Recent Star Formation Histories of Dwarf Galaxies Evan Skillman – U. Minnesota Astro 2001 – 9/2014

State of the art: ACS on HST opened up the Local Group for the study of complete star formation histories from studies of resolved stellar populations. The Local Volume is accessible for studies of recent star formation histories. A large number of galaxies in a variety of environments can be studied in this way.

Sextans A: dIrr, D = 1.4 Mpc, ground-based image, D. Hunter

I. Recent Star Formation Histories Because the lifetime of the helium burning phase is short relative to the main sequence lifetime, helium burning stars are excellent chronometers. The number of blue helium burning stars at a given magnitude can be converted to a star formation rate at a given age via stellar evolution models and an assumed IMF. These stars also provide the spatial distribution of that star formation.

Fe/H = – 0.7 Fe/H = – 1.7 Synthetic CMDs with constant SF from Dolphin et al. (2003)

Synthetic CMDs with constant SF constructed by Dan Weisz using Dolphin’s codes.

Synthetic CMDs with increasing SF in order to highlight the most recent star formation.

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

Sextans A V, V-I color magnitude diagram showing the brightest stars The low dust (metallicity) environment allows accurate photometry to cleanly separate the blue helium burning stars from the main sequence stars. The stellar evolution tracks accurately predict the positions of the red and blue sequences of helium burning stars.

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 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?

Example: Sextans A Recent Star Formation History Recreated from HST V and I Imaging Current sites of active star formation are close to HI maxima (2) These are the sites that host several generations of star formation (3) Perhaps the HI clouds are long lived (i.e., robust against feedback) Needed – a wider field view!

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 IC 2574 in the M81 group Close-up of exciting Association in the SGS

HST ACS imaging of IC 2574 in the M81 group Color-magnitude diagram for the entire ACS field

DDO 165 in the M81 group Recent Star Formation History Identified as post-starburst from EW(H-alpha) by Janice Lee WHISP HI imaging shows strongly interacting system

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)