Mapping the Galactic Halo. III. Simulated Observations of Tidal Streams

Abstract This paper uses statistical and N-body methods to explore a new mechanism to form binary stars with extremely large separations (≳ 0.1 pc), whose origin is poorly understood. Here, ultra-wide binaries arise via chance entrapment of unrelated stars in tidal streams of disrupting clusters. It is shown that (i) the formation of ultra-wide binaries is not limited to the lifetime of a cluster, but continues after the progenitor is fully disrupted, (ii) the formation rate is proportional to the local phase-space density of the tidal tails, (iii) the semimajor axis distribution scales as p(a)da ∼ a1/2da at a ≪ D, where D is the mean interstellar distance, and (vi) the eccentricity distribution is close to thermal, p(e)de = 2ede. Owing to their low binding energies, ultra-wide binaries can be disrupted by both the smooth tidal field and passing substructures. The time-scale on which tidal fluctuations dominate over the mean field is inversely proportional to the local density of compact substructures. Monte-Carlo experiments show that binaries subject to tidal evaporation follow p(a)da ∼ a−1da at a ≳ apeak, known as Öpik’s law, with a peak semi-major axis that contracts with time as apeak ∼ t−3/4. In contrast, a smooth Galactic potential introduces a sharp truncation at the tidal radius, p(a) ∼ 0 at a ≳ rt. The scaling relations of young clusters suggest that most ultra-wide binaries arise from the disruption of low-mass systems. Streams of globular clusters may be the birthplace of hundreds of ultra-wide binaries, making them ideal laboratories to probe clumpiness in the Galactic halo.

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Quantifying tidal stream disruption in a simulated Milky Way

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx1268 ◽

2017 ◽

Vol 470 (1) ◽

pp. 522-538 ◽

Cited By ~ 10

Author(s):

Emily Sandford ◽

Andreas H. W. Küpper ◽

Kathryn V. Johnston ◽

Jürg Diemand

Keyword(s):

Dark Matter ◽

Milky Way ◽

Galactic Halo ◽

Mass Range ◽

Galactic Centre ◽

High Dimensions ◽

Density Profiles ◽

Upper Limits ◽

Tidal Stream ◽

Tidal Streams

Abstract Simulations of tidal streams show that close encounters with dark matter subhaloes induce density gaps and distortions in on-sky path along the streams. Accordingly, observing disrupted streams in the Galactic halo would substantiate the hypothesis that dark matter substructure exists there, while in contrast, observing collimated streams with smoothly varying density profiles would place strong upper limits on the number density and mass spectrum of subhaloes. Here, we examine several measures of stellar stream ‘disruption' and their power to distinguish between halo potentials with and without substructure and with different global shapes. We create and evolve a population of 1280 streams on a range of orbits in the Via Lactea II simulation of a Milky Way-like halo, replete with a full mass range of Λcold dark matter subhaloes, and compare it to two control stream populations evolved in smooth spherical and smooth triaxial potentials, respectively. We find that the number of gaps observed in a stellar stream is a poor indicator of the halo potential, but that (i) the thinness of the stream on-sky, (ii) the symmetry of the leading and trailing tails and (iii) the deviation of the tails from a low-order polynomial path on-sky (‘path regularity') distinguish between the three potentials more effectively. We furthermore find that globular cluster streams on low-eccentricity orbits far from the galactic centre (apocentric radius ∼30–80 kpc) are most powerful in distinguishing between the three potentials. If they exist, such streams will shortly be discoverable and mapped in high dimensions with near-future photometric and spectroscopic surveys.

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The course of the Orphan Stream in the Northern Galactic hemisphere traced with Gaia DR2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz749 ◽

2019 ◽

Vol 486 (1) ◽

pp. 936-949 ◽

Cited By ~ 7

Author(s):

Mark A Fardal ◽

Roeland P van der Marel ◽

Sangmo Tony Sohn ◽

Andres del Pino Molina

Keyword(s):

Proper Motion ◽

Galactic Halo ◽

Dwarf Galaxy ◽

Three Dimensional ◽

Red Giants ◽

Independent Evidence ◽

Horizontal Branch Stars ◽

Red Giant ◽

Tidal Streams ◽

Using Data

Abstract The Orphan Stream is one of the most prominent tidal streams in the Galactic halo. Using data on red giants, RR Lyrae, and horizontal branch stars from Gaia and other surveys, we determine the proper motion of the Orphan Stream over a path of more than 90° on the sky. We also provide updated tracks for the sky position, distance, and radial velocity of the stream. Our tracks in these latter dimensions mostly agree with previous results. However, there are significant corrections to the earlier distance and latitude tracks as the stream approaches the Galactic disc. Stream stars selected with three-dimensional kinematics display a very tight red giant sequence. Concordantly, we find that applying a proper motion cut removes the most metal-rich stars from earlier spectroscopic samples of stream stars, though a significant dispersion remains, indicating a dwarf galaxy origin. The deceleration of the stream towards its leading end suggests a circular velocity of $\sim \!200 \, \mbox{km}\, \mbox{s}^{-1}$ at a galactocentric radius $\sim \!30 \, \mbox{kpc}$, consistent with other independent evidence. However, the track of the stream departs significantly from an orbit; the spatial track does not point along the same direction as the velocity vector, and it exhibits a lateral wiggle that is unlikely to match any reasonable orbit. The low metallicity and small dispersion of the stream in the various coordinates point to a progenitor with a relatively low dynamical mass $\sim \!10^8 \, \mathrm{ M}_{\odot }$.

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The Milky Way, the Galactic Halo, and the Halos of Galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s174392131600003x ◽

2015 ◽

Vol 11 (S317) ◽

pp. 266-271

Author(s):

Ortwin Gerhard

Keyword(s):

Milky Way ◽

Galactic Halo ◽

Large Fraction ◽

Three Dimensional ◽

Dimensional Structure ◽

Galaxy Halos ◽

The Subject ◽

Hierarchical Nature ◽

Tidal Streams ◽

The Many

AbstractThe Milky Way, “our” Galaxy, is currently the subject of intense study with many ground-based surveys, in anticipation of upcoming results from the Gaia mission. From this work we have been learning about the full three-dimensional structure of the Galactic box/peanut bulge, the distribution of stars in the bar and disk, and the many streams and substructures in the Galactic halo. The data indicate that a large fraction of the Galactic halo has been accreted from outside. Similarly, in many external galaxy halos there is now evidence for tidal streams and accretion of satellites. To study these features requires exquisite, deep photometry and spectroscopy. These observations illustrate how galaxy halos are still growing, and sometimes can be used to “time” the accretion events. In comparison with cosmological simulations, the structure of galaxy halos gives us a vivid illustration of the hierarchical nature of our Universe.

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FOSSILs in the Galactic Halo

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318007111 ◽

2018 ◽

Vol 14 (S344) ◽

pp. 73-76

Author(s):

Carl J. Grillmair

Keyword(s):

Galactic Halo ◽

Galactic Center ◽

Dwarf Galaxies ◽

Matched Filter ◽

The North ◽

History Of ◽

Tidal Streams ◽

North Galactic ◽

Size Scales

AbstractWe use a matched filter to detect compact groups of old, metal-poor stars that we term FOSSILs (Fragments of Old Stellar Systems in Limbo). With size scales on the order of 10 arcminutes, distances ranging from 2 to 200 kpc, and memberships ranging from a handful to several dozen stars, these FOSSILs stand out from the surrounding field and are presumably signatures of, or debris from, ancient star clusters and dwarf galaxies. They may be localized concentrations of stars within more extensive tidal streams, and in some cases may be the signatures of extant but heretofore undetected ultrafaint galaxies. Using magnitudes and colors from the Pan-STARRs survey, we detect ∼ 70 such FOSSILs at 5 σ or greater in a 2200 square degree region in the vicinity of the north Galactic pole. A subsample of more populous FOSSILs that could be candidate ultrafaint dwarf galaxies suggests a total population of 200 such objects within 200 kpc of the Galactic center. Spectroscopic and astrometric follow-up of these FOSSILs will be required to determine the nature of these structures, deepen our understanding of the make-up and accretion history of the Galactic halo, and perhaps alleviate the missing satellites problem.

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Tidal Streams in the Galactic Halo: Evidence for the Sagittarius Northern Stream or Traces of a New Nearby Dwarf Galaxy

The Astrophysical Journal ◽

10.1086/319167 ◽

2001 ◽

Vol 549 (2) ◽

pp. L199-L202 ◽

Cited By ~ 60

Author(s):

David Martínez-Delgado ◽

Antonio Aparicio ◽

M. Ángeles Gómez-Flechoso ◽

Ricardo Carrera

Keyword(s):

Galactic Halo ◽

Dwarf Galaxy ◽

Tidal Streams ◽

Northern Stream

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Distribution of R- and S-Process Elements in the Galactic Halo

Symposium - International Astronomical Union ◽

10.1017/s0074180900035555 ◽

1988 ◽

Vol 132 ◽

pp. 501-506

Author(s):

C. Sneden ◽

C. A. Pilachowski ◽

K. K. Gilroy ◽

J. J. Cowan

Keyword(s):

Heavy Element ◽

Galactic Halo ◽

Low Noise ◽

Heavy Elements ◽

Process Synthesis ◽

Element Abundances ◽

Halo Stars ◽

Abundance Patterns ◽

R Process ◽

Process Elements

Current observational results for the abundances of the very heavy elements (Z>30) in Population II halo stars are reviewed. New high resolution, low noise spectra of many of these extremely metal-poor stars reveal general consistency in their overall abundance patterns. Below Galactic metallicities of [Fe/H] Ã −2, all of the very heavy elements were manufactured almost exclusively in r-process synthesis events. However, there is considerable star-to-star scatter in the overall level of very heavy element abundances, indicating the influence of local supernovas on element production in the very early, unmixed Galactic halo. The s-process appears to contribute substantially to stellar abundances only in stars more metal-rich than [Fe/H] Ã −2.

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