Substructure in tidal streams: Tributaries in the anticenter stream

We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65°-long, stellar stream complex previously identified with the Monoceros Ring. The three-stream complex varies in width from 4° to 6° along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies, and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of ≈8 km s^-1. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components that may have included a native population of globular clusters. While the color of the main-sequence turnoff is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions

Another Look at the Eastern Banded Structure: A Stellar Debris Stream and a Possible Progenitor

Using the Sloan Digital Sky Survey Data Release 7, we re-examine the Eastern Banded Structure (EBS), a stellar debris stream first discovered in Data Release 5 and more recently detected in velocity space by Schlaufman et al. The visible portion of the stream is 18° long, lying roughly in the Galactic Anticenter direction and extending from Hydra to Cancer. At an estimated distance of 9.7 kpc, the stream is ≈170 pc across on the sky. The curvature of the stream implies a fairly eccentric box orbit that passes close to both the Galactic center and to the Sun, making it dynamically distinct from the nearby Monoceros, Anticenter, and GD-1 streams. Within the stream is a relatively strong, 2°-wide concentration of stars with a very similar color-magnitude distribution that we designate Hydra I. Given its prominence within the stream and its unusual morphology, we suggest that Hydra I is the last vestige of EBS's progenitor, possibly already unbound or in the final throes of tidal dissolution. Though both Hydra I and the EBS have a relatively high-velocity dispersion, given the comparatively narrow width of the stream and the high frequency of encounters with the bulge and massive constituents of the disk that such an eccentric orbit would entail, we suggest that the progenitor was likely a globular cluster and that both it and the stream have undergone significant heating over time

Four New Stellar Debris Streams in the Galactic Halo

We report on the detection of four new stellar debris streams and a new dwarf galaxy candidate in the Sloan Digital Sky Survey. Three of the streams, ranging between 3 and 15 kpc in distance and spanning between 37 and 84 degrees on the sky, are very narrow and are most probably tidal streams originating in extant or disrupted globular clusters. The fourth stream is much broader, roughly 45 kpc distant, at least 53 degrees in length, and is most likely the tidal debris from a dwarf galaxy. As the streams each span multiple constellations, we extend tradition and designate them the Acheron, Cocytos, Lethe, and Styx streams. At the same distance and apparently embedded in the Styx stream is a ~1 kpc-wide concentration of stars with a similar color-magnitude distribution which we designate Bootes III. Given its very low surface density, its location within the stream, and its apparently disturbed morphology, we argue that Bootes III may be the progenitor of Styx and in possibly the final throes of tidal dissolution. While the current data do not permit strong constraints, preliminary orbit estimates for the streams do not point to any likely progenitors among the known globular clusters and dwarf galaxies.Comment: 10 pages, accepted for publication in the Astrophysical Journa

Substructure in Tidal Streams; Tributaries in the Anticenter Ring

We report on the detection in Sloan Digital Sky Survey data of at least three, roughly parallel components in a 65 degree-long stellar stream complex previously identified with the Anticenter or Monoceros Ring. The three-stream complex varies in width from 4 to 6 degrees along its length and appears to be made up of two or more narrow substreams as well as a broader, diffuse component. The width and complexity of the stream indicate that the progenitor was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc distant and is oriented almost perpendicularly to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies and a preliminary orbit does not point to any viable progenitor candidates. Orbits for the narrower substreams can be modeled with velocity offsets from the broad component of about 8 km/s. We suggest that the broad component is likely to be the remains of a dwarf galaxy, while the narrower streams constitute the remnants of dynamically distinct components which may have included a native population of globular clusters. While the color of the main sequence turn-off is not unlike that for the Monoceros Ring, neither the visible stream nor any reasonable projection of its orbit passes through Monoceros or Canis Major, and we conclude that this stream is probably unrelated to the overdensities found in these regions.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter

Two New Halo Debris Streams in the Sloan Digital Sky Survey

Using photometry from Data Release 10 of the northern footprint of the Sloan Digital Sky Survey, we detect two new stellar streams with lengths of between 25° and 50°. The streams, which we designate Hermus and Hyllus, are at distances of between 15 and 23 kpc from the Sun and pass primarily through Hercules and Corona Borealis. Stars in the streams appear to be metal-poor, with [Fe/H]   ~   − 2.3, though we cannot rule out metallicities as high as [Fe/H] = −1.2. While Hermus passes within 1° (in projection) of the globular cluster NGC 6229, a roughly one magnitude difference in distance modulus, combined with no signs of connecting with NGC 6229's Roche lobe, argue against any physical association between the two. Though the two streams almost certainly had different progenitors, similarities in preliminary orbit estimates suggest that those progenitors may themselves have been a product of a single accretion event

Detection of a 60 Degree-Long Dwarf Galaxy Debris Stream

We report on a 60 degree-long stream of stars, extending from Ursa Major to Sextans, in the Sloan Digital Sky Survey. The stream is approximately 2 degrees wide and is clearly distinct from the northern tidal arm of the Sagittarius dwarf galaxy. The apparent width of the stream suggests a progenitor with a size and mass similar to that of a dwarf galaxy. The stream is about 21 kpc distant and appears to be oriented almost perpendicular to our line of sight. The visible portion of the stream does not pass near any known dwarf galaxies, though we cannot rule out that the stream may form the inner part of a known dwarf galaxy's orbit. The most likely explanation is that the stream constitutes the remains a dwarf galaxy that has been completely disrupted at some point in the past. We also briefly report on the discovery of a diminutive Galactic satellite which lies near the projected path of the new stream, but is unlikely to be related to it.Comment: Modified to conform to published versio

A 22 Degree Tidal Tail for Palomar 5

Using Data Release 4 of the Sloan Digital Sky Survey, we have applied an optimal contrast, matched filter technique to trace the trailing tidal tail of the globular cluster Palomar 5 to a distance of 18.5 degrees from the center of the cluster. This more than doubles the total known length of the tail to some 22 degrees on the sky. Based on a simple model of the Galaxy, we find that the stream's orientation on the sky is consistent at the 1.7 sigma level with existing proper motion measurements. We find that a spherical Galactic halo is adequate to model the stream over its currently known length, and we are able to place new constraints on the current space motion of the cluster.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter

Velocity Variations in the Phoenix-Hermus Star Stream

Measurements of velocity and density perturbations along stellar streams in the Milky Way provide a time-integrated measure of dark matter substructure at larger galactic radius than the complementary instantaneous inner-halo strong lensing detection of dark matter sub-halos in distant galaxies. An interesting case to consider is the proposed Phoenix–Hermus star stream, which is long, thin, and on a nearly circular orbit, making it a particular good target to study for velocity variations along its length. In the presence of dark matter sub-halos, the stream velocities are significantly perturbed in a manner that is readily understood with the impulse approximation. A set of simulations shows that only sub-halos above a few 10^7 M_⊙ lead to reasonably long-lived observationally detectable velocity variations of amplitude of order 1 km s^(−1), with an average of about one visible hit per (two-armed) stream over a 3 Gyr interval. An implication is that globular clusters themselves will not have a visible impact on the stream. Radial velocities have the benefit of being completely insensitive to distance errors. Distance errors scatter individual star velocities perpendicular and tangential to the mean orbit, but their mean values remain unbiased. Calculations like these help build the quantitative case to acquire large, fairly deep, precision velocity samples of stream stars