the least luminous galaxies: faint but not dull daniel zucker macquarie university/ anglo-australian...

The Least Luminous Galaxies: Faint But Not

Dull

Daniel ZuckerMacquarie University/ Anglo-

Australian Observatory

I. Introduction

27 July 2009 Daniel Zucker • Extreme Star Formation in Dwarf Galaxies • Ann Arbor, Michigan

The “Missing Satellite” Problem, circa 2005

• CDM models predict far more low-mass dark subhalos and substructure than dwarfs and streams observed -- “missing satellites”†

• Some theoretical solutions: inhibited star formation; observed satellites much more massive; observed satellites originally more massive but tidally stripped‡

Luminous Matter

Bullock & Johnston 2005

Dark Matter

Diemand+ 2006

† Klypin+ 1999, Moore+ 1999, Benson+ 2002

‡ Somerville 2002, Benson+ 2002; Stoehr+ 2002; Kravtsov+ 2004


The Observational Perspective, circa 2005

• 2003: only 9 Milky Way (MW) dwarf spheroidal galaxies (dSphs), still fewer around M31

• 2004: Andromeda IX (Zucker+)

• 2005: Ursa Major (Willman+), Andromeda X (Zucker+); runts or tip of an iceberg?

Cetus dSph

Whiting+ 1999Subaru g,r,i

And IX


The Sloan Digital Sky Survey

• SDSS-I: Large-area imaging and spectroscopic survey, covered ~25% of the sky

• SDSS-II: just finished, SDSS-III now underway


Stellar Density in SDSS Data

• SDSS data for the North Galactic Cap cover over 8000 sq. deg.• ~56 million stellar objects identified by SDSS pipeline in DR5


The SDSS “Field of Streams”

Density composite from magnitude slices:probing distances in upper main sequence and turn-off stars


A Field of Streams...and Dots

NGC 5466

NGC 5272

Canes Venatici I


The Ultra-Low-Luminosity Explosion

• Wide-area surveys (SDSS,CFHT) ~20 new low-luminosity LG dwarfs since 2004, almost all dSphs

• Many of the new dwarfs have very low measured masses (< 108 M) most sensitive to reionisation and feedback processes Walsh+ 2008


And Now, for Something Different: Leo T, The Smallest Star-Forming Galaxy

• Leo T not dead yet: stars formed within few x 108 yr

• HIPASS, GMRT, WSRT: ~3 x 105 M H I, RV ~ 35 km s-1

• MH I/M ~ 2, (Virialized) Mdyn~ 7x106 M -- how did it keep/accrete gas? Are there many more like Leo T?

INT g,rIrwin+ 2007, Ryan-Weber+ 2008

GMRTWSRT


MV vs. rh for the New Dwarfs

• Few objects with rh between ~40 pc and ~100 pc characteristic size scale for objects with dark matter? Implications+?

• What is the nature of the objects in the “gap” --star clusters or dwarfs?

+ e.g. Gilmore+ 2008, Strigari+2008

Lum

inosi

ty

Size

Gap?

No DM DM


A Common (Minimum) Mass for Galaxies?

Strigari+ 2008Matteo+ 1993



Some Big Questions about Little Galaxies

• How many low-luminosity dwarfs are there? What are their properties, and how are they distributed?

• Is there a minimum galaxy mass? (Is there a common galaxy mass?) If so, what does this tell us about dark matter/ star formation/ both?

• Is there a minimum galaxy size? What are the objects in the “gap”?

• What are their stellar populations, and what can they tell us about the conditions of star formation in such extreme systems over a cosmic time?

II. Stellar Populations of Low-Luminosity Dwarfs*


“The stellar populations of ultra-faint dwarfs are still a mystery” – O. Gnedin (2008)

Stellar Populations with HST

• 101-orbit HST program to observe 4 new Milky Way satellites, 3 M31 satellites and Leo T, using F606W (broad ~V) and F814W (~ I) filters

• WFPC2 (RIP): 4-chip batwing, 160” on a side

And XIII F606W

Zucker+, in prep; Martin+, in prep


Hercules MV = -6.6


CVn I MV = -8.6


Leo T MV = -8.0


Star Formation/Chemical Enrichment Histories

• Hercules: purely old (~14 Gyr), metal poor

• CVn I: extended star formation (14 - 8+ Gyr), no clear sign of younger population (cf. Martin et al. 2008), increasing metallicity

• Leo T: extended, multi-episodic star formation (14 - 6 Gyr, 4 Gyr - 1 Gyr, 250 Myr)

HerCVn ILeo T


Current Star Formation in Extreme Dwarfs

• Leo T: a unique (?) laboratory

• H (Gemini) and UV (pointed GALEX + Swift/UVOT) observations to study recent star formation: no detected H II regions no ongoing SF… or no O stars?

INT g+r, GALEX NUV, FUV

Leo T

Zucker+, in prep.

Gemini Hα, Swift UVW1, UVW2

Leo T


Star Formation History vs. DG

DG(kpc) MV

>10 Gyr (z>2)

5-8 Gyr(0.5<z<1)

<~1 Gyr(z<0.1)

And XI ≥100 -7.3 Old

And XIII ≥100 -6.9 Old+

Her 140 -6.6 Old

CVn II 150 -4.9 Old+

Leo IV 160 -5.0 Old+

CVn I 220 -8.6 Old+ Inter+

Leo T 420 -8.0 Old+ Inter+ Young!


Summary


Epilogue: Some Things Coming Up Down Under…

• Skymapper: southern all-sky 6-filter photometric survey

• HERMES: 400 fiber high-res (R~30000) spectrograph with 2° FOV for AAT

• WALLABY: southern all-sky H I survey proposed for ASKAP


the least luminous galaxies: faint but not dull daniel zucker macquarie university/ anglo-australian...

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