MC2^2: Multi-wavelength and dynamical analysis of the merging galaxy cluster ZwCl 0008.8+5215: An older and less massive Bullet Cluster

We analyze a rich dataset including Subaru/SuprimeCam, HST/ACS and WFC3, Keck/DEIMOS, Chandra/ACIS-I, and JVLA/C and D array for the merging galaxy cluster ZwCl 0008.8+5215. With a joint Subaru/HST weak gravitational lensing analysis, we identify two dominant subclusters and estimate the masses to be M$_{200}=\text{5.7}^{+\text{2.8}}_{-\text{1.8}}\times\text{10}^{\text{14}}\,\text{M}_{\odot}$ and 1.2$^{+\text{1.4}}_{-\text{0.6}}\times10^{14}$ M$_{\odot}$. We estimate the projected separation between the two subclusters to be 924$^{+\text{243}}_{-\text{206}}$ kpc. We perform a clustering analysis on confirmed cluster member galaxies and estimate the line of sight velocity difference between the two subclusters to be 92$\pm$164 km s$^{-\text{1}}$. We further motivate, discuss, and analyze the merger scenario through an analysis of the 42 ks of Chandra/ACIS-I and JVLA/C and D polarization data. The X-ray surface brightness profile reveals a remnant core reminiscent of the Bullet Cluster. The X-ray luminosity in the 0.5-7.0 keV band is 1.7$\pm$0.1$\times$10$^{\text{44}}$ erg s$^{-\text{1}}$ and the X-ray temperature is 4.90$\pm$0.13 keV. The radio relics are polarized up to 40$\%$. We implement a Monte Carlo dynamical analysis and estimate the merger velocity at pericenter to be 1800$^{+\text{400}}_{-\text{300}}$ km s$^{-\text{1}}$. ZwCl 0008.8+5215 is a low-mass version of the Bullet Cluster and therefore may prove useful in testing alternative models of dark matter. We do not find significant offsets between dark matter and galaxies, as the uncertainties are large with the current lensing data. Furthermore, in the east, the BCG is offset from other luminous cluster galaxies, which poses a puzzle for defining dark matter -- galaxy offsets.Comment: 22 pages, 19 figures, accepted for publication in the Astrophysical Journal on March 13, 201

Neutral hydrogen gas, past and future star-formation in galaxies in and around the 'Sausage' merging galaxy cluster

CIZA J2242.8+5301 ($z = 0.188$, nicknamed 'Sausage') is an extremely massive ($M_{200}\sim 2.0 \times 10^{15}M_\odot$ ), merging cluster with shock waves towards its outskirts, which was found to host numerous emission-line galaxies. We performed extremely deep Westerbork Synthesis Radio Telescope HI observations of the 'Sausage' cluster to investigate the effect of the merger and the shocks on the gas reservoirs fuelling present and future star formation (SF) in cluster members. By using spectral stacking, we find that the emission-line galaxies in the 'Sausage' cluster have, on average, as much HI gas as field galaxies (when accounting for the fact cluster galaxies are more massive than the field galaxies), contrary to previous studies. Since the cluster galaxies are more massive than the field spirals, they may have been able to retain their gas during the cluster merger. The large HI reservoirs are expected to be consumed within $\sim0.75-1.0$ Gyr by the vigorous SF and AGN activity and/or driven out by the out-flows we observe. We find that the star-formation rate in a large fraction of H$\alpha$ emission-line cluster galaxies correlates well with the radio broad band emission, tracing supernova remnant emission. This suggests that the cluster galaxies, all located in post-shock regions, may have been undergoing sustained SFR for at least 100 Myr. This fully supports the interpretation proposed by Stroe et al. (2015) and Sobral et al. (2015) that gas-rich cluster galaxies have been triggered to form stars by the passage of the shock.Comment: Accepted to MNRAS, 14 pages, 9 figures, 7 table

MC2^2: Dynamical Analysis of the Merging Galaxy Cluster MACS J1149.5+2223

We present an analysis of the merging cluster MACS J1149.5+2223 using archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from Keck/DEIMOS and Gemini/GMOS. We employ two and three dimensional substructure tests and determine that MACS J1149.5+2223 is composed of two separate mergers between three subclusters occurring $\sim$1 Gyr apart. The primary merger gives rise to elongated X-ray morphology and a radio relic in the southeast. The brightest cluster galaxy is a member of the northern subcluster of the primary merger. This subcluster is very massive (16.7$^{+\text{1.25}}_{-\text{1.60}}\times\text{10}^{\text{14}}$ M$_{\odot}$). The southern subcluster is also very massive (10.8$^{+\text{3.37}}_{-\text{3.54}}\times\text{10}^{\text{14}}$ M$_{\odot}$), yet it lacks an associated X-ray surface brightness peak, and it has been unidentified previously despite the detailed study of this \emph{Frontier Field} cluster. A secondary merger is occurring in the north along the line of sight with a third, less massive, subcluster (1.20$^{+\text{0.19}}_{-\text{0.34}}\times\text{10}^{\text{14}}$ M$_{\odot}$). We perform a Monte Carlo dynamical analysis on the main merger and estimate a collision speed at pericenter of 2770$^{+\text{610}}_{-\text{310}}$ km s$^{-\text{1}}$. We show the merger to be returning from apocenter with core passage occurring 1.16$^{+\text{0.50}}_{-\text{0.25}}$ Gyr before the observed state. We identify the line of sight merging subcluster in a strong lensing analysis in the literature and show that it is likely bound to MACS J1149 despite having reached an extreme collision velocity of $\sim$4000 km s$^{-\text{1}}$.Comment: 17 pages, 12 figure

The role of cluster mergers and travelling shocks in shaping the Hα\alpha luminosity function at z∼0.2\bf z\sim0.2: `sausage' and `toothbrush' clusters

The most extreme cluster mergers can lead to massive cluster-wide travelling shock waves. The CIZA J2242.8+5301 ('sausage') and 1RXS J0603.3+4213 (`toothbrush') clusters ($z\sim0.2$) host enormous radio-emitting shocks with simple geometry. We investigate the role of mergers and shocks in shaping the H$\alpha$ luminosity function, using custom-made narrow-band filters matching the cluster redshifts mounted on the INT. We surveyed $\sim0.28$ deg$^2$ for each cluster and found $181$ line emitters in the `sausage' (volume of $3.371\times10^3$ Mpc$^3$ for H$\alpha$ at $z=0.1945$) and $141$ in the `toothbrush' ($4.546\times10^3$ Mpc$^3$ for H$\alpha$ at $z=0.225$), out of which $49$ (`sausage') and $30$ (`toothbrush') are expected to be H$\alpha$. We build luminosity functions for the field-of-view down to an average limiting star formation rate of $0.14$ M$_{\odot}$ yr$^{-1}$, find good agreement with field luminosity functions at $z=0.2$, but significant differences between the shapes of the luminosity functions for the two clusters. We discover extended, tens-of-kpc-wide H$\alpha$ haloes in galaxies neighbouring relics, which were possibly disrupted by the passage of the shock wave. By comparing the `sausage' cluster with blank fields and other clusters, we also uncover an order of magnitude boost (at $9\sigma$ level) in the normalisation $\phi^*$ of the luminosity function in the relic areas. Our results suggest that cluster mergers may play an important role in the evolution of cluster galaxies through shock-induced star formation.Comment: Accepted for publication in MNRAS, 14 pages, 9 figure

Surface brightness discontinuities in radio halos. Insights from the MeerKAT Galaxy Cluster Legacy Survey

Dynamical motions in the ICM can imprint distinctive features on the X-ray images that map the thermal emission from clusters, such as sharp surface brightness discontinuities due to shocks and cold fronts. The gas dynamics during cluster mergers may also drive large-scale turbulence in the ICM which in turn generates extended synchrontron sources known as radio halos. The presence of surface brightness edges in the thermal gas of clusters has been established by a number of X-ray observations. In contrast, edges in radio halos have been observed only in a handful of cases. Our goal is to search for new radio surface brightness discontinuities in the ICM. We inspected the images of the Bullet Cluster and the other 25 radio halos reported in the MeerKAT Galaxy Cluster Legacy Survey. To aid the identification of surface brightness discontinuities, we applied a gradient filtering edge detection method to the radio images. We found that the adopted filtering technique is helpful to identify surface brightness edges in radio images, allowing us to identify at least one gradient in half of the radio halos studied. For the Bullet Cluster, we found excellent agreement between the locations of the 4 radio discontinuities detected and X-ray edges. This similarity informs us that there is substantial interplay between thermal and non-thermal components in galaxy clusters. This interplay is likely due to the forzen-in ICM magnetic field which mediates the advection of cosmic rays while being dragged by thermal gas flows. We conclude that radio halos are shaped by dynamical motions in the ICM and that they often display surface brightness discontinuities apparently co-located with edges in the thermal gas emission. Our results demonstrate that new and future generations of radio telescopes will provide a complementary approach to X-rays to efficiently detect shocks and cold fronts in the ICM.Comment: 10 pages, 5 figures, 1 table (excluding Appendixes). Abstract abridged to meet arXiv requirements. Submitted to A&

The rise and fall of star-formation in z∼0.2\bf z\sim0.2 merging galaxy clusters

CIZA J2242.8+5301 (`Sausage') and 1RXS J0603.3+4213 (`Toothbrush') are two low-redshift ($z\sim0.2$), massive ($\sim2\times10^{15}M_\odot$), post-core passage merging clusters, which host shock waves traced by diffuse radio emission. To study their star-formation properties, we uniformly survey the `Sausage' and `Toothbrush' clusters in broad and narrow band filters and select a sample of $201$ and $463$ line emitters, down to a rest-frame equivalent width ($13${\AA}). We robustly separate between H$\alpha$ and higher redshift emitters using a combination of optical multi-band (B, g, V, r, i, z) and spectroscopic data. We build H$\alpha$ luminosity functions for the entire cluster region, near the shock fronts, and away from the shock fronts and find striking differences between the two clusters. In the dynamically younger, $1$ Gyr old `Sausage' cluster we find numerous ($59$) H$\alpha$ emitters above a star-formation rate (SFR) of $0.17$ M_{\sun} yr$^{-1}$ surprisingly located in close proximity to the shock fronts, embedded in very hot intra-cluster medium plasma. The SFR density for the cluster population is at least at the level of typical galaxies at $z\sim2$. Down to the same star-formation rate, the possibly dynamically more evolved `Toothbrush' cluster has only $9$ H$\alpha$ galaxies. The cluster H$\alpha$ galaxies fall on the SFR-stellar mass relation $z\sim0.2$ for the field. However, the `Sausage' cluster has an H$\alpha$ emitter density $>20$ times that of blank fields. If the shock passes through gas-rich cluster galaxies, the compressed gas could collapse into dense clouds and excite star-formation for a few $100$ Myr. This process ultimately leads to a rapid consumption of the molecular gas, accelerating the transformation of gas-rich field spirals into cluster S0s or ellipticals.Comment: Accepted for publication in MNRAS after minor referee report. 21 pages, 15 figures, 5 table