What is an ultra-faint Galaxy?

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1 What is an ultra-faint Galaxy? UCSB KITP Feb Beth Willman (Haverford College)

2 Large Magellanic Cloud, M V = -18 ~ 1/10 Milky Way luminosity image credit: Yuri Beletsky (ESO) and APOD

3 NGC 205, M V = ~ 1/40 Milky Way luminosity image credit:

4 Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration M V = ~ 1/300 Milky Way luminosity

5 Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration M V = ~ 1/2700 Milky Way luminosity

6 Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration M V = ~ 1/14,000 Milky Way luminosity

7 ~ 1/40,000 Milky Way luminosity

8 ~ 1/1,000,000 Milky Way luminosity Ursa Major 1

9 Finding Invisible Galaxies bright faint blue red Willman et al 2002, Walsh, Willman & Jerjen 2009; see also e.g. Koposov et al 2008, Belokurov et al.

10 Finding Invisible Galaxies Blue, hot, bright Red, bright, cool V-band apparent brightness bright faint Red, faint, cool blue red From ARAA, V26, 1988 Willman et al 2002, Walsh, Willman & Jerjen 2009; see also e.g. Koposov et al 2008, Belokurov et al.

11 Finding Invisible Galaxies Ursa Major I dwarf 1/1,000,000 MW luminosity Willman et al 2005

12 ~ 1/1,000,000 Milky Way luminosity Ursa Major 1

13 CMD of Ursa Major I Okamoto et al 2008

14 Distribution of the Milky Wayʼs dwarfs Milky Way dwarfs classical dwarfs M V L Sun -6 ultra-faint dwarfs -4 Canes Venatici II Pisces II Leo V -2 Segue I Willman I Distance from Milky Way (kpc)

15 Luminosities and sizes of nearby dwarf galaxies log(r half (pc)) L Sun 10 5 L Sun 10 7 L Sun MW dwarfs M31 dwarfs Pisces II Bootes II Segue II Segue I Willman I Hercules Bootes I Ursa Major II Leo IV Canes Venatici II Coma Berenices Leo V Ursa Major I Sextans Canes Venatici I Ursa Minor Carina Draco Leo II Leo T Leo I Sculptor Fornax M V MW dwarf galaxy discovery papers: Willman et al 05a,b; Zucker et al 06a,b; Belokurov et al 06,07,08,09,10; Walsh et al 07, Irwin et al 07; Detection limits: Walsh, Willman & Jerjen 2009, Koposov et al 2008

16 Luminosities and sizes of nearby dwarf galaxies log(r half (pc)) L Sun 10 5 L Sun 10 7 L Sun MW dwarfs M31 dwarfs undetectable Pisces II Bootes II Segue II Segue I Willman I Hercules Bootes I Ursa Major II Leo IV Ursa Major I Canes Venatici II Coma Berenices Leo V Sextans Canes Venatici I Ursa Minor Carina Draco Leo II Leo T Leo I Sculptor Fornax M V MW dwarf galaxy discovery papers: Willman et al 05a,b; Zucker et al 06a,b; Belokurov et al 06,07,08,09,10; Walsh et al 07, Irwin et al 07; Detection limits: Walsh, Willman & Jerjen 2009, Koposov et al 2008

17 Luminosities and sizes of nearby dwarf galaxies log(r half (pc)) L Sun 10 5 L Sun 10 7 L Sun MW dwarfs M31 dwarfs Pisces II Bootes II Segue II Segue I Willman I Hercules Bootes I Ursa Major II Leo IV Canes Venatici II Coma Berenices Leo V Ursa Major I Sextans Canes Venatici I Ursa Minor Carina Draco Leo II Leo T Leo I Sculptor limit of searches outside the Local Group Fornax M V MW dwarf galaxy discovery papers: Willman et al 05a,b; Zucker et al 06a,b; Belokurov et al 06,07,08,09,10; Walsh et al 07, Irwin et al 07; Detection limits: Walsh, Willman & Jerjen 2009, Koposov et al 2008

18 Luminosities and sizes of nearby dwarf galaxies 104 LSun 100 LSun 3.0 le b cta 2.5 te e d un 2.0 log(rhalf(pc)) 106 LSun 1.5 Segue I Willman I Segue III 0.0 globular clusters MV I know it when I see it is no longer a sufficient definition for galaxy

Luminosities and sizes of nearby dwarf galaxies 104 LSun 100 LSun 3.0 le b cta 2.5 te e d un 2.0 log(rhalf(pc)) 106 LSun 1.5 Segue I Willman I 1.0 0.

19 Luminosities and sizes of nearby dwarf galaxies 104 LSun 100 LSun 3.0 le b cta 2.5 te e d un 2.0 log(rhalf(pc)) 106 LSun 1.5 Segue I Willman I Segue III 0.0 globular clusters MV A self-bound stellar system whose properties cannot be explained by baryons + Newton s laws Willman & Strader in prep

20 Kinematics donʼt reveal huge dark matter reservoirs 12 Gyr [Fe/H] = -1.7 Segue 3: Magellan/ IMACS photometry + Keck/DEIMOS spectroscopy d ~ 17 kpc, M V ~ -0.1, r half ~2 pc σ vel = km/sec Fadely, Willman, Geha, Walsh et al 2011

21 Kinematics donʼt reveal huge dark matter reservoirs 12 Gyr [Fe/H] = -1.7 (probably) NOT A GALAXY Segue 3: Magellan/ IMACS photometry + Keck/DEIMOS spectroscopy d ~ 17 kpc, M V ~ -0.1, r half ~2 pc σ vel = km/sec Fadely, Willman, Geha, Walsh et al 2011

22 Kinematic studies reveal huge dark matter reservoirs σ = km/sec (incl. binary correction) Mass within 30 pc ~ 6 x 10 5 M Sun, M/L ~ stars with [Fe/H] estimates from -1.7 to -3.3 Keck/DEIMOS observations of Segue 1 99% complete sample of stars with r < 22 within 2 r eff (60 pc) Simon, Geha... Kirby... Willman et al (2011)

23 Kinematic studies reveal huge dark matter reservoirs σ = km/sec (incl. binary correction) Mass within 30 pc ~ 6 x 10 5 M Sun, M/L ~ stars with [Fe/H] estimates from -1.7 to -3.3 DWARF GALAXY Keck/DEIMOS observations of Segue 1 99% complete sample of stars with r < 22 within 2 r eff (60 pc) Simon, Geha... Kirby... Willman et al (2011)

24 Kinematic studies complicated by irregular dynamics v helio [km/sec] probable members high [Fe/H] candidates Running v sys (9 star window) probable members including high [Fe/H] projected distance from center [r half,ell ] 10 v helio [km/sec] projected major axis distance from center [pc] Keck/DEIMOS and KPNO observations of Willman I (M V ~ -2, d ~ 38 kpc) Willman, Geha... Simon, Kirby et al (2011)

25 [Fe/H] spread can distinguish galaxies from clusters Willman & Strader in preparation Dwarf data from: Kirby et al 08, 10; Norris et al 10, Simon et al 11, Willman et al 11; GC data from: Carretta et al 06,07,09,10, Johnson & Pilachowski 2010, Cohen et al 2010, Gratton et al 07, Marino et al 11

26 [Fe/H] spread can distinguish galaxies from clusters: Willman 1 16 filled candidate Wil 1 members open MW stars 18 r bright RGB faint RGB MS/BHB (g r) 0 Keck/DEIMOS and KPNO observations of Willman I (M V ~ -2, d ~ 38 kpc) Willman, Geha... Simon, Kirby et al (2011) Two red giant branch star members There is an [Fe/H] ([Ca/Fe]) spread: -1.7 and -2.7 (-0.4 and +0.1)

27 [Fe/H] spread can distinguish galaxies from clusters: Willman 1 16 filled candidate Wil 1 members open MW stars 18 r 0 20 DWARF GALAXY? 22 bright RGB faint RGB MS/BHB (g r) 0 Keck/DEIMOS and KPNO observations of Willman I (M V ~ -2, d ~ 38 kpc) Willman, Geha... Simon, Kirby et al (2011) Two red giant branch star members There is an [Fe/H] ([Ca/Fe]) spread: -1.7 and -2.7 (-0.4 and +0.1)

28 Distribution of the Milky Wayʼs dwarfs Milky Way dwarfs M V -8-6 Low luminosity from nurture or nature? 10 5 L Sun -4 Canes Venatici II Pisces II Leo V -2 Segue I Willman I Distance from Milky Way (kpc) see e.g. Willman et al 2002; Walsh, Willman & Jerjen 2009

29 Outer halo dwarfs: Nature vs Nurture Leo V, Pisces II, CVn II Morphological evidence for tidal evolution? Sand, Strader, Willman et al submitted

30 Hints of disturbed structure: 2d distribution Sand, Strader, Willman et al submitted

31 Hints of disturbed structure: 2d distribution 10 Leo V 5 Dec (arcmin) 0 d ~ 190 kpc -5 To Leo IV RA (arcmin) Sand, Strader, Willman et al submitted

32 Hints of disturbed structure: 2d distribution Sand, Strader, Willman et al submitted

33 Hints of disturbed structure: 2d distribution 10 Pisces II 5 Dec (arcmin) 0 d ~ 170 kpc RA (arcmin) Sand, Strader, Willman et al submitted

34 Hints of disturbed structure: 2d distribution CVn II 10 Dec (arcmin) d ~ 160 kpc RA (arcmin) Sand, Strader, Willman et al submitted

35 Hints of disturbed structure: Ellipticity esdss = / eclassical = / Sand, Strader, Willman et al submitted

36 Hints of disturbed structure: Orientation CVn II 10 Dec (arcmin) RA (arcmin) Sand, Strader, Willman et al submitted

37 Challenging pathological explanations: Luminosity-[Fe/H] relation Segue 1 (7 stars): /- 0.4 dex Wil 1 (3 stars): /- 0.4 dex Kirby et al 2011

38 Connecting observations to theory: How many dwarfs vs subhalos Corrections for SDSS footprint and luminosity bias suggest ~ dwarfs (e.g. Tollerud et al 2008, Koposov et al 2008; loads of assumptions and caveats) The devil is in the details (e.g. densities, spatial distributions)

39 Connecting observations to theory: How many dwarfs vs subhalos 5 log 10 (Cumulative N subhalos ) Via Lactea 2 data mass range of known dwarfs log 10 (M Sun in central 300 pc) The devil is in the details (e.g. densities, spatial distributions)

40 Current and Future Optical Surveys Area (deg 2 ) PS SM LSST 1exp DR8 DES RCS2 Stripe82 LSST MW Volume Fraction SM = SkyMapper PS = PanStarrs 1 DES PS DR8 SM RCS2 LSST 1exp LSST Approx. r band Limit Stripe MSTO Detection Dist (kpc)

41 What we have learned since 1999 A large number of Milky Way dwarf galaxies remain to be found Galaxies exist with mere hundreds of Solar luminosities Complexities of their stellar populations and morphologies yield vital clues to the origin of these objects

42 Open questions The very least luminous objects: tip of the iceberg or tidal remnants? How faint is the faintest galaxy? What is the spatial distribution, luminosity function, mass function of the least luminous galaxies?

43 The End

The Structural Properties of Milky Way Dwarf Galaxies. Ricardo Muñoz (Universidad de Chile) Collaborators:

The Structural Properties of Milky Way Dwarf Galaxies Ricardo Muñoz (Universidad de Chile) Milky Way inner 100 kpc Collaborators: Marla Geha (Yale) Patrick Côté (HIA/DAO) Peter Stetson (HIA/DAO) Josh Simon