# Presentation: 313817

## GIANT TO DWARF RATIO OF RED-SEQUENCE GALAXY CLUSTERSAbhishesh N AdhikariMentor-Jim AnnisFermilab IPM / SDSSAugust 8, 2007

## OUTLINE OF TALK

**Introduction to SDSS**
**The physics of my project: Giant to Dwarf Ratio in Red Galaxy Clusters**
**MaxBCG: A cluster detection algorithm**
**My procedure and programming**
**Initial Results for the ratio and issues I had with it**
**Latest Results: Explanation and Conclusion**
**Comparing my results with the rest of the scientific community**
**Further Steps**

## INTRODUCTION TO SDSS

- Sloan Digital Sky Survey is one of the most ambitious astronomical surveys ever undertaken
- When complete, it will have mapped more than a 100 million objects and 25 percent of the sky
- The SDSS red-shift limit for galaxies is 0.4 (a distance of a few billion light years)
- The survey releases all its data on the internet and constantly updates it

## GIANT TO DWARF RATIO OF RED GALAXY CLUSTERS

- Galaxy
- Cluster: A group of galaxies that are gravitationally bounded
- Field Galaxies: Galaxies that are not a part of any cluster
- Blue Galaxies: Blue, Star forming galaxies
- Red Galaxies: Red, Very little star formation, bigger
- Giant Galaxies: Bright galaxies
- Dwarf Galaxies: Faint galaxies

## GIANT TO DWARF RATIO OF RED GALAXY CLUSTERS

- Luminosity Function of present day Red galaxy clusters (take note of the curve shape only)

## GIANT TO DWARF RATIO OF RED GALAXY CLUSTERS

- Two theories regarding the evolution of red sequence galaxy clusters:

- 1) Red Galaxies form very early on and evolve very passively afterwards

- 2) Red galaxies constantly evolve, and are formed from the merger of blue field galaxies

## GIANT TO DWARF RATIO OF RED GALAXY CLUSTERS

- Theory 2 would mean an increase in giant to dwarf ratio at higher red-shifts

- Theory 1 would mean a constant giant to dwarf ratio over time

## GIANT TO DWARF RATIO OF RED GALAXY CLUSTERS

- Why is this important:
- Theory of evolution of galaxies
- Cosmological Parameters and the Large Scale Structure
- Useful for other surveys

## GIANT TO DWARF RATIO OF RED GALAXIES

- Only a handful of people have done investigation in this particular area, the reason being there has never been a huge cluster catalog.
- Nevertheless, a few people have tried to figure out if the ratio changes using a handful of clusters:
- Stott et al. Compares 5 clusters
- De Lucia et al. Compares 14 clusters
- (Both conclude increasing giant to dwarf ratio at high red-shifts)
- Sloan Digital Sky Survey, with its huge database was ideal to conduct this research on a large number of clusters.
- But first, we needed an algorithm of detecting red cluster galaxies in the SDSS database

## MaxBCG ALGORITHM

- A red-sequence galaxy cluster finding algorithm, described in Koester et al.
- Based on identifying the brightest galaxy that exists at a cluster’s center
- Once the center is located, search for member red-sequence galaxies using the following criteria:
- A Standard Radius
- Magnitude Cuts
- Color Cuts (Getting just the red-sequence galaxies)

## MaxBCG ALGORITHM: Color Cuts

- It has been observed that red sequence cluster galaxies inhabit a tight sequence in the color-magnitude diagram. This is called the E/S0 ridgeline.

- In MaxBCG color cuts, you select galaxies whose colors are within 2 sigma of the central color value of the ridgeline, where sigma is:

## MaxBCG ALGORITHM

- Koester had already performed the algorithm earlier on the SDSS database, and he had a list of centers (about 14000).
- But his magnitude limits weren’t faint enough for us to be able to use in our project
- I also did not have access to any of his codes he used for the algorithm

## MY PROCEDURE

- Study the theory behind the algorithm to understand how exactly it is supposed to work.
- Then convert the algorithm to programming codes so that I could do something useful with it.
- Extract all the cluster centers from the MaxBCG catalog (using TCL)
- Use those centers and the required cuts to do a search in the SDSS database ( using SQL) to get a members catalog for the clusters
- Count the number of galaxies in each of the clusters, find the required ratios and analyze

## INITIAL RESULTS AND ISSUES

- This was my initial result for giant to dwarf ratio vs red-shift

## INITIAL RESULTS AND ISSUES

- This was quite an unexpected result, and it seemed that something was going wrong

## INITIAL RESULTS AND ISSUES

- I re-assessed all of my procedures, and figured out that my problem existed in the color cuts. I created color-magnitude diagrams for some of my clusters

## COLOR MAGNITUDE DIAGRAMS

- I should not have ignored the slope in the color-magnitude diagram

## COLOR MAGNITUDE DIAGRAMS

- The error in the colors seemed to be way too high for higher redshifts

## COLOR MAGNITUDE DIAGRAMS

## COLOR MAGNITUDE DIAGRAMS

- At higher red-shifts, objects are further away, and hence they appear fainter
- The fainter the object appears on the telescope, its photometric precision gets less accurate

- Looking at the graph, we can’t trust the data for red-shift more than about 0.2

## NEW RESULTS

- After making the adjustment for the slope, and understanding that I can’t trust the data for red-shift range 0.2-0.3, I plotted the graph and got the following result

## MY CONCLUSION

- So ignoring data for red-shift range 0.2-0.3, and only looking at the range of 0.1 to 0.2, I came to the conclusion that there is **no significant change in the giant to dwarf ratio** of red galaxies in clusters at different red-shifts.

## COMPARING MY RESULTS WITH OTHER PEOPLE

- Strength
- My data is very precise. I use 14000 clusters as opposed to 10 or 20 by others

- Weakness
- My red-shift range is small (0.1 to 0.2) compared to De Lucia whose clusters range from 0.02 to 0.7

## FURTHER STEPS

- Background subtraction
- Modifying MaxBCG algorithm to make it more efficient for my project
- A better way of selecting galaxies at higher red-shifts ( 0.2 to 0.3)
- Analyzing clusters with red-shifts higher than the SDSS range ( >0.4 )
- Extensive theoretical analysis of the explanation and consequences of my results

## QUESTIONS???