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Boötes-I, Segue 1, the Orphan Stream and CEMP-No Stars: Extreme Systems Quantifying Feedback and Chemical Evolution in the Oldest and Smallest Galaxies

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Boötes-I, Segue 1, the Orphan Stream and CEMP-No Stars: Extreme Systems Quantifying Feedback and Chemical Evolution in the Oldest and Smallest Galaxies Astronomical Science Boötes-I, Segue 1, the Orphan Stream and CEMP-no Stars: Extreme Systems Quantifying Feedback and Chemical Evolution in the Oldest and Smallest Galaxies Gerard Gilmore1 function has been extended down to most enigmatic of the ultra-faint systems. Sergey Koposov1 luminosities three orders of magnitude These targets include: the lowest luminos- John E. Norris2 below previous limits in recent years. ity system Segue 1 (Belokurov, 2007a; Lorenzo Monaco3 Remarkably, these extremely low lumi- see Simon et al. [2011] for a detailed Keck David Yong2 nosity objects, the dwarf spheroidal study) and the Orphan stream (Belokurov 4 Rosemary Wyse (dSph) galaxies with total luminosities as et al., 2007b), both of which are at similar 1 Vasily Belokurov low as 1000 LA, comparable to a moder- distances to the bifurcated tidal tail of Doug Geisler5 ate star cluster, are quite unlike star Sgr (Ibata, Gilmore & Irwin, 1994); the N. Wyn Evans1 clusters — they are real galaxies. Fortu- common-distance and similar-velocity Michael Fellhauer5 nately, with their very few red giants but pair Leo-IV and Leo-V (Belokurov et al., Wolfgang Gieren5 more populous main-sequence turn- 2008); and the inner regions of Boötes-I Mike Irwin1 off stars, they are within range of detailed (Belokurov et al., 2006b), a surviving ex - Matthew Walker1, 6 study, with considerable efforts currently ample of one of the first bound objects Mark Wilkinson7 underway at the Very Large Telescope to form in the Universe, providing a touch- Daniel Zucker8 (VLT) and Keck. stone to test the chemical evolution of the earliest low-mass stars. These lowest-luminosity galaxies provide 1 Institute of Astronomy, Cambridge, a unique opportunity to quantify the Our FLAMES GIRAFFE and UVES spec- United Kingdom formation and chemical enrichment of the tra are beginning to quantify the kine- 2 Mount Stromlo Observatory, Australian first bound structures in the Universe. matics and abundance distribution func- National University, Canberra, Australia At present, there are no convincing mod- tions in these systems, including several 3 ESO els for the origin and evolution of these element ratios, providing the first quan- 4 Johns Hopkins University, Baltimore, extreme objects — observations lead the titative study of what we find to be survi- USA way. The ultra-faint dSphs certainly have vors of truly primordial systems that 5 Departamento de Astronomia, extreme properties. They are clustered in ap parently formed and evolved before the Universidad de Concepcion, Chile groups on the sky, their velocity disper- time of reionisation. Our target fields are 6 Harvard University, Cambridge, USA sions are tiny — no more than 3–4 km/s summarised in Figure 1. 7 University of Leicester, Leicester, United at the lowest luminosities — yet the ob - Kingdom jects themselves are very extended, with 8 Macquarie University, Sydney, Australia half-light radii of hundreds of parsecs, Segue 1 and the Orphan Stream implying extreme dark matter dominance. Their chemical abundances are also Segue 1 is the lowest luminosity galaxy Galactic satellite galaxies provide a extreme, with dispersions of several dex, known. It has a wide abundance range, unique opportunity to map the history and containing stars down to [Fe/H] = –4. including hosting a very carbon-enhanced of early star formation and chemical There are hints that they are associated metal-poor star with no enhancement evolution, the baryonic feedback on gas with, or possibly entangled in, kinematic of heavy neutron-capture elements over and dark matter, and the structure of streams and superimposed on — or in — Solar ratios (called a CEMP-no star: c.f., low-mass dark matter halos, in surviv- the tidal tails of the more luminous Sagit- Norris et al. [2010a] for our VLT study and ing examples of the first galaxies. We tarius dSph (Sgr) galaxy. Beers & Christlieb [2005] for the defini- are using VLT–FLAMES spectroscopy tions of metal-poor star subclasses). Such to map the kinematics and chemical The lowest luminosity dSphs do not look stars, which are like those in the Milky abundances of stars in several ultra- like the tidal debris of larger systems, Way Halo field, are suspected to be suc- faint dwarf spheroidal galaxies and the they look like the most primordial galax- cessors of the very first supernovae — enigmatic Orphan Stream in the Halo. ies of all. Arguably even more interesting see below. The velocity distribution Two paths of early chemical enrichment than their relevance as galaxy building measured in Segue 1, which is consistent at very low iron abundance are observed blocks is to understand the objects them- with the Keck study of Simon et al. (2011), directly: one rapid and carbon-rich selves. Are they the first objects? Did they shows a very narrow distribution, consist- (CEMP-no), one slow and carbon-nor- contribute significantly to reionisation? ent with a dispersion of less than 4 km/s. mal. We deduce long-lived, low-rate What do they tell us of the first stars? In spite of this low dispersion, Segue 1 is star formation in Boötes-I, implying What was the stellar initial mass function completely dark-matter dominated, with insignificant dynamical feedback on the (IMF) at near-zero metallicity? How are a mass-to-light ratio in excess of 1000. structure of its dark matter halo, and the faintest dSphs related to more lumi- The velocity distribution function, in which find remarkably similar kinematics in the nous dSphs and the Milky Way galaxy? Segue 1 has its radial velocity near apparently discrete systems Segue 1 V = 200 km/s, also indicates the pres- and the Orphan Stream. In order to address these questions, we ence of stars from the Sagittarius tidal are obtaining VLT FLAMES observations stream (at V = 0 km/s), which is at a very using both the spectrographs GIRAFFE similar distance, and stars in a cold With the discoveries from the Sloan and UVES, with exposures of up to more kinematic structure with radial velocity Digital Sky Survey, the galaxy luminosity than 15 hours, of member stars of the V = 300 km/s. This cold highest-velocity The Messenger 151 – March 2013 25 Astronomical Science Gilmore G. et al., Boötes-I, Segue 1, the Orphan Stream and CEMP-no Stars Figure 1. The background shows the “Field of Streams” (Belokurov et al., 2006a), the distribution of turn-off stars observed by the Sloan Digital Sky Survey, statistically colour-coded by distance with the bluer colour showing more nearby, the redder more distant streams. The fields observed for this project by the VLT are marked in grey, with representative HNM velocity distributions indicated. The @S velocity distribution for Boötes-I is marked by its extreme narrowness. 6WUHDP #DBKHM The low surface brightness Orphan V Stream is very evident in velocity space, again with very low velocity 6HJXH %RÓWHV, dispersion. Segue 1 shows three dis- tinct velocity peaks, corresponding 0RQRFHUR to the Sgr Stream, Segue 1 itself, and 6DJLWWDULXV6WUHDP the “300 km/s” stream, evident only in /HR,9 2USKDQ 6WUHDP velocity space. l 1HFGS@RBDMRHNM stream is not yet well defined, and remains pens to be passing through a busy part 240 pc), at 60 kpc Galactocentric dis- under study. of the outer Galaxy. The metal-poor tance, whose special features are its ([Fe/H] = –3.5) CEMP-no star Segue 1-7, low surface brightness and low total lumi- The Orphan Stream provides another which we have studied with the VLT, nosity (Mv = –6). Boötes-I has low mean example of a Halo stream, although in lies almost four half-light radii from the metallicity (with a range from [Fe/H] = this case it has a sufficiently high surface centre of Segue 1, while all the other well- –3.7 to –1.9) and hosts a CEMP-no star brightness to allow it to be traced over studied members are inside 2.3 half- that we have studied with the VLT more than 60 degrees of arc. The peri- light radii (70 pc). Does this hint at tidal ([Fe/H] = –3.3; Norris et al., 2010b). Our galacticon of the Orphan Stream orbit truncation of an earlier, much larger (and GIRAFFE spectra have been analysed for passes close to Segue 1, and hence to more luminous?) predecessor? Segue 1 a kinematic study (see Koposov et al., the Sagittarius tidal tail. As Figure 1 illus- is very deep inside the Galactic tidal 2011). Using careful data reduction tech- trates, the internal velocity dispersion field, well inside the (disrupting) Sgr dSph, niques, Koposov et al. (2011) showed in the Orphan Stream is unresolved at the and the Large Magellanic Cloud–Small that GIRAFFE spectra obtained in single resolution of the observations, being Magellanic Cloud pair, with their gaseous one-hour observing blocks over several less that 3–4 km/s. The Orphan Stream tidal stream. All the dSph galaxies that years can be combined to deliver radial and Segue 1 kinematics and distance, are not deep in the Galactic tidal field velocities with an accuracy floor ap - the latter determined from main-sequence have much larger half-light radii (Gilmore proaching 0.1 km/s. From this study we turn-off fitting, provide an interesting et al., 2007). We also have the remarka- demonstrated that Boötes-I has a smaller illustration of the complexity of the outer ble similarity (indeed, near identity) of the velocity dispersion than suggested Galactic Halo. Both Segue 1 and the distances and radial velocities of Segue 1 by previous studies. We showed the kine- Orphan Stream have similarly low internal and the Orphan Stream as further clues. matics to be best de s cribed by a two- velocity dispersions.
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