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

2. 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.

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

5. 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.

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

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

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

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

10. 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.

11. Example: Sextans A
Local Group Dwarf Irregular
Distance Mpc
Recent Star Formation
History Recreated from
HST V and I Imaging
(Dohm-Palmer et al. 2002)

12. 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?

13. 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!

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

15. 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

16. HST ACS imaging of IC 2574 in the M81 group
Close-up of exciting
Association in the SGS

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

19. 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

22. 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)