Color and Metallicity Distributions of M81 Globular Clusters

Ma Jun

Two Peaks of Metallicity Ditribution of Milky Way Globular Clusters

Metal-poor peak is –1.59

Metal-rich peak is –0.55

Two Peaks of Metallicity Distribution of M31 Globular Clusters

For all M31 GCs including metallicity obtained using the metallicity-color relation:

Metal-poor peak is –1.48, 169

Metal-rich peak is –0.63, 78

For M31 GCs metallicity obtained by spectrumu:

Metal-poor peak is –1.42, 135

Metal-rich peak is –0.64, , 43

For M31 GCs including metallicity obtained using the metallicity-color relation:

Metal-poor peak is –1.43, 110

Metal-rich peak is –0.60, 56

For M31 GCs metallicity obtained by spectrumu:

Metal-poor peak is –1.36, 94

Metal-rich peak is –0.53, 31

Two Peaks of V-I Color Distribution of Some Extragalactic Globular Clusters

Data are from Hubble telescope observations

V－ I

V－ I

N

N

Relationship between Metallicity and Color

There are three explanations for two peaks of metallicity

or color distribution!

The first explanations for the bimodality of metallicity and color

1.Mutiphase in situ formation of globular clusters:

The indigenous GCs form in two distinct phases of star formation from gas of differing metallicity, giving rise to the bimodal GC metallicity distributions. The metal-poor GCs form at an early stage in the collapse of the protogalactic cloud, the metal-rich GCs form out of more enriched gas, roughly contemporaneously with the galaxy stars (Forbes et al. 1997, AJ, 113, 1652);

The second explanations for the bimodality of metallicity and color

2. Major mergers:

GCs form in galaxy mergers. If elliptical galaxies form through mergers of spiral galaxies, the merger model can predict that the GC systems of normal elliptical galaxies should have at least two peaks in the metallicity distribution. The metal-rich GCs form during the merger process. (Zepf & Ashman 1993, MNRAS, 264, 611);

The third explanations for the bimodality of metallicity and color

3. Tidal stripping or capture:The metal-rich clusters represent the galaxy’s intrinsic GC population and metal-poor component of the observed GC metallicity distribution arises from the capture of GCs from other galaxies, either through mergers or through tidal stripping (Cote et al. 1998, ApJ, 501, 554).

Two Peaks of Color and Metallicity Distribution of M81 Globular Clusters

Distribution of M81 GCs in BATC image

1. Sample: The sample of M81 GCs is from Perelmuter et al. (1995), Schrode et al. (2002) and Chandar et al. (2001). There are 95 confirmed GCs.

2. Reddening correction: Kong (2000) obtained the reddening maps of M81 based on the images in 13 BATC intermediate-band filters from 3800 to 10000 A. We used the reddening data of Kong (2000).

GC Sample and reddening correction

Two Peaks of Color Distribution of M81 Globular Clusters

Two peaks:

98.0)( 0 VB

66.0)( 0 VB

Relationship between Metallicity and Color

26 M81 GCs with metallicity uncertainties smaller than 1.0 dex.

The linear correlation coefficient is r=0.73.

Two Peaks of Metallicity of M81 Globular Clusters

We use the color-metallicity correlation to derive metallicities for the sample GCs not having spectroscopic metallicities or having metallicity uncertainties larger than 1dex.

Metal-poor peak is –1.59

Metal-rich peak is –0.55

Projected spatial distribution of the metal-poor and metal-rich GCs

Correlation between absolute magnitude and metallicity

Slope: 0.015+/--0.04

GC formation theory:If cooling from metals determines the temperature in the cluster forming clouds, massive clusters should be more metal-poor.

GC metallicity as a function of projected radius

Slope:0.009+/--0.009

Galaxy formation theory: If galaxies form in rapid monolithic dissipative collapse in which the enrichment timescale is shorter than the collapse time, the halo stars and GCs should show a radial metallicity gradient.

Conclusions:

1. The relation between spectroscopic metallicity and intrinsic B and V color also exists as it does for the Milky Way and M31.

2. We first present that the distributions of intrinsic B and V colors and metallicities of M81 GCs are bimodal, with metallicity peaks at [Fe/H]=-1.45 and -0.53, respectively, as has been demonstrated for our Milky Way and M31.

3. The distribution of metal-rich GCs is central concentrated than metal-poor.

4. There is a weak correlation between metallicity and absolute magnitude.

5. There exists a weak slope of metallicity along the projected radius.

a KMM test is applied to the data. This test uses a maximum likelihood method to estimate the probability that the data distribution is better modeled as a sum of two Gaussians than as a single Gaussian. Here we use a homoscedastic test (i.e., the two Gaussians are assumed to have the same dispersion). The (B-V)_0 of the two peaks, the P-value, and the numbers of GCs assigned to each peak by the KMM test are (B-V)_0=0.98 and 0.66, 0.071, and 67 and 27. The P-value is in fact the probability that the data are drawn from a single Gaussian distribution.