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Register a new userA Faint New Milky Way Satellite in Bootes
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V. Belokurov
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In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.
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[abridged] We present MMT/Megacam imaging in Sloan $g$ and $r$ of the extremely low luminosity Bootes II Milky Way companion. We use a bootstrap approach to perform robust measurements of, and uncertainties on, Bootes IIs distance, luminosity, size, and morphology. We show that Bootes IIs stellar population is old and metal-poor ([Fe/H] $lta$ -2). Assuming a stellar population like that of M92, Bootes II is at a distance of 42 $pm$ 2 kpc, closer than the initial published estimate of 60 $pm$ 10 kpc. This distance revision, combined with a more robust measurement of Bootes IIs structure with a Plummer model (exponential model) results in a more compact half-light size of $r_hsimeq 36 (33) pm 9 (10)$ pc and lower luminosity of $M_Vsimeq-2.4 (-2.2) pm 0.7 (0.7)$ mag. This revised size and luminosity move Bootes II into a region of size-luminosity space not previously known to be occupied by old stellar populations, but also occupied by the recently discovered Milky Way satellites Willman 1 and SEGUE 1. We show that the apparently distorted morphology of Bootes II is not statistically significant given the present data. We use a tidal argument to support a scenario where Bootes II is a dwarf galaxy (dark matter dominated) rather than a globular cluster (not dark matter dominated). However, we can not rule out that Bootes II is a star cluster on the verge of disruption, such as Palomar 5.
We present the discovery of a faint Milky Way satellite, Laevens 2/Triangulum II, found in the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS 1) 3 pi imaging data and confirmed with follow-up wide-field photometry from the Large Binocular Cameras. The stellar system, with an absolute magnitude of M_V=-1.8 +/-0.5, a heliocentric distance of 30 +2/-2 kpc, and a half-mass radius of 34 +9/-8 pc, shows remarkable similarity to faint, nearby, small satellites such as Willman 1, Segue 1, Segue 2, and Bootes II. The discovery of Laevens 2/Triangulum II further populates the region of parameter space for which the boundary between dwarf galaxies and globular clusters becomes tenuous. Follow-up spectroscopy will ultimately determine the nature of this new satellite, whose spatial location hints at a possible connection with the complex Triangulum-Andromeda stellar structures.
In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD) shows a well-defined sub-giant branch, main sequence and turn-off, from which we estimate a distance of ~30 kpc and a projected size of ~250 x 125 pc. The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of M_V ~ -3.8, which would make it the faintest known satellite galaxy. UMa IIs isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.
In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellites color-magnitude diagram shows a well-defined red giant branch, as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of ~220 kpc. Based on the SDSS data, we estimate an absolute magnitude of Mv ~ -7.9, a central surface brightness of mu_0,V ~ 28 mag arcsecond^-2, and a half-light radius of ~ 8.5 (~ 550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.
We report on the discovery of a new Milky Way (MW) satellite in Bootes based on data from the on-going Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Bootes IV, is the third ultra-faint dwarf that we have discovered in the HSC-SSP. We have identified a statistically significant (32.3$sigma$) overdensity of stars having characteristics of a metal-poor, old stellar population. The distance to this stellar system is $D_{odot}=209^{+20}_{-18}$ kpc with a $V$-band absolute magnitude of $M_V=-4.53^{+0.23}_{-0.21}$ mag. Bootes IV has a half-light radius of $r_h=462^{+98}_{-84}$ pc and an ellipticity of $0.64^{+0.05}_{-0.05}$, which clearly suggests that this is a dwarf satellite galaxy. We also found another overdensity that appears to be a faint globular cluster with $M_V=-0.20^{+0.59}_{-0.83}$ mag and $r_h=5.9^{+1.5}_{-1.3}$ pc located at $D_{odot}=46^{+4}_{-4}$ kpc. Adopting the recent prediction for the total population of satellites in a MW-sized halo by Newton et al. (2018), which combined the characteristics of the observed satellites by SDSS and DES with the subhalos obtained in $Lambda$CDM models, we estimate that there should be about two MW satellites at $M_Vle0$ in the $sim676$ deg$^2$ covered by HSC-SSP, whereas that area includes six satellites. Thus, the observed number of satellites is larger than the theoretical prediction. On the face of it, we have a problem of too many satellites, instead of the well-known missing satellites problem whereby the $Lambda$CDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinements for the assignment of subhalos to satellites such as considering those found by the current deeper survey. [abridged]
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