What Is an Ultra-Faint Galaxy?

Home , Canes Venatici, Leo II (dwarf galaxy), Leo IV (dwarf galaxy), Leo T (dwarf galaxy), Leo V (dwarf galaxy), Pisces II (dwarf galaxy)

What is an ultra-faint Galaxy?

UCSB KITP Feb 16 2012 Beth Willman (Haverford College) ~ 1/10 Milky Way luminosity Large Magellanic Cloud, MV = -18 image credit: Yuri Beletsky (ESO) and APOD NGC 205, MV = -16.4 ~ 1/40 Milky Way luminosity image credit: www.noao.edu Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration ~ 1/300 Milky Way luminosity

MV = -14.2 Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration ~ 1/2700 Milky Way luminosity

MV = -11.9 Image credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration ~ 1/14,000 Milky Way luminosity

MV = -10.1 ~ 1/40,000 Milky Way luminosity ~ 1/1,000,000 Milky Way luminosity Ursa Major 1 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. Finding Invisible Galaxies

Red, bright, cool bright Blue, hot, bright V-bandbrightness apparent

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. Finding Invisible Galaxies

Ursa Major I dwarf 1/1,000,000 MW luminosity Willman et al 2005 ~ 1/1,000,000 Milky Way luminosity Ursa Major 1 CMD of Ursa Major I

Okamoto et al 2008 Distribution of the Milky Wayʼs dwarfs

-14 Milky Way dwarfs 107 -12

-10 classical dwarfs

V -8 5

10 Sun M L

-6 ultra-faint dwarfs

Canes Venatici II -4 Leo V Pisces II

Willman I 1000 -2 Segue I 0 50 100 150 200 250 300 Distance from Milky Way (kpc) Luminosities and sizes of nearby dwarf galaxies

5 7 1000 LSun 10 LSun 10 LSun 3.5 MW dwarfs M31 dwarfs 3.0

Sextans Fornax Canes Venatici I Ursa Minor Leo I Ursa Major I (pc)) 2.5 Sculptor Carina HerculesBootes I

half Draco Leo II Ursa Major II Leo IV log(r 2.0 Leo T Canes Venatici II Coma Berenices Pisces II Leo V Bootes II Segue II 1.5 Segue I Willman I

0 -2 -4 -6 -8 -10 -12 -14 MV 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 Luminosities and sizes of nearby dwarf galaxies

5 7 1000 LSun 10 LSun 10 LSun 3.5 MW dwarfs M31 dwarfs 3.0

Sextans Fornax Canes Venatici I Ursa Minor Leo I Ursa Major I (pc)) 2.5 Sculptor Carina HerculesBootes I

half Draco Leo II undetectableUrsa Major II Leo IV log(r 2.0 Leo T Canes Venatici II Coma Berenices Pisces II Leo V Bootes II Segue II 1.5 Segue I Willman I

5 7 1000 LSun 10 LSun 10 LSun 3.5 MW dwarfs M31 dwarfs 3.0

Sextans Fornax Canes Venatici I Ursa Minor Leo I Ursa Major I (pc)) 2.5 Sculptor Carina HerculesBootes I

half Draco Leo II Ursa Major II Leo IV log(r 2.0 Leo T Canes Venatici II Coma Berenices Pisces II Leo V Bootes II Segue II

1.5 Local

Segue I Group limit of of limit

Willman I searches outside the 0 -2 -4 -6 -8 -10 -12 -14 MV 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 Luminosities and sizes of nearby dwarf galaxies

4 6 100 LSun 10 LSun 10 LSun 3.0

2.5

2.0 undetectable

(pc)) 1.5 Segue I Willman I half 1.0 log(r

0.5 Segue III 0.0 globular clusters

0 -2 -4 -6 -8 -10 -12 MV “I know it when I see it” is no longer a sufﬁcient deﬁnition for galaxy Luminosities and sizes of nearby dwarf galaxies

4 6 100 LSun 10 LSun 10 LSun 3.0

2.5

2.0 undetectable

(pc)) 1.5 Segue I Willman I half 1.0 log(r

0.5 Segue III 0.0 globular clusters

0 -2 -4 -6 -8 -10 -12 MV A self-bound stellar system whose properties cannot be explained by baryons + Newton’s laws Willman & Strader in prep Kinematics donʼt reveal huge dark matter reservoirs

Segue 3: Magellan/ 12 Gyr [Fe/H] = -1.7 IMACS photometry + Keck/DEIMOS spectroscopy

d ~ 17 kpc, MV ~ -0.1, rhalf ~2 pc

+1.5 σvel = 1.5 -1.0 km/sec

Fadely, Willman, Geha, Walsh et al 2011 Kinematics donʼt reveal huge dark matter reservoirs

Segue 3: Magellan/ 12 Gyr [Fe/H] = -1.7 IMACS photometry + Keck/DEIMOS (probably) NOT A GALAXYspectroscopy

d ~ 17 kpc, MV ~ -0.1, rhalf ~2 pc

+1.5 σvel = 1.5 -1.0 km/sec

Fadely, Willman, Geha, Walsh et al 2011 Kinematic studies reveal huge dark matter reservoirs

+1.4 σ = 3.7 -1.1 km/sec (incl. binary correction)

5 Mass within 30 pc ~ 6 x 10 MSun, M/L ~ 3000 6 stars with [Fe/H] estimates from -1.7 to -3.3

Keck/DEIMOS observations of Segue 1 Simon, Geha ... Kirby... 99% complete sample of stars with r < 22 within 2 Willman et al (2011) reff (60 pc) Kinematic studies reveal huge dark matter reservoirs

+1.4 σ = 3.7 -1.1 km/sec (incl. binary correction)

5 Mass within 30 pc ~ 6 x 10 MSun, M/L ~ 3000 DWARF 6GALAXY stars with [Fe/H] estimates from -1.7 to -3.3

Keck/DEIMOS observations of Segue 1 Simon, Geha ... Kirby... 99% complete sample of stars with r < 22 within 2 Willman et al (2011) reff (60 pc) Kinematic studies complicated by irregular dynamics

10 probable members Running vsys (9 star window) high [Fe/H] candidates probable members including high [Fe/H] 0

ï10 [km/sec] helio

v ï20

ï30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 projected distance from center [rhalf,ell]

ï10 [km/sec] helio

v ï20

ï30 50 0 ï50 projected majorïaxis distance from center [pc]

Keck/DEIMOS and KPNO observations of Willman, Geha... Simon,

Willman I (MV ~ -2, d ~ 38 kpc) Kirby et al (2011) [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 [Fe/H] spread can distinguish galaxies from clusters: Willman 1

filled ï candidate Wil 1 members open ï MW stars 16

18 0 r

22 bright RGB faint RGB MS/BHB

ï0.5 0.0 0.5 1.0 (g ï r) 0 Willman, Geha... Simon, Kirby et al (2011) Keck/DEIMOS and KPNO Two red giant branch star members observations of Willman I There is an [Fe/H] ([Ca/Fe]) spread: -1.7 (MV ~ -2, d ~ 38 kpc) and -2.7 (-0.4 and +0.1) [Fe/H] spread can distinguish galaxies from clusters: Willman 1

filled ï candidate Wil 1 members open ï MW stars 16

18 0 r 20 DWARF GALAXY?

22 bright RGB faint RGB MS/BHB

-14 Milky Way dwarfs 107 -12

-10

V -8 5

10 Sun

M Low luminosity from nurture or L nature? -6

Canes Venatici II -4 Leo V Pisces II

Willman I 1000 -2 Segue I 0 50 100 150 200 250 300 Distance from Milky Way (kpc)

see e.g. Willman et al 2002; Walsh, Willman & Jerjen 2009 Outer halo dwarfs: Nature vs Nurture

Leo V, Pisces II, CVn II

Morphological evidence for tidal evolution?

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

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

0 d ~ 190 kpc Dec (arcmin) -5 To Leo IV

-10

10 5 0 -5 -10 RA (arcmin) Sand, Strader, Willman et al submitted Hints of disturbed structure: 2d distribution

10 5 0 -5 -10

10 5 0 -5 -10 10 5 0 -5 -10 10 5 0 -5 -10 10 5 0 -5 -10

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

0 d ~ 170 kpc Dec (arcmin) -5

-10

10 5 0 -5 -10 RA (arcmin) Sand, Strader, Willman et al submitted Hints of disturbed structure: 2d distribution

CVn II

0 d ~ 160 kpc

-5Dec (arcmin)

-10

15 10 5 0 -5 -10 -15 RA (arcmin) Sand, Strader, Willman et al submitted Hints of disturbed structure: Ellipticity

eSDSS = 0.44 +/- 0.05 eclassical = 0.32 +/- 0.03 Sand, Strader, Willman et al submitted Hints of disturbed structure: Orientation

CVn II

-5Dec (arcmin)

-10

15 10 5 0 -5 -10 -15 RA (arcmin) Sand, Strader, Willman et al submitted Challenging pathological explanations: Luminosity-[Fe/H] relation

Segue 1 (7 stars): -2.7 +/- 0.4 dex

Wil 1 (3 stars): -2.1 +/- 0.4 dex

Kirby et al 2011 Connecting observations to theory: How many dwarfs vs subhalos

Corrections for SDSS footprint and luminosity bias suggest ~100-300 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) Connecting observations to theory: How many dwarfs vs subhalos

mass range of

) 4 known dwarfs subhalos 3

2 (Cumulative N 10

log 1 Via Lactea 2 data 0 5.0 5.5 6.0 6.5 7.0 7.5 8.0 log10(MSun in central 300 pc)

The devil is in the details (e.g. densities, spatial distributions) Current and Future Optical Surveys

LSS SM = SkyMapper PS LSST SM LSSTï1exp PS = PanStarrs 1 DR8 LSST 0.1000 10000

) DES 2 LSSTï1exp 0.0100 DES PS 1000 Area (deg RCS2 DR8 0.0010 SM

MW Volume Fraction RCS2 Stripe82

100 0.0001 Stripe82 21 22 23 24 25 26 27 28 0 50 100 150 200 250 300 Approx. rïband Limit MSTO Detection Dist (kpc) 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 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? The End