Recent Star Formation Histories of Dwarf Galaxies

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Recent Star Formation Histories of Dwarf Galaxies

• Evan Skillman U. Minnesota
• Gas Accretion and Star Formation in Galaxies -
  Munich ????/2007

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Collaborators
This talk draws on experiences from the following
collaborative projects
2 HST WFPC2 GO Programs -- 11 papers on studies
of nearby dwarf galaxies Andrew Cole, Robbie
Dohm-Palmer, Andy Dolphin, Jay Gallagher, Mario
Mateo Abi Saha, Eline Tolstoy
HST Archival Legacy Program The History of Star
Formation in the Local Group Andy Dolphin, Jon
Holtzman, and Dan Weisz
HST GO Program on Star Formation Histories of M81
Group Dwarfs John Cannon, Cole, Dolphin, Rob
Kennicutt, Janice Lee, Fabian Walter, Weisz

HST Large Program on Star Formation Histories of
Isolated Dwarfs in the LG PIs Carme Gallart,
Andrew Cole
HST Treasury and Archival Legacy Programs ANGST
and ANGRRR PI Julianne Dalcanton
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Prelude

• Technique Deriving star formation
  histories from the observations of resolved stars
  in nearby galaxies.
• Ultimate Goal Comparing present day star
  and gas distributions with spatially resolved
  star formation histories allows us to measure the
  impact of star formation on the galaxy and
  thereby quantify feedback.

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Outline

• Introduction Snapshots from
• Dohm-Palmer et al. (1997, 1998, 2002)
• II. The M81 Dwarfs Project
• Recent Star Formation Histories
• of 9 SINGS Dwarf Galaxies
• ACS Nearby Galaxies Survey Treasury Program
  (ANGST) and ANGRRR (Archive of Nearby Galaxies
  Reduce, Reuse, Recycle)

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Synthetic CMDs created by D. Weisz using
Dolphins codes
Left Painted young to old Right Painted
old to young
Top panels Increasing SFR to emphasize recent
star formation
Bottom panels constant SFR.
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CMDs of Sextans A from HST WFPC2 (Dohm-Palmer
et al. 1997, 2002)
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Recent Star Formation Histories Recreated from
HST V and I Imaging
(Dohm-Palmer et al. 1998) A larger sample
allows us to see what is normal and what is
exceptional.
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Example Sextans A Local Group Dwarf
Irregular Distance 1.4 Mpc Recent Star
Formation History Recreated from HST V and I
Imaging (Dohm-Palmer et al. 2002)
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• Example Sextans A
• Recent Star Formation
• History Recreated from
• HST V and I Imaging
• SF is migratory
• Returning to same sights
• 100 Myr time scale
• Lack of strong bursts
• Only a few cells covered
• Can these results be
• generalized?

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II. M81 Dwarfs Project

• Combining spatially resolved star formation
  histories with H-alpha, UV, and far-IR imaging
  allows a comparison of the different measures of
  star formation rates and feedback.
• Comparisons as a function of metallicity and SFR
  are possible.
• Our cycle 14 HST/ACS program concentrates on 9
  dwarfs in the M81 group (SINGS/THINGS galaxies).
• Co-Is Cannon, Dolphin, Kennicutt, Lee, Walter,
  Weisz

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Two supergiant shells in IC 2574 in the M81 group
VLA HI observations from Walter Brinks (1999)
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HST ACS imaging of IC 2574 in the M81
group Placement of M81 group in the CVZ leads
to very efficient observing
Cycle 12 program PI Walter Co-Is
Cannon, Brinks, Weisz
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HST ACS imaging of 3 dwarfs in the M81
group Color-magnitude diagrams for the entire
ACS fields
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DDO 165 in the M81 group Complete Star Formation
History For details, see poster by D. Weisz
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DDO 165 in the M81 group Highlighting the AGB
stars Access to intermediate ages
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9 M81 group dwarfs Complete Star Formation
Histories For details, see poster by D. Weisz
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III. ANGST/ANGRRR

• HST Project to Survey Star Formation Histories of
  all Galaxies within 4 Mpc. PI Dalcanton
• ANGST program observations in hand will provide
  recent SFHs for 40 dwarfs
• This is a statistically significant sample
• The galaxies are located in a range of
  environments

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SFRs for three starburst galaxies identified from
the surveys of nearby galaxies by Karachentsev et
al. Note that the bursts are well resolved in
time and all have durations in excess of 100
Myr For relic starbursts, we see the whole
event. Short timescales for bursts probably
measure flickering, not actual burst duration.
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Summary

• Nearby Dwarfs can provide us with recent,
  spatially resolved SFHs for a large sample of
  galaxies.
• The recent star formation histories allow us to
  observe how star formation proceeds and to test
  simple ideas (efficiencies, feedback, metallicity
  dependences, thresholds).
• What fraction of stars are formed due to local
  processes versus global processes? (use
  spatio-temporal statistics to quantify)
• Major goal of ANGST/ANGRRR is to do this type of
  work for larger galaxies.