The Intracluster Medium

1. The Intracluster Medium Ryan Howie Patrick Foley

2. What is the ICM? Visual picture of Pereus Cluster. X-ray picture revealing the ICM Both pictures are 1Mpc

3. Basics of the ICM • The ICM is the warmer gas from the formation of galaxies and stars in the cluster • This gas exists at a very high temperature: 1-10 keV Temperature Simulation • It’s also at a low density ~ 10-27 g/cm3 (about 1 particle* per cm3) Density Simulation • It is the dominant baryonic mass of the cluster • Overall cluster mass is still dominated by dark matter * The term particle is misleading. Since the ICM is very hot, nearly all of the medium is ionized.

4. ICM Radiation • ICM emits primarily Bremsstrahlung radiation • Electrons gain energy when they go whizzing past nuclei. The extra energy is then emitted off as an X-ray photon

5. X-ray Simulation • X-ray distribution matches the simulated distribution well

6. Radiation • Inverse Compton scattering is another effect that results from the ICM • A higher energy electron hits a low energy photon and gives it energy

7. The Problem • The hot gas could not have stayed hot all this time • Cooling times vary depending on location ex: core of Perseus has a cooling time of about 250 million years • Gas in the outer regions has cooling times longer then the age of the clusters

8. The problem • Gas in the cores of clusters should be well below 1keV, but we do not observe this • Need to get energy from somewhere • Solutions: • Radiative cooling • Supernovae • AGN

9. Radiative Cooling • The coolest gas condenses into stars • This increases the average temperature of the gas • Observations can be used to see approximate fraction of stars made in this way

10. Supernovae • Stellar cores explode releasing a large amount of energy • Depending on location in the galaxy and the density of gas around the supernova, some of the gas can escape the galaxy. This is called a blowout • The gas has a large amount of energy that it will deposit into the ICM

11. Fixing the problem • Using N-body simulations, ideas can be tested to see if they match up to the observed results • Using radiative cooling and SN, the simulations condensed too much gas into stars to keep the temperatures at observed values

12. Active Galactic Nuclei • AGN are massive black holes (~106 Msolar) at the center of a galaxy • Matter falling into the black hole forms an accretion disk • This releases a large amount of energy (mostly gravitational) • This energy is released as radio, IR, UV, and X-rays

13. AGN in picture form

14. Fixing the problem • Still need more energy, so adding AGN was introduced • AGN seem to provide enough energy to account for the temperature of the gas • Many people have run models and AGN seem to work, but there is much more work to be done in the future

15. How AGN heat the ICM • Each model varies a little bit • All of them use a bubble as the primary mechanism • The jet rapidly heats an area of the ICM • A compression wave moves through the medium as a shock • This is how most of the energy is transmitted to the ICM

16. Perseus Cluster basics • Distance: about 250 million light years (z = 0.0183) • Consists of about 190 galaxies • Brightest X-ray cluster • Has a massive AGN at its core

17. Center of Perseus • X-ray photo of the center galaxy of the cluster (NGC 1275) Red = lower temperatures Green = intermediate temperatures Blue = hottest temperatures Enhanced image to show the small changes in brightness of the gas

18. What we see in the picture • The ripples are expanding sound waves caused by the AGN • These effects have been seen in other galaxy clusters • These features were observed and quickly were linked to the AGN inserting energy, confirming the idea that AGN must be the source of the missing energy

19. ICM Chemical Abundance Why Should we study this? “N-body simulations have shown that the baryon fraction in a rich cluster does not change appreciably during its evolution. Clusters of galaxies can thus be considered as closed systems, retaining all information about their past star-formation and metal-production histories. This suggests that direct observations of elemental abundances in the intracluster medium (ICM) can constrain the history of star formation in clusters, the efficiency with which gas was converted into stars, the relative importance of different types of supernovae, and the mechanisms responsible for the ejection and the transport of metals” -De Lucia et al., 2003

20. Chemical Abundance of the ICM • Iron in the ICM is of the same order of magnitude as the iron in galaxies. • Average metallicity of the ICM: 0.2-0.3 solar • Primordial gas cannot account for this • Significant amount of the ICM must have been processed from the cluster galaxies and then transferred to the ICM.

21. ICM Metal Enrichment Processes • Ram Pressure Stripping (RPS) • Galactic Winds • Galaxy-Galaxy Interaction • AGN Outflows • Intra-Cluster Supernovae

22. Ram-Pressure Stripping • Two (or more) interacting galaxies cause the ISM to be stripped away from the galaxies and accumulate in the ICM • Many clusters show observational evidence of RPS • Probably not very efficient in clusters due to short interaction times - Example of RPS - Most notable feature are the tails seen http://burro.cwru.edu/Talks/TidalDebris/clusters.html

23. Galactic Winds • Many supernova explosions can provide abundant thermal energy and shock waves. The ISM can absorb the excess energy and reach speeds high enough to leave the galaxy (around hundreds of km/s). • The expelled gas becomes deposited into the ICM, including the metals • Amount expelled dependent on many factors (e.g. galactic mass, ICM pressure)

24. Galaxy-Galaxy Interaction • Two galaxies passing near each other can trigger starbursts. • More stars mean more supernovae • More supernovae mean more galactic winds. • Countering effect: RPS can strip away the ISM that would form into those stars • Either way, the ISM is removed from the galaxies and will eventually get deposited into the ICM

25. Other ICM Metal Contributors: • AGN Outflows • Numerous observational evidence shows AGN jets and winds interacting with the ICM • Pressure of relativistic jets can push away some of the ISM into the ICM • Intra-cluster Supernovae • ICM contains stars stripped from galaxies or directly formed in the ICM • When these stars explode (mainly Type Ia), they enrich the ICM directly and efficiently

26. Simulating ICM Enrichment • Methods are not independent of each other – one mechanism can affect the efficiency of the other • Scales involved are not similar • Different processes have different metallicity distributions along with different timescales • Obviously, ICM enrichment is a complex process of many different mechanisms

27. ICM Enrichment Simulations • Simulations take into account RPS and Galactic Winds individually, then shows them both • 3-D Metallicity Simulation • X-Ray Metallicity Simulation

28. Metallicity Map of Abell 1060 • Red circled areas are metallicity inhomogeneous regions of the ICM • Agrees well with simulated distribution Hayakawa et al. PASJ 2004, 56, 743

29. Model results • At least 10% of the ICM is due gas removed from RPS • Inhomogeneous distribution of the enriched material is seen (plumes and stripes of metal rich material) • Agrees with recent observations of the ICM • Centrally concentrated region of metal-enriched material can be fully accounted for from RPS • Galactic winds tend to have metals more dispersed around the ICM • Believed to be the primary source of the ICM

30. ICM and Supernovae History • Supernovae Type II and Type Ia provide different chemical abundances • X-ray observations can distinguish the emission lines of the different elements produced from the supernovae • Analyzing the abundance of each element can then reveal the supernovae history of the galaxy clusters

31. Type Ia vs. Type II • Define xi as the abundance pattern of elements • ySN as the total heavy-element mass of the SNe, y i, SN as the mass of the ith element • ζ as the mass fraction of SN Ia’s matter in the ICM • Compare xi to the ICM in question • However, different values of ζ can produce similar results…

32. Finding the Most Probable ζ • The most probable ζ can be found by finding the minimum of the function where xi is each theoretical element abundance compared to the observed ICM abundance xi,icm

33. g(ζ) Fits for Two Clusters Abell 2199 AWM 7 • Similar results seen for two different kinds of supernovae models • Minima for most seen between 0 < ζ < 0.1 Nagataki and Sato, 1998

34. Finding the Most Probable ζ-Another approach • Chi-squared fits using many elements xi • σi is the standard deviation for the ICM • However, there are none available, • Instead, authors use error bars of 90% confidence values

35. ζ-Fits for Two Clusters Abell 496 Able 1060 • Similar results seen for two different SNe models Nagataki and Sato, 1998

36. Results… • Supernovae chemical abundances released into the ICM are similar to the solar system abundances • The most probable ζ for almost all clusters is 0 < ζP < 0.1 • In general, the contribution of Type II SNe enrichment outnumber Type Ia SNe by about 10 to 1

37. SNe Sources of Error • Element abundance is critically dependent on the stellar model used • Some models predict Type Ia SNe to contribute ~50% • Some models predict a top-heavy Initial Mass Function (Salpeter Function) in order to account for metal abundances • X-ray abundance patters typically have large errors associated with them

38. ICM Enrichment over Time • Depends somewhat upon the ICM deposit-model used A.) ICM enrichment occurs at same time as star-formation (metals are directly deposited) B.) Ejected mass goes into cluster halo then later falls back in (ICM enrichment lags behind star-formation) C.) Enrichment occurs somewhere in between the A.) and B.) timeframes

39. ICM Metal Enrichment over Time • Comparing these values to the ICM can reveal the star-formation history of a cluster. A.) B.) C.) De Lucia et al., 2003

40. Star Formation Rate History • SFR normalized to total stars in the galaxies • Shows star formation peak at high redshift (z ~ 5) • Results are similar for all three models De Lucia et al., 2003

41. Metal Enrichment by Galaxy Mass • Lower mass galaxy will allow more material to escape, but it will have less material to contribute to the ICM A.) B.) C.) De Lucia et al., 2003

42. Model Results: • 40-50% of metals come from galaxies with a total baryonic mass > 1x1010h-1 MSun • ICM enrichment occurs at high redshift 60-80% at redshifts >1 35-60% at redshifts >2 20-45% at redshifts >3 • Lower values for lag-model (B), higher for instantaneous model (A), in between for (C)

43. Odd ICM Use • Used to prove the existence of dark matter • Recall ICM is the dominant baryonic component of a cluster • If dark matter was false, then the ICM distribution should be matching the gravitational lensing mass-contours Clowe et al., 2004

44. References • Fabian, A. C., et. al., “A very deep Chandra observation of the Perseus cluster: shocks, ripples and conduction.” November 22, 2005 • Zanni, C., et. al., “Heating groups and clusters of galaxies: The role of AGN jets.” September 8, 2004 • Sijack D. and Springel V., “Hydrodynamical simulations of cluster formation with central AGN heating.” February 2006 • S. Schindler, “Metal Enrichment in the Intra-Cluster Medium.” astro-ph/0611631 v1: 20 Nov 2006 • Rosemary Wyse, “The intracluster Medium: An Invariant Stellar Initial Mass Function.” The Astrophysical Journal, 490:L69-L72; 20 November 1997 • Balestra et al., “Tracing the Evolution in the Iron Content of the ICM.” astro-ph/0609664 v2: 31 October 2006 • De Lucia et al., “Chemical Enrichment of the Intracluster and Intergalactic Medium in a Hierarchical Galaxy Formation Model.” MNRAS 349, 1101-1116: 2004 • S. Schindler, “Metal Enrichment Process in the Intra-Cluster Medium.” A&A 435, L25-L28: 2005 • W. Domainko, “Enrichment of the ICM of Galaxy Clusters due to Ram-Pressure Stripping.” A&A 452, 795-802: 2006 • Clowe et al., “A Direct Empirical Proof of the Existence of Dark Matter.” astro-ph/0608407 v1: 19 August 2006 • http://wikipedia.org/wiki/Intracluster_medium • http://astro.uibk.ac.at/astroneu/hydroskiteam/ HYDRO-SKI Team, Univeristy of Innsbruck • http://venables.asu.edu/quant/proj/compton.html Dept of Physics and Astronomy, Arizona State University • http://chandra.harvard.edu/photo/2003/perseus/ Harvard-Smithsonian Center for Astrophysics