Recent Star Formation Histories of Dwarf Galaxies Evan Skillman – U. Minnesota Gas Accretion and Star Formation in Galaxies - Munich – 

Collaborators 2 HST WFPC2 GO Programs papers on studies of nearby dwarf galaxies Andrew Cole, Robbie Dohm-Palmer, Andy Dolphin, Jay Gallagher, Mario Mateo Abi Saha, Eline Tolstoy HST Large Program on Star Formation Histories of Isolated Dwarfs in the LG PIs: Carme Gallart, Andrew Cole 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 Treasury and Archival Legacy Programs ANGST and ANGRRR PI: Julianne Dalcanton This talk draws on experiences from the following collaborative projects:

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”.

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

Synthetic CMDs created by D. Weisz using Dolphin’s codes Top panels: Increasing SFR to emphasize recent star formation Bottom panels: constant SFR. Left: “Painted” young to old Right: “Painted” old to young

CMDs of Sextans A from HST WFPC2 (Dohm-Palmer et al. 1997, 2002)

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.

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)

Example: Sextans A Recent Star Formation History Recreated from HST V and I Imaging (1)SF is “migratory” (2)Returning to same sights (3)~100 Myr time scale (4)Lack of strong bursts Only a few “cells” covered – Can these results be generalized?

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

Two supergiant shells in IC 2574 in the M81 group VLA HI observations from Walter & Brinks (1999)

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

HST ACS imaging of 3 dwarfs in the M81 group Color-magnitude diagrams for the entire ACS fields

DDO 165 in the M81 group Complete Star Formation History For details, see poster by D. Weisz

DDO 165 in the M81 group Highlighting the AGB stars Access to intermediate ages

9 M81 group dwarfs Complete Star Formation Histories For details, see poster by D. Weisz

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

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.

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.