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

2. How old are galaxies? 2/21 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? An old elliptical galaxy A young spiral galaxy To first order, they are the age of the universe. Through their stars

3. Age, distance, as well as mass and chemistry, can be reasonably well characterized for a stellar cluster. Clusters are like time capsules In star clusters, all stars are the same age 3/21 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. We rely on stellar evolution theory to derive ages of star clusters

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

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

6. NGC 628 6/21 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.

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

8. 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. Stellar clusters range in age from millions to billions of years old 8/21 But That’s a big IF M51 How much can you confidently determine from these blobs? This is what we hope to improve upon! S.S. Larson, 2000, MNRAS Improving mass and age estimates of unresolved stellar clusters (Talk title)

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

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

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

12. Clusters will have quite varied properties 12/21 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 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.

13. Mass Kroupa-Salpeter IMF Mass Distribution Geneva/Padova Database, metalicity Stellar Evolution r t, r c King Model Spatial Distribution A V, R V, d CCM Model Extinction Cluster Model Age Stellar density, magnitude limits “SKY” Model Stellar Field 13/21 MASS ive Cluster Evolution and Analysis Package ( MASSCLEAN )

14. Real clusters versus Simulated cluster  Stolte et al. (2004): 1x10 6 yrs 7x10 3 M  Av=4.5 6 kpc 14/21 NGC 3603 at the distance of M31 (as seen by HST/ACS)

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

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? 17/21 Showing the 1-sigma range is deceptive when the distribution is non-Gaussian. Popescu & Hanson, 2010, ApJ Letters Popescu & Hanson, in prep. `Real clusters’ (Monti Carlo results) lie here:

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

19. There is power in information… 19/21 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. Popescu & Hanson, in prep.

20. Hunter et al. (2003) Improved ages (& masses) based on photometry alone 20/21 Popescu & Hanson, in prep.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.

21. Take Away Message 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 to better constrain the mass and age of all stellar clusters. 21/21