Major dry-merger rate

and

extremely massive major dry-mergers

of BCGs

Deng Zugan

June 31st 2008. Taiwan

Outline

● Introduction

● Sample selection

● Analysis and results

● Summary and discussion

● Introduction

Why brightest Cluster Galaxies - BCGs

- Most luminous and most massive galaxies

- Oldest stellar population with little stellar formation

Does Hierarchical model work for these galaxies?

- Many have stellar halo

How do the stellar halos form?

The modern model: (De Lucia et al. 2006 ; Renzini 2007; Condelice 2007)

Strong starburst happened at early time

As the star formation is downsizing, dark matter halos continue to merge. Which result in Upsizing in galactic mass assembly.

Question: What is the processes of upsizing?

Dry mergers?

How often they merged recently?

Whether major-dry merger can happen in BCGs or not is still in controversy. (Kauffmann & Haehnelt 2000, but Makino & Hut 1997)

Merger rate of early type galaxies have been estimated by number of works.

Their results are diverse and the samples are not uniformly selected, Most did not focused on BCGs and luminous elliptical galaxies. (Renzini 2007)

However, analyses have shown that major merger would have stronger effects on the evolution of luminous early type galaxies. (Bell, 2006)

One of the possibilities that the stellar halo even the intra-cluster light may also come from major dry-mergers. (Rines et al. 2007)

However, dry mergers do not have prominent tracers as wet mergers. Wet mergers often have prominent tide tails and star forming regions.

From NOAO Deep Wide-Field Survey

Examples of dry(red) mergers

(Dokkum et al. 2005)

2.Samples

A rather big and complete sample would be needed to investigate problem mentioned above.

We use C4 cluster catalogue based on DR2 of SDSS as our parent sample. The reason are, (Miller et al. 2005)

1. They are sorted out from Sloan redshift survey samples with uniform way.

2. Rather large coverage ～ 2600 square degree.

3. It is rather complete between redshift 0.03-0.12. (contained ～ 90 Abell clusters and ～ 98 X-ray clusters)

Close pairs are considered as on going mergers under some conditions. (Bell et al. 2006)

Criteria of early type BCGs selected from C4 catalogue

1. Redshift between 0.03 and 0.12. completeness

2. Removed duplicated, misidentified sources and sources in cluster with richness less than 10.

3. They should be red, (g-r)> 0.7. dry merger

There are 515 BCGs meeting these conditions

4. Search those sources which have companions within 20kpc with comparable luminosities.

possible on going major merging

5. Not all have redshift for both objects. We adopted the way used by Bell et al. to probe merger candidates. really close (Bell et al 2006)

Use GALFIT with multiple Sérsic models to build model of the sources in both g- and r- bands.

Probe the residuals obtained by subtract models from images.

Checking the residual carefully to see if it has signals of strong interaction, such as short stellar tide tails, stellar bridges, asymmetric stellar plumes physical pairs

The color of each component can be checked again.

From model we can get magnitude for each component, and we required the difference of V band absolute magnitudes smaller than 1.5 (or, ratio of luminosities smaller than 1:4)

major dry-mergers

Non interacting

Comparing with selection cretiria used by those have redshift for both objects.

13 sources have redshift for both galaxies.

10 in them have differences in redshift smaller than 200 km/s/Mpc .

All these ten galaxies have clear features of dry major mergers, and have been classification to be physically close pairs.

3. Analysis and results

There are totally 17 close pairs meet all conditions, and have separation less than 20 kpc.

Dry major-merger rates

Comparison of our results to Bell et al. (1995)

Bell et al. studied luminous red galaxies major merger rate in GEMS (Galaxy Evolution from Morphology and SEDs)

379 red early type galaxies with redshift 0.1 < z < 0.7.

They found 6 close pairs meet conditions of dry major-mergers which almost the same as ours.

If we consider those only with projected separation smaller than 20 km/s/Mpc in our sample.

There would be 17 major dry-mergers in 515 galaxies with redshift between 0.03-0.12 in our sample.

1. Dry merger fraction in GEMs

6/379 = 1.58% redshift from 0.1 to 0.7

2. For BCGs in C4 catalogue

17/515 = 3.3% redshift from 0.01 to 0.12

One can say that BCGs have higher Dry major -merger fraction than common luminous red galaxies.

To give more quantitative comparison, we have to know

1. How the merger rate changes with redshift

Merger rate (1+z)∝ α 3 <α< 0

2. The time scale τ from closed pair to completely merged

0.15-0.2 Gyr Bell et al. (1995)

However,

≈1 Gyr White et al. (2007)

If we assume

α=0 and τ= 1.5 Myr, Then, from redshift 0 to 0.7. Early type galaxy would merger

1.3 times for sample of Bell et al. (2006)

And

2.8 times for BCGs for α= 0

The merger rate would be significantly higher if α > 0.

We can estimate the merger rates of our sample for different dependence of merger properties on the redshift

Suppose merger probability ∝(1+z)α and estimate the number of merger pairs between z = 0.1 and z = 0.7

α pair number No. of mergers

0 17 2.8

1 22 3.6

2 30 4.9

3 40 6.6

Comparing with results from luminous red galaxies analysed by Bell et al., there are 6 pairs in 379 galaxies 1.3 mergers for each in redshift from 0.1 to 0.7.

Even the time scale of dry merger is ~ 1 Gyr , the merger rates would be 0.68, 0.93 and 1.24 in redshift 0.1 – 1.0

Extremely massive mergers of BCGs

Rines et al. reported (2007) that they detected an extremely massive dry galaxy merger in a cluster with redshift z = 0.39. CL0958+4702.

It is a dry merger with stellar plume of surface brightness 24.8 mag.arcsec-2 at 50 kpc, and luminosity in the plume is at least 4L* with a diffuse halo extends at least to 110 kpc.

They considered this merger galaxy would connect with formation of fossil group and intra-cluster light.

We have found similar objects from our candidates of dry major-mergers.

1015-50 kpc

1015-200 kpc

22 mag.arcsec-2

25 mag.arcsec-2

C41015 22 mag arcsec-2 25 mag arcsec-2 C41176 22 mag arcsec-2 25 mag arcsec-2

Semi major 19 kpc 107 kpc 15 kpc 66 kpc

-23.49 Mag. -22.87 Mag.

~ 6 L* ~4.5 L*

4. Summary and discussion

● Major dry-merger do happened in BCGs with significant higher rate than non-BCGs

● BCGs in richer clusters have higher probabilities to have dry major-merger than in poorer clusters

● Luminosities of dry BCGs intend to be larger in richer clusters

● So-called extremely massive dry major-mergers are not rare even in local universe

Discussions

● Cretiria of selection merger objects should be justified

carefully

● Evolution of merger rate with redshift is largely

unknown, but should not be very simple

● How can major dry-merger can appear in center of

clusters need to be investigated with more simulation

Thank you

Correlation between dry

major-merger and

environment

Merger fraction vs. richness