Improving mass and age estimates of unresolved stellar clusters

1. Improving mass and age estimates of unresolved stellar clusters Margaret Hanson & BogdanPopescu Department of Physics

2. How old are galaxies? To first order, they are the age of the universe. An old elliptical galaxy But we can see galaxies which contain mostly OLD or mostly YOUNG stars. Why does star formation cease? What triggers star formation? This is something we wish to understand: Galaxies, their formation and evolution. How is this done? Through their stars 2/21 A young spiral galaxy

3. In star clusters, all stars are the same age We rely on stellar evolution theory to derive ages of star clusters We need to study the colors of the individual stars in the cluster to derive the age. The stars lie in different regions based on their intrinsic mass. Single aged stellar cluster Color-Magnitude Diagram of stars in a single (old) cluster. Age, distance, as well as mass and chemistry, can be reasonably well characterized for a stellar cluster. Clusters are like time capsules 3/21

4. For nearby galaxies, we can still make out the individual stars.. A bit. Hubble Telescope images of star clusters in M33. M33 San Roman et al., 2009, ApJ 4/21

5. What about field populations? More Hubble Space Telescope Images. A: ACS, W: WFPC2). M33 A1 field Mixed stellar populations Red Giant Branch Young blue stars Red clump Barker et al., 2007, AJ CMD of a mixed population of stars It works, but more accurate ages are found in a same age star cluster 5/21

6. What if you can’t see individual stars? Single stars in galaxies more than 1 million light years away can not typically be resolved. However, single stellar clusters can be seen. NGC 628 6/21

7. Stellar clusters exist in all galaxies If you understand the star clusters.. you understand their host galaxy. 7/21

8. Stellar clusters range in age from millions to billions of years old If we can derive accurate ages and masses for the clusters, they provide us with a means to measure the star formation history of a distant galaxy. M51 But That’s a big IF How much can you confidently determine from these blobs? Improving mass and age estimates of unresolved stellar clusters This is what we hope to improve upon! (Talk title) S.S. Larson, 2000, MNRAS 8/21

9. How is it done presently? The clusters are measured in individual bands, (U, B, V, etc.) (ApJ, 1985) Young blue Blue Blue Red Red Old red The location of clusters in these diagrams are compared to predictions by Simple Stellar Population (SSP) models to get age. 9/21

10. Deriving age and mass of clusters Blue, young Red, old Girardi et al., 1995, A&A Mass comes from apparent magnitude once age has been constrained 10/21

11. This method remains in use today ACS imaging of star clusters Mora et al., 2009, A&A Why is this method still used? There is no other way to do it 11/21

12. SSP models use statistics of stellar evolutionary lifetimes to assign the fractional light expected from the different stars as the cluster ages. They assume, light is ONLY a function of age, not mass. Clusters will have quite varied properties Our new models derive light as fxn of age and mass. This allows for the stochastic variation expected to be seen in real clusters (particularly low mass clusters). Our SSP-like model is called MASSCLEAN 12/21

13. MASSive Cluster Evolution and Analysis Package (MASSCLEAN) Mass Geneva/Padova Database, metalicity Mass Distribution Stellar Evolution Kroupa-Salpeter IMF rt , rc Spatial Distribution King Model AV , RV, d Extinction CCM Model Cluster Model Stellar density, magnitude limits Stellar Field “SKY” Model Age 13/21

14. Real clusters versus Simulated cluster NGC 3603 at the distance of M31 (as seen by HST/ACS) Stolte et al. (2004): 1x106 yrs 7x103 M Av=4.5 6 kpc 14/21

15. 50 million Monte Carlo models The B-V color lines up along the expected SSP line for a high mass cluster. But, as cluster mass decreases the variance on B-V begins to increase. 50,000 M ----------- 25,000 M --------- -- 10,000 M ----------- 5,000 M ----------- 1,000 M ----------- 200 M Among low mass clusters, B-V colors are found far from SSP predictions. 15/21

16. All together.. 13/15

17. What about the color-color plot? Note the broad range of real cluster colors. Is it at all surprising? Showing the 1-sigma range is deceptive when the distribution is non-Gaussian. `Real clusters’ (Monti Carlo results) lie here: Popescu & Hanson, 2010, ApJ Letters Popescu & Hanson, in prep. 17/21

18. What good is this? How is this helping?? (Talk title) Improving mass and age estimates of unresolved stellar clusters Popescu & Hanson, in prep. 18/21

19. We now have a database of over 50 million models. We can work back, using a statistical inference code, to determine what the most likely mass and age is for a cluster, given its observed light properties. There is power in information… Popescu & Hanson, in prep. 19/21

20. Based on Hunter et al. (2003) UBV Magnitudes, we are re-calculating all ages and masses for 900+ LMC clusters.. Note how few clusters fall in the ‘SSP-friendly’ range of mass. Popescu & Hanson, in prep. Hunter et al. (2003) Improved ages (& masses) based on photometry alone 20/21

21. Only extremely massive clusters (> 50,000 Msun) can be approximated well with current SSP models Monte Carlo simulations show the remarkable and extreme range of observed values demonstrated by moderate mass clusters. Output from our Monte Carlo simulations allows SIMULTANEOUS solutions tobetter constrain the mass and age of all stellar clusters. Take Away Message 21/21