
}

Journal Article:
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  • As large-scale stellar surveys have become available over the past decade, the ability to detect and characterize substructures in the Galaxy has increased dramatically. These surveys have revealed the Triangulum-Andromeda (TriAnd) region to be rich with substructures in the distance range 20-30 kpc, and the relation of these features to each other, if any, remains unclear. An exploration using Two Micron All Sky Survey (2MASS) photometry reveals not only the faint sequence in M giants detected by Rocha-Pinto et al. spanning the range 100° < l < 160° and –50° < b < –15°, but, in addition, a second, brightermore » and more densely populated sequence. These sequences are likely associated with the distinct main sequences (MSs) discovered (and labeled TriAnd1 and TriAnd2) by Martin et al. in an optical survey in the direction of M31, where TriAnd2 is the optical counterpart of the fainter red giant branch (RGB)/asymptotic giant branch sequence of Rocha-Pinto et al. Here, the age, distance, and metallicity ranges for TriAnd1 and TriAnd2 are estimated by simultaneously fitting isochrones to the 2MASS RGB tracks and the optical MS/MS turn-off features. The two populations are clearly distinct in age and distance: the brighter sequence (TriAnd1) is younger (6-10 Gyr) and closer (distance of ∼15-21 kpc), whereas the fainter sequence (TriAnd2) is older (10-12 Gyr) and at an estimated distance of ∼24-32 kpc. A comparison with simulations demonstrates that the differences and similarities between TriAnd1 and TriAnd2 can simultaneously be explained if they represent debris originating from the disruption of the same dwarf galaxy, but torn off during two distinct pericentric passages.« less
  • We present the latitude-normalized radial velocity (v{sub b} ) distribution of 3318 subsolar metallicity, V {approx}< 13.5 stars from the Grid Giant Star Survey (GGSS) in southern hemisphere fields. The sample includes giants mostly within {approx}5 kpc from the Galactic disks and halo. The nearby halo is found to (1) exhibit significant kinematical substructure, and (2) be prominently represented by several velocity coherent structures, including a very retrograde 'cloud' of stars at l {approx} 285 Degree-Sign and extended, retrograde 'streams' visible as relatively tight l-v{sub b} sequences. One sequence in the fourth Galactic quadrant lies within the l-v{sub b} spacemore » expected to contain tidal debris from the 'star cluster' {omega}Centauri. Not only does {omega}Cen lie precisely in this l-v{sub b} sequence, but the positions and v{sub b} of member stars match those of N-body simulations of tidally disrupting dwarf galaxies on orbits ending with {omega}Cen's current position and space motion. But the ultimate proof that we have very likely found extended parts of the {omega}Cen tidal stream comes from echelle spectroscopy of a subsample of the stars that reveals a very particular chemical abundance signature known to occur only in {omega}Cen. The newly discovered {omega}Cen debris accounts for almost all fourth Galactic quadrant retrograde stars in the southern GGSS, which suggests {omega}Cen is a dominant contributor of retrograde giant stars in the inner Galaxy.« less
  • We reveal the highly structured nature of the Milky Way (MW) stellar halo within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS) photometric survey from blue main sequence (MS) and MS turn-off stars. We map no fewer than five stellar structures within a heliocentric range of ∼5-30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of ∼17more » kpc. With a surface brightness of Σ {sub V} ∼ 32-32.5 mag arcsec{sup –2}, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the MW halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km s{sup –1} at the 90% confidence level. Along with the width of the stream (300-650 pc), its dynamics point to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the MW stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances.« less
  • We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey, a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam Wide-Field Camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H] = -1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (M{sub V} = -9.9 +- 0.6),more » albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II, and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (M{sub V} = -6.5 +- 0.8) and lies a lot closer in projection to M33 than it does to M31 (42 versus 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20.« less
  • We present a sample of {approx}5000 RR Lyrae stars selected from the recalibrated LINEAR data set and detected at heliocentric distances between 5 kpc and 30 kpc over {approx}8000 deg{sup 2} of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We analyze in detail the light curve properties and Galactic distribution of the subset of {approx}4000 type ab RR Lyrae (RRab) stars, including a search for new halo substructures and the number density distribution as a function of Oosterhoff type. We find evidence for the Oosterhoff dichotomy among field RR Lyraemore » stars, with the ratio of the type II and I subsamples of about 1:4, but with a weaker separation than for globular cluster stars. The wide sky coverage and depth of this sample allow unique constraints for the number density distribution of halo RRab stars as a function of galactocentric distance: it can be described as an oblate ellipsoid with an axis ratio q = 0.63 and with either a single or a double power law with a power-law index in the range -2 to -3. Consistent with previous studies, we find that the Oosterhoff type II subsample has a steeper number density profile than the Oosterhoff type I subsample. Using the group-finding algorithm EnLink, we detected seven candidate halo groups, only one of which is statistically spurious. Three of these groups are near globular clusters (M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo Stellar Stream); the remaining three groups do not seem to be near any known halo substructures or globular clusters and seem to have a higher ratio of Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the halo. The extended morphology and the position (outside the tidal radius) of some of the groups near globular clusters are suggestive of tidal streams possibly originating from globular clusters. Spectroscopic follow-up of detected halo groups is encouraged.« less