Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userWith and without spectroscopy: Gaia DR2 proper motions of seven Ultra-Faint Dwarf Galaxies
335
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present mean absolute proper motion measurements for seven ultra-faint dwarf galaxies orbiting the Milky Way, namely Bo{o}tes III, Carina II, Grus II, Reticulum II, Sagittarius II, Segue 2 and Tucana IV. For four of these dwarfs our proper motion estimate is the first ever provided. The adopted astrometric data come from the second data release of the Gaia mission. We determine the mean proper motion for each galaxy starting from an initial guess of likely members, based either on radial velocity measurements or using stars on the Horizontal Branch identified in the Gaia ($G_{rm BP}$-$G_{rm RP}$,$G$) colour-magnitude diagram in the field of view towards the UFD. We then refine their membership iteratively using both astrometry and photometry. We take into account the full covariance matrix among the astrometric parameters when deriving the mean proper motions for these systems. Our procedure provides mean proper motions with typical uncertainties of $sim0.1$ mas/yr, even for galaxies without prior spectroscopic information. In the case of Segue 2 we find that using radial velocity members only leads to biased results, presumably because of the small number of stars with measured radial velocities. Our procedure allows to maximize the number of member stars per galaxy regardless of the existence of prior spectroscopic information, and can therefore be applied on any faint or distant stellar system within reach of Gaia.
rate research
Read More
The second data release from the Gaia mission (DR2) provides a comprehensive and unprecedented picture of the motions of astronomical sources in the plane of the sky, extending from the solar neighborhood to the outer reaches of the Milky Way. I present proper motion measurements based on Gaia DR2 for 17 ultra-faint dwarf galaxies within 100 kpc of the Milky Way. I compile the spectroscopically-confirmed member stars in each dwarf bright enough for Gaia astrometry from the literature, producing member samples ranging from 2 stars in Triangulum II to 68 stars in Bootes I. From the spectroscopic member catalogs I estimate the proper motion of each system. I find good agreement with the proper motions derived by the Gaia collaboration for Bootes I and Leo I. The tangential velocities for 14 of the 17 dwarfs are determined to better than 50 km/s, more than doubling the sample of such measurements for Milky Way satellite galaxies. The orbital pericenters are well-constrained, with a median value of 38 kpc. Only one satellite, Tucana III, is on an orbit passing within 15 kpc of the Galactic center, suggesting that the remaining ultra-faint dwarfs are unlikely to have experienced severe tidal stripping. As a group, the ultra-faint dwarfs are on high-velocity, eccentric, retrograde trajectories, with nearly all of them having space motions exceeding 370 km/s. In a low-mass (M_vir = 0.8 x 10^12 M_sun) Milky Way potential, eight out of the 17 galaxies lack well-defined apocenters and appear likely to be on their first infall, indicating that the Milky Way mass may be larger than previously estimated or that many of the ultra-faint dwarfs are associated with the Magellanic Clouds. The median eccentricity of the ultra-faint dwarf orbits is 0.79, similar to the values seen in numerical simulations, but distinct from the rounder orbits of the more luminous dwarf spheroidals.
With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest mass, ultra-faint satellite galaxies in the Milky Ways (MW) halo for the first time. Many of these faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their 6-dimensional phase space information, we calculate the orbital histories of 13 ultra-faint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: i.) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); ii.) Magellanic satellites that were recently captured by the MCs $<$ 1 Gyr ago (Reticulum 2, Phoenix 2); iii.) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and iv.) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMCs gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites.
We search for RR Lyrae stars in 27 nearby ($<100$ kpc) ultra-faint dwarf satellite galaxies using the Gaia DR2 catalog of RR Lyrae stars. Based on proper motions, magnitudes and location on the sky, we associate 47 Gaia RR Lyrae stars to 14 different satellites. Distances based on RR Lyrae stars are provided for those galaxies. We have identified RR Lyrae stars for the first time in the Tucana II dwarf galaxy, and find additional members in Ursa Major II, Coma Berenices, Hydrus I, Bootes I and Bootes III. In addition we have identified candidate extra-tidal RR Lyrae stars in six galaxies which suggest they may be undergoing tidal disruption. We found 10 galaxies have no RR Lyrae stars neither in Gaia nor in the literature. However, given the known completeness of Gaia DR2 we cannot conclude these galaxies indeed lack variable stars of this type.
We present ten new ultra-cool dwarfs in seven wide binary systems discovered using $textit{Gaia}$ DR2 data, identified as part of our $textit{Gaia}$ Ultra-Cool Dwarf Sample project. The seven systems presented here include an L1 companion to the G5 IV star HD 164507, an L1: companion to the V478 Lyr AB system, an L2 companion to the metal-poor K5 V star CD-28 8692, an M9 V companion to the young variable K0 V star LT UMa, and three low-mass binaries consisting of late Ms and early Ls. The HD 164507, CD-28 8692, V478 Lyr, and LT UMa systems are particularly important benchmarks, because the primaries are well characterised and offer excellent constraints on the atmospheric parameters and ages of the companions. We find that the M8 V star 2MASS J23253550+4608163 is $sim$2.5 mag overluminous compared to M dwarfs of similar spectral type, but at the same time it does not exhibit obvious peculiarities in its near-infrared spectrum. Its overluminosity cannot be explained by unresolved binarity alone. Finally, we present an L1+L2 system with a projected physical separation of 959 au, making this the widest L+L binary currently known.
We have analyzed the differences in positions of 9081 matched sources between the Gaia DR2 and VLBI catalogues. The median position uncertainty of matched sources in the VLBI catalogue is a factor of two larger than the median position uncertainty in the Gaia DR2. There are 9% matched sources with statistically significant offsets between both catalogues. We found that reported positional errors should be re-scaled by a factor of 1.3 for VLBI and 1.06 for Gaia, and in addition, Gaia errors should be multiplied by the square root of chi square per degree of freedom in order to best fit the normalized position differences to the Rayleigh distribution. We have established that the major contributor to statistically significant position offsets is the presence of optical jets. Among the sources for which the jet direction was determined, the position offsets are parallel to the jet directions for 62% of the outliers. Among the matched sources with significant proper motion, the fraction of objects with proper motion directions parallel to jets is a factor of 3 greater than on average. Such sources have systematically higher chi square per degree of freedom. We explain these proper motions as a manifestation of the source position jitter caused by flares that we have predicted earlier. Therefore, the assumption that quasars are fixed points and therefore, differential proper motions determined with respect to quasar photocenters can be regarded as absolute proper motions, should be treated with a great caution.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
