How Massive Are the Least Massive Galaxies?

How Massive are the Least Massive Galaxies?

Rosemary Wyse Johns Hopkins University

Gerry Gilmore, Mark Wilkinson, Vasily Belokurov, Sergei Koposov, Matt Walker, John Norris Wyn Evans, Dan Zucker, Mike Irwin, Anna Frebel, Apoorva Jayaraman, David Yong n Small scales on which nature of dark matter best tested: dwarf galaxies n Least massive but dark-matter dominated and nearby è indirect detection targets n Today: new data concerning Segue 1 and Boo I n Mass determinations from stellar motions require member stars and virial equilibrium n Segue 1: associated streams n Mass determinations require excellent handle on errors n Boo I : central velocity dispersion down by factor > 3, M/L within scale radius down by >10 Field of Streams (and dots) Belokurov et al (inc RW, 2006)

o o Boo I Segue 1

Outer stellar halo is lumpy: but only ~15% by mass (total 9 mass ~ 10 M¤) and dominated by Sgr dSph stream (Deason et al 2012). Dots include galaxies and star clusters. Composite image of SDSS data, 19< r< 22, g-r < 0.4 colour- coded by mag (distance), blue (~10kpc), green, red (~30kpc)

Boo I

Self-gravitating Dark matter, galaxies Star clusters Segue 1

Update from Gilmore et al 07 Add ~20 new satellites, galaxies and star clusters - but note low yield from Southern SEGUE/SDSS imaging : only Segue 2 and Pisces II as candidate galaxies, in 3/8 area (Belokurov et al 09,10) Dark Matter Content n Evidence for dark matter:

n (Relatively) high stellar motions – provided no contamination by non-members and in virial equilibrium; mass within half-light radius robust

Walker et al 2009

Wolf et al 2010

n Broad spread in chemical abundances (metallicity) of member stars -- self-enrichment, from very low initial value if first galaxies

Boötes I n Luminosity ~ 1/2,000,000 that of the Milky Way, 4 M* ~ 4 x 10 M¤, distance of ~ 65kpc, half-light radius ~ 250pc (< dark matter scalelength?), ~ n Pre-2011: central velocity dispersion ~ 6-9km/s, 6-7 estimated mass within half-light radius ~ 10 M¤, with 3 3 M/L ~ 10 , mean dark matter density ~ 0.1M¤/pc or ~ 5 GeV/cc (50 x local MWG) è collapse at z > 10 ~ n Stars are old and metal-poor, some extremely so -- only short-lived, early star formation, some self-enrichment n More luminous dSph have very varied SFHs

Belokurov et al 06; Gilmore et al 07; Martin et al 08; Walker et al 09; Okamoto et al 10; Norris, Wyse et al 2010 Segue 1 n Luminosity 1/100,000,000 that of Milky Way, M* ~ 600 M¤, distance of ~ 25kpc, half-light radius ~ 30pc (?), velocity dispersion ~ 4 km/s (?), estimated mass within half-light 5 3 radius ~ 3 x 10 M¤(?), M/L ~ 2000 (?), <ρ>DM ~ 1 M¤/pc (50 GeV/cc) and high collapse redshift

n Superposed on Sgr tidal debris, close in distance and velocity (?), contamination likely (Niederste-Ostholt et al 09); unlikely (Simon et al 2011) n Again old, metal-poor population, some stars extremely metal-poor – very unusual distribution

Belokurov et al. 07; Martin et al 08; Geha et al 09; Walker et al 09; Norris, Wyse et al 2010; Simon et al 2011 Segue 1, vel disp ~ 4 km/s Simon et al 2011

R_max = 67pc, 2.3 half-light radii But what is the value of the stellar velocity dispersion? Beware underestimated errors….and non-members

® Koposov et al 2011 : reduced dispersion in Boo I by further factor of 2! Koposov, et al (inc RW), 2011

Getting the most from Flames on VLT: Bootes I field, FOV ~1 half-light radius, 130 fibres, 16 x 45min integrations è Repeated observations allow detection of variability: è 110 non-variable (giant) stars (< 1km/s) Analyse spectra in pixel space; Retain full covariance: map model spectra onto data, find ‘best’ match values of stellar parameters (gravity, metallicity, surface temperature) with a Bayesian classifier.

Black: data r=19; red=model Koposov, et al (inc RW), 2011 Identify 37 members, based on line-of-sight velocity, metallicity and stellar gravity (should be giants, dwarfs will be foreground field halo stars)

Previous literature valueè (already lower than initial)

Mass within half-light radius down by factor ~4 from previous, by ~10 from initial estimates; M/L ~ 1700 è120 (need dispersion profile for total mass) n Caveat for Segue 1: in very complex part of the Galaxy n Very flat (bimodal?) metallicity distribution, unlike other dwarf galaxies: contamination? n But note contains carbon-enhanced very metal-poor star (Norris, Gilmore, RW et al 2010) n Extended structure around it n Extremely metal-poor members out to limit of surveys

n Same distance and line-of-sight as Sgr stream, but different velocity (Niederste-Ostholt et al wrong orbit for Sgr stream)

n Distance and velocity, line-of-sight match Orphan stream (Newberg et al 2010, Koposov et al inc RW 2012; Jayaraman et al inc RW 2012)

n What is the `300km/s stream’? n Extremely wide-field mapping needed to be assured of status [Fe/H] distributions and radial dependence Norris, RW et al 2010

Dwarf spheroidals have well-defined peaks, with low metallicity tails: self-enriched, from primordial gas? First bound systems?

Segue 1

Very luminous globular cluster è lacks low-metallicity tail; most clusters do not self-enrich in Fe; Need enough compact baryons Extended structure around Segue 1 BHB stars from SDSS, same ra as Segue 1: streams Niederste-Ostholt et al 2009 declination declination

Sgr â Segue 1 tail Red: Segue 1 Norris, RW et al 2010

Geha et al Black: Boo I Segue 1 is very close to estimated location of Orphan Stream in Newberg et al (2010) line-of-sight position, Galactocentric line-of- sight velocity and distance (SDSS BHB and F-stars)

New spectroscopic data from VLT provide further Insight/confusion – stream is very cold, same mean velocity and dispersion as Segue 1 (Koposov, Gilmore et al.) Segue 1 is not simple, kinematically Tens of half-light isolated system! radii offset: kpc

Sgr Stream D~25kpc è

Segue 1 and/or Orphan Stream D~25kpc è

Heliovel km/s 300km/s ‘stream’

Found within Segue 1 targets New photometry suggests stream a little closer, more metal-rich

CFHT photometry

See also Norris et al. 2010 Kinematic structure surrounding Segue 1 extremely complex: need wide-field mapping

Wider-field sparse sampling from SDSS spectra

Jayaraman et al. 2012

At Segue 1 distance 1deg ~ 400pc Summary n Dwarf spheroidals hold great promise for unlocking nature of dark matter

n Association with streams - chance or causal?

n Need wide area mapping

n Contamination, tidal effects?

n But there exists Metallicity – luminosity correlation n Need accurate and precise data and analysis for mass (profiles) -- and lots of it

n More VLT data arriving now…..Segue 1 and orphan stream n Prime Focus Spectrograph on Subaru soonish