Improving mass and age

estimates of unresolved

stellar clustersMargaret Hanson & Bogdan Popescu

Department of Physics

How old are galaxies?

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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

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

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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

Hubble Telescope images of star clusters in M33.

For nearby galaxies, we can still make out the individual stars.. A

bit.

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M33

San Roman et al., 2009, ApJ

It works, but more accurate ages are found in a same age star cluster

What about field populations?

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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

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.

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Stellar clusters exist in all galaxies

you understand their host galaxy.

If you understand the star clusters..

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

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But That’s a big IFM51

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)

How is it done presently?The clusters are measured in individual bands, (U, B, V, etc.)

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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

Deriving age and mass of clusters

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Blue, young

Red, old

Girardi et al., 1995, A&A Mass comes from apparent magnitude once age has been constrained

This method remains in use today

Why is this method still used?

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ACS imaging of star clustersMora et al., 2009, A&A

There is no other way to do it

Clusters will have quite varied properties

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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.

Mass Kroupa-Salpeter IMF Mass Distribution

Geneva/Padova Database, metalicity

Stellar Evolution

rt , rc King Model Spatial Distribution

AV , RV, d CCM Model Extinction

Cluster Model

Age

Stellar density, magnitude

limits“SKY” Model Stellar Field

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MASSive Cluster Evolution and Analysis Package (MASSCLEAN)

Real clusters versus Simulated cluster

Stolte et al. (2004): 1x106 yrs 7x103 M Av=4.5 6 kpc

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NGC 3603 at the distance of M31(as seen by HST/ACS)

50 million Monte Carlo modelsThe B-V color lines up along the expected SSP line for a high mass cluster.

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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

All together..

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What about the color-color plot?Note the broad range of real cluster colors. Is it at all surprising?

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Showing the 1-sigma range is deceptive when the distribution is non-Gaussian.

Popescu & Hanson, 2010, ApJ LettersPopescu & Hanson, in prep.

`Real clusters’ (Monti Carlo results) lie here:

What good is this?

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Popescu & Hanson, in prep.

How is this helping??

Improving mass and age estimates of

unresolved stellar clusters

(Talk title)

There is power in information…

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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.

Hunter et al. (2003)

Improved ages (& masses) based on photometry alone

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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.

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.

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