Massive Stars and Star Clusters in the Antennae Galaxies

Brad Whitmore

2006, May Workshop

OUTLINE

• The Formation of Young Massive Clusters

• Three Questions about Massive Stars

• The Big Picture

Principle Collaborators:- Rupali Chandar- Francois Schweizer- Mike Fall- Qing Zhang- Barry Rothberg

The Formation of Young Massive Clusters (i.e. Super Star Clusters)

• Most stars are formed in groups and clusters, (e.g., Lada & Lada, 2003)

• Star formation is enhanced in merging galaxies, and most of the star formation is in the form of young massive clusters.

• Hence, understanding what triggers the formation of star clusters in mergers is important for understanding the formation of stars in general.

Toomre, “The Evolution of Galaxies and Stellar Populations”, 1977

NGC 4038/9 is the youngest and nearest galaxy in the Toomre sequence, hence perhaps our best chance for understanding the formation of massive compact star clusters in mergers.

Are They Really Young Globular Clusters ?

Some of the young clusters we see in the Antennae (and other merging galaxies) have the:

• Colors (-0.2 < V-I < 0.6)

• Luminosities (-15 < Mv < ?, power law LF with index ~ -2)

• Sizes (Reff ~ 4 pc)

• Distributions (similar to the field stars)

• Spectra (~ 10 objects age dated at 3 - 20 Myr)

• Velocity Dispersions (10 - 15 km/s)

• Masses (104 - 106)

to be globular clusters with ages in the range 1 to ~ 500 Myr.

Perhaps the most convincing evidence that some clusters will survive to become regular globular clusters is the presence of these ~500 Myr clusters in the NE extension.

Three Questions about Massive Stars

• What triggers the formation of stars (clusters) ?

• What fraction of stars are in clusters, and is this consistent with the idea that essentially all stars form in clusters ?

• What can we say about the stellar content of the super star clusters ?

It is clear that shocks are important (e.g., along spirals arms).

In mergers, one popular model (e.g., Kumai et al. 1993) is that cloud-cloud collisions with velocities ~ 50 –100 km/s are required.

We have used STIS long-slit spectra in 3 positions angle of the Antennae to test this idea.

What triggers the formation of clusters ?

We find the the velocity fields are remarkably quiescent.

RMS dispersions are ~10 km/sec, essentially the same as in the disks of normal spiral galaxies.

This does not favor high velocity cloud-cloud collision models.

Instead, models where a high pressure interstellar medium implodes GMCs without greatly altering their initial velocity distribution are favored (e.g, Jog and Solomon, Elmegreen, ….)

.

Whitmore et al. 2005

What Fraction of Massive Stars in the Antennae are in Clusters?

Studies of starburst and merging galaxies find that 10 – 50 % of the UV light (i.e., young stars) are in clusters (Meuer 1995).

The initial fraction of stars in clusters is even higher since most clusters don’t survive.

Our Antennae model predicts ~ 8 % of the UV luminosity is from clusters, in agreement with observations ( ~10 %).

Hence, our data is consistent with the idea that all stars are born in clusters.

See Fall, Chandar, Whitmore (2005) for a related calculation using total H_alpha flux.

~ median age of observed clusters

Whitmore, Chandar & Fall (2006) model

(details in big picture part of talk)

Even clusters that survive loose a large fraction of their stars from their outer halos.

~ 50 % of the light is beyond 50 pc in Knot S, a typical tidal radius for a globular cluster.

Bastian & Goodwin (2005) find similar profiles in M82, N1569, and N1705, compatible with N-body simulations of clusters with rapid removal of mass due to gas expulsion.

Fall, Chandar & Whitmore (2005) make a similar argument to explain the disruption of ~90 % of clusters in the first 10 Myr (infant mortality).

Whitmore et al. 1999

~ 10 Myr

~ 500 Myr

Other observations that support this basic picture are:

Chandar et al. (2005) find that the integrated spectrum of the field

stars in several local starburst galaxies is consistent with formation

from clusters which have dissolved, with typical time scales of

7 – 10 Myr. (NOTE: Similar result by Tremonti et al. 2001).

Wit et al. (2005) use proper motions from Hipparcus to estimate

that only 4 +/- 2 % of the O and B stars in the Milky Way formed

outside of groups or clusters

• Most of the O and B stars in the field are consistent with

being “runaway” stars from nearby groups and clusters.

What can we say about the Stellar Component in the Antennae ?

In our 1999 paper, one of our primary difficulties was differentiating stars from clusters.

This led us to conclude that the number of young star clusters in the Antennae was between 800 and 8000, a pretty big range !

Our new ACS data provides a better opportunity for making this determination, and for studying the stars in their own right.

NOTE: This is work in progress, and exploratory in nature.

Before starting on the Antennae …

Ubeda, Maiz-Appendiaz, and Mackenty (2006) recently completed an

impressive piece of work on NGC 4314, a nearby (3 Mpc) starburst dwarf

galaxy.

Using CHORIOS (publicly available) , a software package they wrote to

perform maximum likliehood fits to either clusters or stars, they have

taken the game to a new level of sophistication.

There main conclusions are:

• Extinction is quite patchy, but relatively low around all but the

youngest clusters.

• 10 of the 12 clusters they study have ages < 10 Myr.

• The blue-to-red supergiant ratios are consistent with theory.

• The stellar IMF in the field is steeper than –2.8.

They isolate 12 apertures (cluster regions) and also individual field stars.

1. The most massive young clusters in merging galaxies, such as the

Antennae, appear to be excellent candidates for proto-globular

clusters.

They use up to 6 filters (F170, F336W, F555W, F814W, J, H) and make fits

assuming both stellar and cluster SEDs.

They also make confidence contour plots.

Examples for region I-As

Below is an example of a cluster where 2 individual stars (1 RSG and 1

BSG) within the cluster appear to contribute a significant fraction of

the total luminosity.

Ubeda et al. (2006) use this example to demonstrate the stochastic

nature of low mass clusters, where a single star or two can greatly

affect the results.

What would 30 Dor (core Mv ~ -10, 10**5 Msolar) look like at the

distance of the Antennae?

30 Dor

Region S

• Is it possible to distinguish clusters from stars just based on a concentration index (i.e., Vmag(3 pix) – Vmag(1 pix))?

Conclusions:

Past procedure of using Mv < -9 as criteria ( based on Humphreys 1983) to define clusters is ~ 90 % accurate.

Using point-like vs. resolved concentration indexes to define stars is only partially successful. (color-color is better).

> 50 % of the faint objects are clusters (important !)

clusters

starsstars

clusters

Chorizos Fits:

• Help distinguish between stars and clusters

• Provide estimates of Mbol and Log Te.

if cluster

if star

Radial Profiles

• Help distinguish between stars and clusters

Some evidence for triggered

star cluster formation?

Red = greater than 10 Myr

Green = 3 – 10 Myr

Blue = less than 3 Myr

Attempt to make a Mbol vs. Log Te diagram for stars.

• Selecting stars based on size or color alone does not work very

well.

• Using both size and color appears to work pretty well.

How far can a runaway O star get?

250 pc (40 km/s for 5 Myr)

Maximum runaway O star

definition . Wit et al. (2005)

Tentative Summary for Exploratory Study of Region S in the Antennae

1. It is possible to study individual stars brighter than Mv = –6 mag in

the Antennae (using both “size” and color to differentiate stars from

clusters).

2. The brightest (Mv ~ -9.1) and most massive (~100 Msolar) stars

around region S are typical of stars found in other environments

(e.g., MW, 30 Dor).

3. There is tentative evidence for some triggering of star clusters

around Region S.

4. > 50 % of the faint objects with Mv brighter than –9 are clusters.

Hence, the total number of clusters in the Antennae is closer to

8000 than 800. (NOTE: Need to sort out color selection effects).

Other Studies Comparing Stellar and Cluster

Components in External Galaxies.

Saviane, Hibbard, Rich (2004) studied the stellar component in the

dwarf at the end of the tidal tail in the Antennae. They conclude:

• There is a young component of stars with ages 2 – 100 Myr.

• No “super star clusters” are present. They suggest that the

environment is not conducive to their formation.

• They use the tip of the red giant technique to estimate a

distance which is ~ 30 % closer than our previous estimate (we

believe unlikely since peculiar velocity would have to be ~ 500

km/s).

• While there are no Super Star Clusters (i.e., with Mv < -9), many of the

point-like objects are clearly resolved clusters.

• With this low a rate of star formation we wouldn’t expect a cluster brighter

than Mv = -9. The brightest cluster on the PC is Mv = –8.4 (-7.7 if we use

their distance), hence only slightly fainter. Statistics not different physics.

The Big Picture

Roughly 40 gas-rich mergers have now been observed in detail by HST. All show young star clusters.

In addition, we find young, massive, compact clusters in: starburst dwarf galaxies (e.g., Meurer et al., 1995),

barred galaxies (Barth et al., 1995),

spiral galaxies (Larsen & Richtler, 1999)

Milky Way and LMC (e.g., Walborn 2000)

These clusters have properties similar to those seen in the mergers, but always fewer in number, and generally fainter in luminosity.

It appears that wherever you have regions of star formation, you have young massive

clusters being formed, not just in mergers !

Whitmore, 2000

Essentially all the cluster luminosity functions in merging and starbursting galaxies are power laws with index ~ -2.

Power law luminosity functions with index

-2 appear to be the norm, in spiral galaxies

as well as merging and starburst galaxies.

Larsen

(2002)

Mergers and starburst galaxies may have the brightest clusters only because they have the most clusters (i.e., there may be a “universal” luminosity function with the correlation simply being due to statistics, not special physics).

(i.e., a size-of-sample effect)

Larsen (2002) has shown that a similar relation holds versus the total star formation in a galaxy.

Whitmore, 2000

Best fit

Predicted if universal power-law, index = -2

spirals

mergers

Fall, Chandar & Whitmore (2005) show that 90 % of the young clusters in the Antennae are disrupted each decade of time (i.e, infant mortality).

The disruption rate appears to be independent of mass to first order.

The disruption rate

appears to be a power

law with index –1 for the

Antennae, the SMC

(Rafelski & Zaritsky

2005), and the MW

(Lada & Lada 2003).

Universal destruction rate ?

Fall, Chandar & Whitmore 2005

The Big Picture - A General Framework for Understanding the Demographics of Star Clusters

Ingredients (assume all stars form in clusters) :1. A universal initial mass function (power

law, index -2)2. Various star(cluster) formation histories3. Various cluster disruption mechanisms (e.g., T**-1 < 100 Myr, 2-body relaxation > 100 Myr)

4. Convolution with observational artifacts and selection effects

Observations (luminosity and age distributions, color-color diagrams, total luminosity of a galaxy, fraction of field stars, …)

Here is an example of two

of our models based on

observations of the

Antennae.

Whitmore, Chandar, Fall

(2006)

Probably the most relevant output from the model for the present discussion is the prediction of the fraction of stars in the field, as discussed earlier.

Our Antennae model predicts ~ 8 % of the UV luminosity is from clusters, in agreement with observations ( ~10 %).

Hence, our data is consistent with the idea that all stars are born in clusters.

~ median age of observed clusters

Whitmore, Chandar, & Fall (2006)

model

Summary1. The small-scale velocity dispersion between clusters is remarkably

quiescent (e.g, ~ 10 km/sec), indicating that high-velocity cloud-

cloud collisions are not the triggering mechanism for cluster

formation.

2. Observations in both the Milky Way and external galaxies support the

idea that essentially all stars are formed in groups and clusters.

3. The most luminous (massive) stars in external galaxies as far away as

the Antennae (~ 20 Mpc) can be differentiated from the clusters.

2. A general framework has been developed to explain the demographics

of stars and star clusters in galaxies. Important aspects are;

• Assumption that all stars are born in groups and clusters.

• An initial mass distribution which is a power law with index = -2

• Infant mortality of ~90 % of the clusters each decade of time.

1.