Understanding the Physics and Observational Effects of Collisions between Galaxy Clusters

Transcription

1 Understanding the Physics and Observational Effects of Collisions between Galaxy Clusters Daniel R. Wik 1 Advisor: Craig L. Sarazin 1 1 Department of Astronomy, University of Virginia, Charlottesville, VA, drw2x@virginia.edu ABSTRACT Galaxy clusters, the largest gravitationally bound objects in the universe, form over time from collisions and mergers between clusters and between clusters and smaller galaxy groups. The results of these collisions can be detected in a variety of ways. In one case, I have found that clusters which have recently undergone a merger will have an enhanced Sunyaev-Zel dovich (SZ) Effect. Another result of these mergers is the acceleration of relativistic particles, which produce non-thermal emission that should be observable in X-rays. In the Coma cluster, the brightest merging cluster in the sky, detections of this emission have been claimed, though these findings are controversial. Using observations of Coma with the Hard X-ray Detector onboard Suzaku, I find no conclusive non-thermal emission and derive an upper limit on the emission that excludes the most recent previous detection. Finally, I present preliminary results on the direct detection of the relative velocity between two equal mass clusters currently undergoing a major merger in the Cygnus A cluster. The geometry and thermal structure of the clusters agree well with the merger scenario hinted at by previous observations of the thermal gas and the galaxy velocity distribution. Introduction Over the past decade, the history, age, and likely future evolution of the universe have been reliably uncovered for the first time, prompting some to call our time the golden age of cosmology. While past advances have come from observations of the afterglow of the Big Bang and of supernovae in distant galaxies, it will soon be possible to study the largest scales of the cosmos with the largest and most massive objects in the universe, clusters of galaxies. These tightly bound collections of thousands of galaxies grow by pulling in galaxies, groups of galaxies, and even other clusters, which they attract with their immense gravity. When 2 galaxy clusters merge, they release enormous amounts of energy, up to Joules, making cluster mergers the most energetic events since the Big Bang itself. Part of this energy is used to heat up intracluster gas the atmospheres of clusters filling up the space in between the galaxies through shock waves and turbulence. In addition to heating up the gas, shocks and turbulence act to accelerate particles traveling near the speed of light, which can emit light in the form of radio waves and X-rays. The existence of this light indicates that a cluster has recently or is currently undergoing a merger, an often difficult conclusion to draw from the optical light alone, and it can tell us about the dynamics and history of the merger, along with the specific physics necessary to produce that light. As well as studying individual clusters undergoing mergers, we can apply what we learn from these observations to understand how mergers affect large samples of galaxy clusters. Over the history of the universe, the nearby galaxy clusters we see now formed from many merger events, so if we observed one of these clusters in the past, it would hold fewer galaxies, smaller amounts of colder gas, and would overall contain less mass. The number of clusters at a given mass changes over time, and how the number evolves can tell us how much stuff is in the universe, namely the total amount of mass and dark energy the mysterious substance that appears to be accelerating the expansion of the universe. In a cluster that Wik 1

2 has reached equilibrium, the intracluster gas is in balance with the cluster s gravity, so the energy in the particles of this gas directly corresponds to the mass of the cluster, which is the source of its gravity. This gas is very energetic and hot because the gravity in clusters is so strong, so we can observe its emission at X-rays energies. Also, the hot intracluster gas will distort the Cosmic Microwave Background (CMB), the relic radiation left over from the Big Bang, via the Sunyaev-Zel dovich (SZ) effect. The amount of change in the CMB spectrum is also directly related to the energy in the cluster gas, and so can be used to measure the total mass of the cluster. From measurements of the X-ray temperature or SZ signal of many galaxy clusters, we can use their inferred masses to constrain the dark matter and dark energy content of the universe. However, the mass you derive is only valid if the cluster gas is in equilibrium and is energetically the dominant component. During violent cluster mergers, the gas is not in equilibrium but is transiently heated, which temporarily boosts the SZ signal. If a cluster is observed while it is undergoing a boost, the inferred mass will be incorrect. Also, the mass that is inferred assumes nearly all the energy in the cluster is in the thermal, hot gas particles. While this is a reasonable assumption, it is known that clusters contain nonthermal particles as well, in the form of relativistic electrons traveling near the speed of light. The precise amount of energy in these particles and another relativistic quantity, the magnetic field, has so far yet to be decisively measured, though both are required to explain the presence of cluster-wide structures seen at radio frequencies, called radio halos and radio relics, that happen to be found only in merging clusters. The existence of this emission directly implies the presence of Inverse Compton (IC) emission at high X-ray energies, which if observed would uniquely determine the energetic contribution of relativistic electrons and magnetic fields to the intracluster gas. Thus, if clusters are to be used as cosmological probes, the physics taking place during cluster mergers must be well understood. Cluster Mergers and the SZ Effect To assess the effect of mergers on measurements of the SZ effect in galaxy clusters, we used a small suite of binary cluster merger simulations from Ricker & Sarazin (2001). These simulations include mergers between clusters with various mass Fig. 1. Images of the SZ parameter y from 3D snapshots of the 1:3 mass ratio, 2 r s impact parameter merger simulation. The image are viewed from a line-of-sight which is rotated 45 from the merger axis and 45 azimuthally from the merger plane 386 Myr before first core crossing. Fig. 2. Same as Figure 1, but at 114 Myr after first core crossing. ratios (M < : M > = 1 : 1, 1 : 3, 1 : 6.5) and impact parameters (head-on to glancing impacts), totaling 8 unique simulations. From these we calculated the SZ observable y at each time step for Wik 2

3 which data was saved. Sample images of y from the 1:3 mass ratio merger are provided in Figures 1 and 2. In practice, only one characteristic value for the SZ signal will be extracted for an individual cluster in a large sample. Generally in the literature, this value is taken to be either the central (or equivalently the maximum) value of y, y max, or y is integrated over the entire cluster, usually denoted by Y. Though it is not immediately clear from the figures, note that the scale of Figure 2 is twice that of Figure 1, indicating that after the cluster centers have collided for the first time, the value of y increases or experiences a boost. These boosts, which appear in both Y and y max, show up as a peak when the SZ parameter is plotted as a funciton of time. The effect of boosts on the number of observed clusters as a function of redshift can then be calculated. The results were presented in last year s research paper and have been published in the Astrophysical Journal (Wik et al. 2008), so I will not repeat them here. The Search for Non-thermal Emission in the Coma Cluster While cluster mergers themselves can confuse cosmological studies that depend on the galaxy cluster mass distribution, mergers can also be exploited to census another component coincident with the gas relativistic particles and magnetic fields. If these relativistic components contain a significant amount of energy relative to the thermal gas, then they would modify the equilibrium state of the gas upon which conversions between observables, such as Y, y max, and X-ray temperature, and cluster mass are based. It is thought that relativistic particles are accelerated across shock fronts and turbulence induced by mergers (Sarazin 1999, 2002), which may explain why radio halos and relics are observed exclusively in clusters currently or recently undergoing a merger event (Feretti & Giovannini 2007). The radio emission is produced by relativistic electrons spiralling around magnetic field lines, so we know both of these relativistic components exist. However, the strength of this emission depends on both the number of electrons and the strength of the magnetic field, so from the radio data alone these components cannot be disentangled. Certain assumptions, arrived at through equipartition arguments, lead to estimates of the number of relativistic particles and magnetic field strengths that do not contribute significant amounts of energy relative to the thermal gas. This would not remain true if the value of the average magnetic field in clusters was about 10 times larger than its equipartition value, as suggested by Faraday RM measurements from which the magnetic field can be derived along particular lines of sight. The relativistic electrons that produce the radio halos and relics also produce IC emission through interactions with the CMB, and this emission should be observable at hard X-ray energies. If emission is seen at both radio and X-ray frequencies, then the magnetic field can be directly determined. However, the IC X-ray emission has been difficult to observe with past instruments, generally leading to weak or non-detections (Nevalainen et al. 2004). The most recent observatory with hard X-ray capabilities, the Suzaku X-ray Observatory, should be 3 more sensitive where IC emission is expected to be detected. We report here an observation with the Suzaku Hard X-ray Detector (HXD) of the Coma cluster, which hosts the brightest radio halo in the sky. Coma was observed for over 160,000 seconds, or for about 2 days. The greatest difficulty in detecting IC emission from clusters like Coma is that they tend to be hot because they are so massive massive clusters undergo more mergers typically and are thus more likely to host radio halos or relics. The hotter the gas in a cluster, the more thermal emission there is at higher energies, which is where we are looking for the IC emission. So, in order to confidently detect an IC signal, the thermal emission must be well characterized. To accomplish this goal, we extract complimentary data at lower energies, where the thermal emission is completely dominant, from a mosaic observation of Coma with the XMM-Newton Observatory shown in Figure 3 (Schuecker et al. 2004). For this data, we spatially weight the X-rays according to the spatial sensitivity of the HXD (since it is a non-imaging instrument while XMM is) to create a spectrum that exactly corresponds to the spectrum obtained from the Suzaku HXD observation. Specifically, we extracted 10 spectra from the regions shown in Figure 3 and summed them with weights that decreased with the radius of the region from the center. With these two spectra, one at lower energies from XMM and one at higher energies from Suzaku, we now have a long enough lever arm to accurately characterize the thermal emission and see whether there is evidence for IC emission as well. Wik 3

4 Declination (J2000) R1 R2 R Right Ascension (J2000) Fig. 3. The XMM-Newton image of the Coma cluster. The white contours represent the spatial sensitivity of the Suzaku HXD instrument in 10% intervals with the greatest sensitivity in the center. Counts/s/keV S χ APEC XMM Point Sources CXB Energy (kev) Fig. 4. The photon detection rate as a function of energy in kilo-electron volts. The black data points are from XMM (below 12 kev) and the Suzaku HXD (above 12 kev). The bottom panel reports the residuals compared to the best-fit model, relative to the size of the error on the data points. The various histograms represent the contribution of thermal emission (labeled APEC ), the Cosmic X-ray background ( CXB ), and emission from point sources ( XMM point sources ). Even considering only statistical errors, we do not detect a non-thermal component in the total X-ray emission from Coma. When a singletemperature thermal model for the emission is fit 1T to the joint spectra (Figure 4), we obtain a temperature that agrees with the temperatures found for fits to each spectrum individually. Given this initial agreement, it is unlikely any significant excess at higher energies would be detectable in the HXD spectrum. In fact, the inclusion of a nonthermal model component leads to best-fit values that are either unphysical or statistically insignificant. On the other hand, a two-temperature model describes the data slightly better than the single-temperature model, again reinforcing the hypothesis that we have only detected thermal emission from the Coma cluster. While a two-temperature fit is essentially the most complicated thermal model that can be uniquely fit to these spectra due to the coarse spectral resolution of the instruments, cluster gas typically emits over a large range of temperatures that are known to spatially vary. We divided the XMM data into spatial regions and found the temperature of the gas in each region (Figure 5), and then we created a multi-temperature model, being careful to weight each model s contribution by the spatial sensitivity of the HXD, which was extrapolated to higher energies. Localized non-thermal emission will distort the best-fit temperature for a given region so that the extrapolated emission at higher energies will be incorrect, and large residuals should appear in the HXD spectrum. However, if the fit using the temperature map-derived models (Figure 6) is compared with the singletemperature fit (Figure 4), no obvious differences in the residuals are apparent. Therefore, we conclude that we in fact only detect thermal emission in the high energy Suzaku HXD spectrum, and the expected non-thermal contribution is missing. This result is consistent with recent findings from the INTEGRAL observatory that also detected hard X-ray photons from Coma that was entirely consistent with only thermal emission (Renaud et al. 2006; Eckert et al. 2007). To determine an upper limit for the undetected non-thermal emission, we must be careful to also include all potential systematic uncertainties that might affect fits to the data. The main uncertainty lies in the precise determination of the non-x-ray background in the instrument, which is known to an accuracy of about 3%. We also include the variation in the cosmic X-ray background, due to the non-uniformity of structure in the universe on small scales, and the estimated uncertainty in the cross-calibration between the XMM and Suzaku spectra. We determine an upper limit for IC emission by setting these systematic quantities Wik 4

5 Declination (J2000) 45: : : :00: :00.0 to the limit of their potential variation that favors detecting an IC signal. For the background level, we set it to be 3% lower than originally set, for example. We find an upper limit on the IC emission of W/m 2. This value is 2.5 lower than the most recent detections of IC emission in Coma, with the RXTE observatory (Rephaeli & Gruber 2002) and the BeppoSAX observatory (Fusco-Femiano et al. 2004). This result is not entirely surprising, especially given that the BeppoSAX detection was controversial. Rossetti & Molendi (2004) analyzed the same data as Fusco- Femiano et al. (2004) and due to a different background determination, they only found an upper limit consistent with our result. This work has been accepted for publication in the Astrophysical Journal (Wik et al. 2009) and will appear in the May 2009 issue. 27:30: : : :00: : :56:00.0 Right Ascension (J2000) Fig. 5. The temperature map derived from the XMM data. The white contours are the same as in Fig. 3. The colorbar gives the temperature in kilo-electron volts (1 kev = 11.6 million degrees Kelvin). Counts/s/keV S χ T map XMM Point Sources CXB T map Energy (kev) Fig. 6. Same as Fig. 4, except the onetemperature thermal model has been replaced by a multi-temperature model (labeled Tmap ) derived from the temperature map in Fig. 5. The Merging Cygnus A Cluster Cygnus A is one of the brightest sources in the sky at radio wavelengths. This radio galaxy sits at the center of galaxy cluster with extremely extended X-ray emission to its NW (Figure 7). Previous X-ray observations by ASCA (Markevitch et al. 1999) have revealed a higher temperature region in between the part of the cluster surrounding Cygnus A and its extension. If the extension is another cluster colliding with the cluster that hosts Cygnus A, there should be a hotter region in between them due to shocked gas, just as observed. Additional evidence that a major merger is ongoing here is derived from the observed velocities of the galaxies in the clusters (Ledlow et al. 2005). There is some evidence that the galaxies can be divided into 2 groups based on the distribution of their velocities, which would be expected in the merger scenario. The implied collision speed along our line of sight, based on these 2 lines of evidence, is about 2000 km/s, or nearly 1% of the speed of light! However, the small number of galaxies and the low significance of the high temperature region in the X-ray emitting gas cannot firmly establish the merger picture. To truly pin down that this is indeed a merging cluster and to extract its dynamics, we proposed to observe it with the Suzaku X-ray Imaging Spectrometer (XIS). The spectral resolution of the instrument is 3 better than previous and contemporary instruments and should be able to detect the velocity difference if it is this large. Taking spectra from regions along the presupposed merger axis, we are able to confirm the temperatures first observed by ASCA at higher significance, and we find that the relative velocity of the gas does indeed differ by a significant amount about 3000 km/s. The sense of this difference, i.e. which cluster is moving towards us and which is moving away from us, matches the expectation from the galaxy velocities. From this preliminary analysis we obtained the data this spring we are confident that the merger scenario has been confirmed. Also, this Wik 5

6 Shocked, High T Region Cygnus A Cluster NW Cluster non-thermal emission is expected to be found. Using this data set, we will be able to constrain IC emission in all the clusters with known diffuse radio emission, as well as in a complete sample of nearby clusters. The complete sample will allow us to evaluate the prevalence of non-thermal emission in massive clusters at the current epoch for the first time, and the radio sample will determine or constrain the relative amount of non-thermal components in the ICM of clusters. Currently, spectra for all 100 clusters have been extracted from the BAT survey. This work will allow a better determination of the relativistic energy contribution in clusters, which will address the usefulness of current observable quantity-mass relations as well as probe the physical processes that occur during cluster mergers. Fig. 7. A ROSAT PSPC X-ray image of the Cygnus A cluster. The 2 white circles indicate the locations of the 2 merging clusters, and the ellipse in between them marks the rough location of the hot region where the gas has already collided and been heated by a merger shock. additional data will allow us, in conjunction with simulations of galaxy cluster mergers, to uncover the details of such a rarely observed event. Observations like this are critical to our understanding of the physics of cluster mergers, which are typically estimated from simple dynamical arguments with little direct evidence from actual observed cases to back them up. Future Work Upper limits on the flux of IC emission in clusters with radio halos or relics provide a lower limit on the magnetic field strength, which sets a lower limit on the amount of energy in relativistic components in galaxy clusters. Even if no IC emission is detected, the derived lower limit could still indicate that a non-negligible amount of energy resides in relativistic components, which would modify observable quantity-mass relations necessary to use clusters to study cosmology. Because cluster surveys, both in X-rays and in the SZ effect, are achieving senstitivity to cosmological parameters, it is critical that merger effects are allowed for and that we fully understand the energetics of the intracluster gas. To that end, I have data from the 3- year Swift/BAT all-sky survey, which has mapped the sky in the high energy X-ray regime where REFERENCES Eckert, D., Neronov, A., Courvoisier, T. J.-L., & Produit, N. 2007, A&A, 470, 835 Feretti, L. & Giovannini, G. 2007, ArXiv Astrophysics e-prints Fusco-Femiano, R., Orlandini, M., Brunetti, G., et al. 2004, ApJ, 602, L73 Ledlow, M. J., Owen, F. N., & Miller, N. A. 2005, AJ, 130, 47 Markevitch, M., Sarazin, C. L., & Vikhlinin, A. 1999, ApJ, 521, 526 Nevalainen, J., Oosterbroek, T., Bonamente, M., & Colafrancesco, S. 2004, ApJ, 608, 166 Renaud, M., Bélanger, G., Paul, J., Lebrun, F., & Terrier, R. 2006, A&A, 453, L5 Rephaeli, Y. & Gruber, D. 2002, ApJ, 579, 587 Ricker, P. M. & Sarazin, C. L. 2001, ApJ, 561, 621, (RS) Rossetti, M. & Molendi, S. 2004, A&A, 414, L41 Sarazin, C. L. 1999, ApJ, 520, 529 Sarazin, C. L. 2002, The Physics of Cluster Mergers (ASSL Vol. 272: Merging Processes in Galaxy Clusters), 1 38 Schuecker, P., Finoguenov, A., Miniati, F., Böhringer, H., & Briel, U. G. 2004, A&A, 426, 387 Wik, D. R., Sarazin, C. L., Finoguenov, A., et al. 2009, ArXiv e-prints Wik 6

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