For Ursa Minor, larger points represent smaller measurement uncertainties. Contaminants could affect the measured velocity dispersion. It is important to measure metallicities for individual stars in SegueÂ 2 to test whether the average metallicity for a larger sample remains above the LZR. 2007; Martin et al. It turned out that our data cannot resolve the velocity dispersion of SegueÂ 2. For example, a star at [Fe/H] = â3 is slightly bluer than the [Fe/H] = â2 isochrone, but a star at [Fe/H] = â1 is much redder. The contours show the 1Ï (68.3%), 90%, 2Ï (95.4%), 99%, and 3Ï (99.7%) confidence levels. 2011a), a dSph with an average metallicity similar to SegueÂ 2. However, they used a metallicity indicator that has since been shown to be unreliable for very metal-poor stars. If you have a user account, you will need to reset your password the next time you login. If SDSS J021904.48+200218.4 were counted as a member, SegueÂ 2's velocity dispersion would be Ïv = 4.3Â kmÂ sâ1. Our prices are in USD and may vary depending on prevailing exchange rates. by Iliki Music | Apr 5, 2011. The other metallicities came from Willman et al. The bottom panel shows an average of the available [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe] ratios for each star. If the MW is tidally stripping SegueÂ 2, then it falls into the latter category of subtle disruption, although B09 suggested that SegueÂ 2 was deposited into the MW halo by a larger host, just as Sagittarius has deposited globular clusters. References. In Section 6.1, we suggested that SegueÂ 2 may be the remnant of a galaxy that was previously 100â1000Â times more luminous than it is now. Instead, the dispersion in metallicity indicates that SegueÂ 2 retained SN ejecta despite its small mass. It relates the mass within the half-light radius to the velocity dispersion and the deprojected half-light radius. For Gaussian distributed errors, a membership cut of 2.58Î´vr rejects non-members in addition to 1% of members. We cross-correlated each one-dimensional spectrum with template spectra obtained by Simon & Geha (2007). In the last steps, the four abundance ratios mentioned above were measured by restricting the spectral matching separately to neutral Mg, Si, Ca, and Ti lines. The fact that our upper limitâbased on a larger sampleâis lower than B09's measurement indicates that our velocity uncertainties are not underestimated. Funny 4. (2012). B09 found evidence for the stream with a spectroscopic sample covering an area of sky six times larger than our DEIMOS survey. Because SegueÂ 2 is such a sparse galaxy, it was necessary to remove contaminants from the spectroscopic sample. There is a very big difference between a claim and an opinion. aB09. Therefore, we attempted to quantify the expected level of residual contamination. 2008), which overlaps the lower luminosity bound of the previously known satellites. Therefore, we chose a central isochrone more metal-rich than the measured mean metallicity. One of our membership cuts was that the metallicity of each star, where measurable, needed to be [Fe/H] < â1. Segue 2 has a luminosity just 900 times that of our sun, the researchers report. Without their generous hospitality, none of the observations presented herein would have been possible. Seeing was measured from the FWHM of the profiles of the alignment stars when the grating was angled for zeroth order (no spectral dispersion). You will only need to do this once. Embed review. The presence of the stream is exciting because it could be that SegueÂ 2 was deposited in the MW halo as a satellite or subcomponent of a larger galaxy that has been tidally disrupted. The similarity of chemical properties, especially mean metallicity, to a more luminous satellite leads to two possible explanations for the origin of SegueÂ 2. The upper limit on the mass estimate assumed that the galaxy is in dynamical equilibrium and is supported by velocity dispersion. Dwarf spheroidal galaxies (dSphs) obey a universal relationship between average stellar metallicity and luminosity or stellar mass (Kirby et al. They are plotted versus stellar mass, which may be more relevant to these quantities, and luminosity, which is directly observable. That is about the magnitude where our estimated uncertainty on [Fe/H] exceeded 0.4 dex. Finally, we shifted all velocities to the heliocentric frame. 2002), which are available in the SDSS filters. By using an instance property (A → B) 2. These galaxies are near enough that their stellar populations are resolved. The histogram has also been scaled to reflect the expected level of contamination for the effective area of sky observed. All of the stars shown here are members of SegueÂ 2. Astrophysics - Cosmology and Extragalactic Astrophysics. Figure 4.Â Comparison of radial velocities from this work to B09. The Dynamics and Metallicity Distribution of the Distant Dwarf Galaxy VV124, Spectroscopic Confirmation of the Dwarf Galaxies Hydra II and Pisces II and the Globular Cluster Laevens 1, Triangulum II. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. By number, such objects should be rareâbut not extremely rareâin the MW at z = 0. In reality, the galaxy needed to sustain several explosions of Type Ia SNe to achieve a steady decline in [Î±/Fe]. The stars' [Î±/Fe] ratios decline with increasing [Fe/H], indicating that SegueÂ 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100Â Myr. However, PeÃ±arrubia et al. 2011; Vargas et al. We sampled 10 metallicities from a probability distribution based on the closed box model of chemical evolution: P([Fe/H]) = (lnâ10/p)10[Fe/H]expâ(â 10[Fe/H]/p) (Lynden-Bell 1975; Pagel 1997). The tiny and faint collection of 1,000 stars called Segue 2 orbits the Milky Way 35 kilo-parsecs (114,170 light years) away from our Sun. This criterion eliminated many objects with i0 > 20. The low stellar densities of the SDSS ultra-faint dwarf galaxies make it difficult to detect them. The solid angle was 10Â deg2, but the model parameters were based only on the coordinates of SegueÂ 2. Despite our fairly stringent membership criteria, some unidentified contaminants might have remained in our sample. 2013). Evolution of Segue, Part 2. vr: inappropriate radial velocity. Figure 8.Â Upper limit on the velocity dispersion as a function of confidence level. We found a dispersion in [Fe/H] and a decline of [Î±/Fe] as a function of [Fe/H]. 2012; Wise et al. Those subhalos would spend most of their lives at large Galactocentric distances. The slightly larger systematic error did not change any of our qualitative conclusions, and it changed the quantitative limits we placed on the velocity dispersion and mass by only a few percent. Hardware development is painfully slow. Notice, Smithsonian Terms of However, one star (SDSS J021904.93+200715.4) has [Fe/H]syn = â2.29 Â± 0.12 and [Fe/H]CaT = â3.23 Â± 0.05. We applied all of the membership cuts described above to the BesanÃ§on model simulation. Kirby et al. The presence of stars in a subhalo as small as vmax = 8Â kmÂ sâ1 is especially remarkable because the threshold for atomic cooling to form stars is vmax > 17Â kmÂ sâ1 (Rees & Ostriker 1977). 2. From these 35 stars, we calculated Î´sysvr = 1.95Â kmÂ sâ1. (2013) measured ã[Fe/H]ã = â2.03 Â± 0.06. If SegueÂ 2 is in equilibrium, then its total mass is related to the square of the velocity dispersion. Inhomogeneous mixing is especially important for low-mass, low-metallicity galaxies (Greif et al. 2 How to Use Segue in a Sentence To use segue in a sentence, first decide if you would rather use the word in its verb or noun form. We reiterate that it is unwise to make much of the sparse sampling of the LZR at L < 104âLâ. From the co-added spectrum of five red clump giants, B09 determined their average metallicity to be ã[Fe/H]ã = â2.0 Â± 0.25. The corresponding limit on the mass within the three-dimensional half-light radius (46Â pc) is M 1/2 < 1.5 (2.1) × 105 M â. We also determined that the average metallicity of SegueÂ 2 places it more metal-rich than the LZR defined by the more luminous MW satellite galaxies. In Interface Builder, you specify the segue’s identifier string in the attributes inspector. Additional satellites may be found by deeper surveys or by surveys that target parts of the 75% of the sky that SDSS-I did not observe.