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Table 1 details the exposure times for and conditions under which each slit each slitmask was observed. We chose to rely on the spectral synthesis measurements rather than the Ca triplet because spectral synthesis provides independent estimates of both [Fe/H] and [α/Fe] ratios. The fact that our upper limit—based on a larger sample—is lower than B09's measurement indicates that our velocity uncertainties are not underestimated. These included three quartz flat lamp exposures and one arc lamp exposure per slitmask. We measured radial velocities and chemical abundances from the co-added spectra. Metz et al. The quality of the bottom spectrum is good enough to measure its radial velocity but not metallicity or chemical abundance ratios to the required precision. In order to be more inclusive of a range of metallicities and ages and to account for modeling errors in the isochrones, an additional buffer of 0.1 mag in color was allowed beyond the blue and red isochrones. Any stars with such high surface gravities at the distance of Segue 2 would be far too faint to be included in our spectroscopic sample. If the MW is tidally stripping Segue 2, then it falls into the latter category of subtle disruption, although B09 suggested that Segue 2 was deposited into the MW halo by a larger host, just as Sagittarius has deposited globular clusters. E.N.K. Both photometric and kinematic errors obeyed exponential forms as a function of i0 magnitude. These tiny galaxies and their relevance to dark matter physics make a compelling case for building high-resolution spectrographs for the next generation of extremely large telescopes. Figure 1 shows a color–magnitude diagram (CMD) for the region of the sky enclosed by the DEIMOS slitmasks. The editor-at-large for Merriam-Webster acknowledges the choice “probably isn’t a big shock.” Peter Sokolowski says because it’s a technical term that has gone into everyday use, pandemic is an obvious choice. 2012). All but four stars agree to within the combined 1σ uncertainties, and three of those four stars agree to within the 2σ uncertainties. However, DLAs have much larger gas masses than the stellar mass of Segue 2. 2011). It relates the mass within the half-light radius to the velocity dispersion and the deprojected half-light radius. Be among the first to get notified when we launch. Or they could hint at a metallicity floor for galaxy evolution. All of the radial velocities were checked by visually comparing the template spectrum to the velocity-corrected observed spectrum. The velocity dispersion is too small to be measured with our data. We measured vr from the noise-added array. The mean metallicity, lang[Fe/H]rang = -2.22 ± 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. For stars where the lines at 8542 and 8662 Å were observed, we fit separate Lorentzian profiles to each line. Because this star may belong to the stream, we excluded it from membership consideration. (2009), is a galaxy with a luminosity of only 900 L_sun. Thus, we found no evidence for a tidal stream. Wolf et al. We chose the mean metallicity of the distribution according to the LZR. In Section 6.2, we presented an alternate scenario, wherein Segue 2 is not a heavily stripped remnant of a more massive galaxy but instead a relatively undisturbed dwarf galaxy whose properties indicate the existence of a metallicity floor in galaxy formation. Our prices are in USD and may vary depending on prevailing exchange rates. Because we could not resolve the velocity dispersion, our estimate of the mass within the half-light radius is an upper limit. The probability distribution from the MCMC is shown in Figure 7. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2—a threefold increase in spectroscopic sample size. Our upper limit on the mass of Segue 2 makes it the least massive galaxy known. Its membership of about 7,000 individuals also includes physicists, mathematicians, geologists, engineers, and others whose research and educational interests lie within the broad spectrum of subjects comprising contemporary astronomy. The typical wavelength range for a single object was 2700 Å. It is possible that our survey merely missed the stream stars. The stars' [α/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. (2011a). 2010; Ritter et al. A similar origin has been proposed for stellar systems that may have arrived via the tidal dissolution of the Sagittarius dwarf galaxy (e.g., Law & Majewski 2010). 2011). 2009) has revolutionized the concept of a galaxy. (2009). During spectral observations, DEIMOS's active flexure compensation system kept the spectra stationary on the detector within a precision of 0.07 Å in the dispersion direction and 002 in the spatial direction. The quality of the top two spectra is high enough to measure both radial velocities (Section 3.1) and chemical abundances (Section 3.3). Regardless of whether tides affected Segue 2 in the past, they are not affecting it now. Only a small fraction of subhalos have orbital eccentricities that large. In the initial stages of the measurement, the effective temperature, surface gravity, and metallicity were based on photometry. It seems to be consistent with a universal relationship between its dynamical mass and stellar mass. Segue 2 is about 1000 stars, held together by dark matter, orbiting our Milky Way, and is only about 800 times brighter than our Sun! Just as for the observational data, we allowed model stars to pass membership cuts if colors, magnitudes, and velocities were consistent with membership. 2003) on the Keck II telescope. The MW cannot be presently disrupting Segue 2 at its current location. The SN would unbind M_{\rm ej} = E_{\rm ej} / (6 \sigma _v^2) = 1.5 \times 10^5\, M_{\odot } of gas from the galaxy. G: spectrum shows emission lines or redshifted Ca H and K lines, indicating that the object is a galaxy. 2013; Angulo et al. It was excluded because the measurement uncertainty on vhelio was small, whereas the member star with similar velocity had a larger measurement uncertainty. Figure 8 shows the constraint on σv, marginalized over 〈vhelio〉. We chose such a large area so that the sample size was large. 2002), which are available in the SDSS filters. It would lose stellar mass and luminosity as it dissolved, but its stars' chemical properties would not have changed. Figure 1. Color–magnitude diagram from SDSS (Abazajian et al. It would be wise to expand the sample sizes in Segue 1, Segue 2, and Willman 1 before we conclude that the linearity of the LZR disappears below a few thousand L☉. B09 considered some red giants just above the subgiant branch to be possible members, and they are denoted by a question mark (?). 's measurements are marginally consistent with the LZR, taking into account the error on the mean and the intrinsic scatter in the relation. 2006; Martin et al. 2013) to test whether tidal stripping of a dSph with the stellar mass of Ursa Minor could produce Segue 2. Contaminants could also affect the chemical abundances because those measurements assumed that all stars were at the distance of Segue 2. 2008), which overlaps the lower luminosity bound of the previously known satellites. It works to advance physics research, application and education; and engages with policy makers and the public to develop awareness and understanding of physics. Sohn et al. The procedures for both measurements closely mimic those described by Kirby et al. The word you want to use in such cases is segue.This use of segue is defined in our Online Dictionary as "to make a transition without interruption from one activity, topic, scene, or part to another." (2008b) showed that intense tidal stripping of 99%–99.9% of the stellar mass of a dSph decreases the surface brightness by a factor of ~300, increases the mass-to-light ratio by a factor of ~10, decreases the velocity dispersion by a factor of ~6, and decreases the half-light radius by a factor of ~1.5. From the line-of-sight stellar velocity dispersion measured with MMT/Hectospec, they determined that its mass has a much larger fraction of dark matter than of stars. The spec2d reduction software produced an estimate of the noise in each pixel. We found a dispersion in [Fe/H] and a decline of [α/Fe] as a function of [Fe/H]. The dotted line shows the maximum likelihood value of 〈vhelio〉. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. (2012) calculated δsysvr = 2.21 km s−1. 2013). However, more recent spectroscopy (Simon et al. The velocity dispersion is too small to be measured with our data. The most luminous of the new satellites, Canes Venatici I, has an absolute magnitude of MV = −8.6 (L = 2.3 × 105 L☉; Zucker et al. Notes. The target density in this region of the sky allowed about 80 spectroscopic targets per slitmask. 7/9/2015. Segue 2 has a measurable spread in metallicity (σ([Fe/H]) = 0.43). The spectral fitting for chemical abundances excludes the poorly modeled Ca triplet. It is not practical to draw strong conclusions from only three galaxies that disobey the LZR. Vargas et al. However, Segue 1, Segue 2, and Willman 1 together invoke the suggestion of a metallicity floor for galaxy formation. B09 identified a possible stellar stream at the same coordinates and radial velocity as Segue 2. Please note, The Astrophysical Journal Letters (ApJL) and Research Notes of the AAS (RNAAS) 2012, Table 1). From the co-added spectrum of five red clump giants, B09 determined their average metallicity to be 〈[Fe/H]〉 = −2.0 ± 0.25. We also ruled out stars with EW(Na) > 0.4 Å and [Fe/H] < −2. 2011a), a dSph with an average metallicity similar to Segue 2. Although the doublet is in a spectral region affected by telluric absorption, the equivalent width (EW) in a typical dwarf star is 1 Å, strong enough to be noticed easily through the telluric absorption. However, their estimate of δsysvr = 0.35 km s−1 was based on low-S/N spectra in the distant Leo V ultra-faint dwarf galaxy (Walker et al. We observed stars in and around Segue 2 in order to enlarge the available spectroscopic sample, to refine the velocity dispersion and metallicities, and to measure detailed abundances. 2012). Regardless, our upper limit on the velocity dispersion is already at the limits of the precision of the radial velocities. (2008) showed that the resolution of DEIMOS spectra hardly limits the ability to measure iron abundances in red giants compared to high-resolution spectroscopy. Because their low surface brightnesses limit the distance out to which SDSS can detect them, almost all of the new SDSS dwarfs are satellites of the Milky Way (MW). We also excluded stars with log g ≥ 4.8, which would have been ruled as non-members based on the Na i 8190 EW membership cut. Notes. by Followin! As a result, galaxies similar to Segue 2 would be difficult to detect, and Segue 2 would likely be one of the closest such objects to the MW's center. Segue 2是銀河系最小和最黯淡的衛星星系之一 —它的集成發光度大約是太陽的800倍(絕對視星等大約是-2.5),這遠低於大多數球狀星團的發光度 。 That is about the magnitude where our estimated uncertainty on [Fe/H] exceeded 0.4 dex. By Oscar Wells Gabriel II November 30, 2020 GMT. One such survey was the Sloan Extension for Galactic Understanding and Evolution (SEGUE; Yanny et al. In the top two panels, the best-fitting synthetic spectra are plotted in red. 2008; Springel et al. 2003), and it likely deposited many of the MW halo's globular clusters in the process of dissolution (Law & Majewski 2010). High-resolution spectrographs like Keck/HIRES and Subaru/HDS could possibly measure the velocity distribution of ~10 stars in Segue 2 over three to four nights. The low stellar densities of the SDSS ultra-faint dwarf galaxies make it difficult to detect them. This metallicity is directly related to the EWs of the Ca ii 8542 and 8662 absorption lines. As a result, the [α/Fe] ratios declined over time as [Fe/H] increased. Evolved stars and galaxies also contaminated the sample. Select your desired journals and corridors below. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo ( v max < 10 km s –1 ), below the atomic hydrogen cooling limit. The relation predicts that a galaxy of Segue 2's luminosity should have 〈[Fe/H]〉 = −2.83. The source of this mass could be dark matter or stars that were part of the galaxy before any possible tidal stripping (Section 6.1). 2012; Wise et al. 2003; Newman et al. From these 35 stars, we calculated δsysvr = 1.95 km s−1. Therefore, it would move to the left in the bottom panels of Figure 9. The systematic error is added in quadrature with the random uncertainty such that the total error on vr is \delta v_r = \sqrt{\delta _{\rm MC}v_r^2 + \delta _{\rm sys}v_r^2}. Furthermore, extremely tidally stripped galaxies likely would not survive their next one to two pericentric passages, which would make Segue 2 a transient phenomenon that will survive for only 2–3 Gyr. The stream occupies a larger area of sky than the galaxy. Vida Que Segue. An alternative explanation for the chemical composition and low luminosity of Segue 2 is that the stellar content of the galaxy is bound and largely unaffected by tidal stripping. Segue 1 is a dwarf spheroidal galaxy or globular cluster situated in the Leo constellation and discovered in 2006 by Sloan Digital Sky Survey. 2013). Figure 11. Distribution of individual [α/Fe] ratios in Segue 2 (black points) compared to Ursa Minor (magenta points; Kirby et al. Simon & Geha (2007) first noticed that the ultra-faint galaxies lie slightly above the LZR. The selection area was bounded on the red side of the red giant branch by a Yonsei–Yale isochrone with an age of 14.1 Gyr and [Fe/H] = −1. Frebel & Bromm (2012) described the effect on chemical abundance patterns of "one-shot" chemical enrichment from a single generation of long-lived stars. In the event that none of the Hα or the three strong Ca ii triplet lines at 8498, 8542, or 8662 Å could be recognized, the spectrum was flagged as "Bad," meaning that it was of insufficient quality to measure vr confidently. Figure 9 shows various dynamical quantities for MW satellite galaxies. The figure legend lists reasons for which stars were excluded from consideration as members. These stars have large surface gravities, which are reflected in the strength of some absorption lines. Of these stars, both we and B09 classified 10 as members. 2. ASCIITypeset image. We cross-correlated each one-dimensional spectrum with template spectra obtained by Simon & Geha (2007). bS denotes stars that B09 classified as members of a stellar stream coincident with Segue 2. In the case of a cored dark matter profile, Segue 2 could not have been stripped to its present state without being completely disrupted. Assuming an MW mass interior to Segue 2's Galactocentric distance (41 kpc) of 1011 M☉, the upper limit on the Roche radius is 1.1 kpc, which is well beyond the extent of the stars. The other metallicities came from Willman et al. The American Astronomical Society (AAS), established in 1899 and based in Washington, DC, is the major organization of professional astronomers in North America. In particular, they found a systematic error floor of δsysvr = 2.2 km s−1. That mass is 150 times larger than the present stellar mass. Receive alerts on all new research papers in American Astronomical Society claimed that the existence of the plane was inconsistent with CDM structure formation. It is unlikely that we rejected any members with the radial velocity cut because our sample has only 25 members. The American Astronomical Society. Segue 2, discovered by Belokurov et al. The area of sky covered is about 0.12 deg2, which is about one-sixth of the area covered by B09's Hectospec survey.

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