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Register a new userEvidence of a dwarf galaxy stream populating the inner Milky Way Halo
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Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 dataset using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. (2020). We detect LMS-1 as a $60deg$ long stream to the north of the Galactic bulge, at a distance of $sim 20$ kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from LAMOST, SDSS and APOGEE, we infer that the stream is very metal poor (${rm langle [Fe/H]rangle =-2.1}$) with a significant metallicity dispersion ($sigma_{rm [Fe/H]}=0.4$), and it possesses a large radial velocity dispersion (${rm sigma_v=20 pm 4,km,s^{-1}}$). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of $75deg$ to the Galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024 and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.
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The Eastern Banded Structure (EBS) and Hydra~I halo overdensity are very nearby (d $sim$ 10 kpc) objects discovered in SDSS data. Previous studies of the region have shown that EBS and Hydra I are spatially coincident, cold structures at the same distance, suggesting that Hydra I may be the EBSs progenitor. We combine new wide-field DECam imaging and MMT/Hectochelle spectroscopic observations of Hydra I with SDSS archival spectroscopic observations to quantify Hydra Is present-day chemodynamical properties, and to infer whether it originated as a star cluster or dwarf galaxy. While previous work using shallow SDSS imaging assumed a standard old, metal-poor stellar population, our deeper DECam imaging reveals that Hydra~I has a thin, well-defined main sequence turnoff of intermediate age ($sim 5-6$ Gyr) and metallicity ([Fe/H] = $-0.9$ dex). We measure statistically significant spreads in both the iron and alpha-element abundances of $sigma_{[Fe/H]} = 0.13 pm 0.02$ dex and $sigma_{[alpha/{rm Fe}]} = 0.09 pm 0.03$ dex, respectively, and place upper limits on both the rotation and its proper motion. Hydra~Is intermediate age and [Fe/H] -- as well as its low [$alpha$/Fe], apparent [Fe/H] spread, and present-day low luminosity -- suggest that its progenitor was a dwarf galaxy, which subsequently lost more than $99.99%$ of its stellar mass.
We present the analysis of 1,207 RR Lyrae found in photometry taken by the Catalina Surveys Mount Lemmon telescope. By combining accurate distances for these stars with measurements for ~14,000 type-AB RR Lyrae from the Catalina Schmid telescope, we reveal an extended association that reaches Galactocentric distances beyond 100 kpc and overlaps the Sagittarius streams system. This result confirms earlier evidence for the existence of an outer halo tidal stream resulting from a disrupted stellar system. By comparing the RR Lyrae source density with that expected based on halo models, we find the detection has ~8 sigma significance. We investigate the distances, radial velocities, metallicities, and period-amplitude distribution of the RR Lyrae. We find that both radial velocities and distances are inconsistent with current models of the Sagittarius stream. We also find tentative evidence for a division in source metallicities for the most distant sources. Following prior analyses, we compare the locations and distances of the RR Lyrae with photometrically selected candidate horizontal branch stars and find supporting evidence that this structure spans at least 60 deg of the sky. We investigate the prospects of an association between the stream and unusual globular cluster NGC 2419.
Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.
The following is a comment on the recent letter by Iocco et al. (2015, arXiv:1502.03821) where the authors claim to have found ...convincing proof of the existence of dark matter.... The letter in question presents a compilation of recent rotation curve observations for the Milky Way, together with Newtonian rotation curve estimates based on recent baryonic matter distribution measurements. A mismatch between the former and the latter is then presented as evidence for dark matter. Here we show that the reported discrepancy is the well known gravitational anomaly which consistently appears when dynamical accelerations approach the critical Milgrom acceleration a_0 = 1.2 times 10^{-10} m / s^2. Further, using a simple modified gravity force law, the baryonic models presented in Iocco et al. (2015), yield dynamics consistent with the observed rotation values.
We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180 deg to l = 227 deg with turnoff magnitude $g sim 19.5$; the ring could encircle the Galaxy. We infer that the low Galactic latitude structure is at a fairly constant distance of R = 18 +/- 2 kpc from the Galactic Center above the Galactic plane, and has R = 20 +/- 2 kpc in the region sampled below the Galactic plane. The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 degrees of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b) = (198, -27), 22 km/s at (l,b) = (225, 28), 30 km/s at (l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure rotates in the same prograde direction as the Galactic disk stars, but with a circular velocity of 110+/-25 km/s. The narrow measured velocity dispersion is inconsistent with power law spheroid or thick disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l, b) = (165, -55) deg. We estimate a preliminary metallicity from the Ca II (K) line and color of the turnoff stars of [Fe/H] = -1.6 with a dispersion of 0.3 dex and note that the turnoff color is consistent with that of the spheroid population. We interpret our measurements as evidence for a tidally disrupted satellite of $2 times 10^7$ to $5 times 10^8 rm M_odot$ which rings the Galaxy.
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