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Register a new userStellar kinematics in the remote Leo II dwarf spheroidal galaxy -- Another brick in the wall
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We present the projected velocity dispersion profile for the remote (d=233kpc) Galactic dwarf spheroidal (dSph) galaxy Leo II, based on 171 discrete stellar radial velocities that were obtained from medium-resolution spectroscopy using the FLAMES/GIRAFFE spectrograph at the European Southern Observatory, Chile. The dispersion profile of those stars with good membership probabilities is essentially flat with an amplitude of 6.6+-0.7 km/s over the full radial extent of our data, which probe to the stellar boundary of this galaxy. We find no evidence of any significant apparent rotation or velocity asymmetry which suggests that tidal effects cannot be invoked to explain Leo IIs properties. From basic mass modeling, employing Jeans equation, we derive a mass out to the limiting radius of (2.7+-0.5) 10^7 Msun and a global mass to light ratio of 27-45 in solar units, depending on the adopted total luminosity. A cored halo profile and a mild amount of tangential velocity anisotropy is found to account well for Leo IIs observed kinematics, although we cannot exclude the possibility of a cusped halo with radially varying velocity anisotropy. All in all, this galaxy exhibits dark matter properties which appear to be concordant with the other dSph satellites of the Milky Way, namely a halo mass profile which is consistent with a central core and a total mass which is similar to the common mass scale seen in other dSphs.
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Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy. Methods. By combining first-epoch $Hubble$ $Space$ $Telescope$ observations and second-epoch $Gaia$ Data Release 2 positions, we measured the proper motions of $149$ sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of $81$ red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of $45$ Draco members with high-precision and accurate 3D motions, which we present as a table in this paper. Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of $sim120$ pc from the centre of Draco to be $sigma_{R} =11.0^{+2.1}_{-1.5}$ km/s, $sigma_{T}=9.9^{+2.3}_{-3.1}$ km/s and $sigma_{LOS}=9.0^{+1.1}_{-1.1}$ km/s in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy $beta=0.25^{+0.47}_{-1.38}$ at $r gtrsim120$ pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than $approx 1/3$ of the escape velocity of the system. In this case, we constrain the maximum circular velocity $V_{max}$ of Draco to be in the range of $10.2-17.0$ km/s. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only. Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming $Gaia$ releases.
We present low-resolution spectroscopy of 120 red giants in the Galactic satellite dwarf spheroidal (dSph) Leo I, obtained with the GeminiN-GMOS and Keck-DEIMOS spectrographs. We find stars with velocities consistent with membership of Leo I out to 1.3 King tidal radii. By measuring accurate radial velocities with a median measurement error of 4.6 km/s we find a mean systemic velocity of 284.2 km/s with a global velocity dispersion of 9.9 km/s. The dispersion profile is consistent with being flat out to the last data point. We show that a marginally-significant rise in the radial dispersion profile at a radius of 3 is not associated with any real localized kinematical substructure. Given its large distance from the Galaxy, tides are not likely to have affected the velocity dispersion, a statement we support from a quantitative kinematical analysis, as we observationally reject the occurrence of a significant apparent rotational signal or an asymmetric velocity distribution. Mass determinations adopting both isotropic stellar velocity dispersions and more general models yield a M/L ratio of 24, which is consistent with the presence of a significant dark halo with a mass of about 3x10^7 M_sun, in which the luminous component is embedded. This suggests that Leo I exhibits dark matter properties similar to those of other dSphs in the Local Group. Our data allowed us also to determine metallicities for 58 of the targets. We find a mildly metal poor mean of -1.31 dex and a full spread covering 1 dex. In contrast to the majority of dSphs, Leo I appears to show no radial gradient in its metallicities, which points to a negligible role of external influences in this galaxys evolution.
In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of the early-type dwarf systems, we study one of the few isolated dwarf spheroidal galaxies (dSphs) of the Local Group (LG): Cetus. We obtained VLT/FORS2 spectra ($Rsim2600$) in the region of the nIR CaII triplet lines for 80 candidate red giant branch stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. The kinematic analysis shows that Cetus is a mainly pressure-supported ($sigma_v = 11.0_{-1.3}^{+1.6}$ km/s), dark-matter-dominated system ($M_{1/2}/L_V = 23.9_{-8.9}^{+9.7} M_odot/L_odot$) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant [Fe/H] spread (median [Fe/H] = -1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dSphs of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream. This study represents a first comprehensive analysis of Cetus chemo-kinematic properties. Our results add Cetus to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similar systems inhabiting different environments may hint at metallicity gradients in LG early-type dwarfs being driven by internal mechanisms.
We present metallicities and ages for 52 red giants in the remote Galactic dwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surface area of Leo II and are radial velocity members. We obtained medium-resolution multi-fiber spectroscopy with ESO/VLTs FLAMES spectrograph. The metallicities were determined based on the near-infrared Ca II triplet. The resulting metallicity distribution (MD) is asymmetric and peaks at [Fe/H]=-1.74 dex on the Carretta & Gratton scale. The full range in metallicities extends from -2.4 to -1.1 dex. As in other dSphs, no extremely metal-poor red giants were found. We compare Leo IIs observed MD with model predictions for several other Galactic dSphs from the literature. Leo II clearly exhibits a lack of more metal poor stars, in analogy to the classical G-dwarf problem, which may indicate a comparable `K-giant problem. Moreover, its evolution appears to have been affected by galactic winds. We use our inferred metallicities as an input parameter for isochrone fits to SDSS photometry and derive approximate ages. The resulting age-metallicity distribution covers the full age range from 2-15 Gyr on our adopted isochrone scale. During the first 7 Gyr relative to the oldest stars [Fe/H] appears to have remained almost constant. The almost constant metallicity at higher ages and a slight drop by about 0.3 dex thereafter may be indicative of rejuvenation by low metallicity gas. Overall, the age-metallicity relation appears to support the formation of Leo II from pre-enriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr. Our findings support earlier photometric findings of Leo II as a galaxy with a prominent old and a dominant intermediate-age population. We do not find a significant radial metallicity gradient nor age gradient in our data.(Abridged)
We carried out a wide-field V, I imaging survey of the Local Group dwarf spheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2-m Subaru Telescope. The survey covered an area of 26.67 x 26.67 arcmin^2, far beyond the tidal radius of Leo II (8.63 arcmin), down to the limiting magnitude of V ~26, which is roughly 1 mag deeper than the turn-off point of the main sequence stars of Leo II. Radial number density profiles of bright and faint red giant branch (RGB) stars were found to change their slopes at around the tidal radius, and extend beyond the tidal radius with shallower slopes. A smoothed surface brightness map of Leo II suggests the existence of a small substructure of globular cluster luminosity beyond the tidal radius. We investigated the properties of the stellar population by means of the color-magnitude diagram. The horizontal branch (HB) morphology index shows a radial gradient in which red HB stars are more concentrated than blue HB stars, which is common to many Local Group dwarf spheroidal galaxies. The color distribution of RGB stars around the mean RGB sequence shows a larger dispersion at the center than in the outskirts, indicating a mixture of stellar populations at the center and a more homogeneous population in the outskirts. Based on the age estimation using subgiant branch (SGB) stars, we found that although the major star formation took place ~8 Gyr ago, a considerable stellar population younger than 8 Gyr is found at the center; such a younger population is insignificant in the outskirts.
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