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

Matching LOFAR sources across radio bands

Aims. With the recent preliminary release of the LOFAR LBA Sky Survey (LoLSS), the first wide-area, ultra-low frequency observations from LOFAR were published. Our aim is to combine this data set with other surveys at higher frequencies to study the spectral properties of a large sample of radio sources. Methods. We present a new cross-matching algorithm taking into account the sizes of the radio sources and apply it to the LoLSS-PR, LoTSS-DR1, LoTSS-DR2 (all LOFAR), TGSS-ADR1 (GMRT), WENSS (WSRT) and NVSS (VLA) catalogues. We then study the number of matched counterparts for LoLSS radio sources and their spectral properties. Results. We find counterparts for 22 607 (89.5%) LoLSS sources. The remaining 2 640 sources (10.5%) are identified either as an artefact in the LoLSS survey (3.6%) or flagged due to their closeness to bright sources (6.9%). We find an average spectral index of $\alpha = -0.77 \pm 0.18$ between LoLSS and NVSS. Between LoLSS and LoTSS-DR2 we find $\alpha = -0.71 \pm 0.31$. The average spectral index is flux density independent above $S_{54} = 181$ mJy. Comparison of the spectral slopes from LoLSS--LoTSS-DR2 with LoTSS-DR2--NVSS indicates that the probed population of radio sources exhibits evidence for a negative spectral curvature.Comment: 13 pages, 22 figures and 2 tables. Accepted for publication in 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

Probing Very Bright End of Galaxy Luminosity Function at z >~ 7 Using Hubble Space Telescope Pure Parallel Observations

We report the first results from the Hubble Infrared Pure Parallel Imaging Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble Space Telescope to do deep imaging along a large number of random sightlines. To date, our analysis includes 26 widely separated fields observed by the Wide Field Camera 3, which amounts to 122.8 sq.arcmin in total area. We have found three bright Y098-dropouts, which are candidate galaxies at z >~ 7.4. One of these objects shows an indication of peculiar variability and its nature is uncertain. The other two objects are among the brightest candidate galaxies at these redshifts known to date L>2L*. Such very luminous objects could be the progenitors of the high-mass Lyman break galaxis (LBGs) observed at lower redshifts (up to z~5). While our sample is still limited in size, it is much less subject to the uncertainty caused by "cosmic variance" than other samples because it is derived using fields along many random sightlines. We find that the existence of the brightest candidate at z~7.4 is not well explained by the current luminosity function (LF) estimates at z~8. However, its inferred surface density could be explained by the prediction from the LFs at z~7 if it belongs to the high-redshift tail of the galaxy population at z~7.Comment: ApJL in press (accepted Dec. 27, 2010); minor corrections and one figure added to address referee's comment

The CFHT Legacy Survey: The Morphology-Density Relation of Galaxies out to Z~1

We study the relationships between galaxy total luminosity (M_g), morphology, color and environment as a function of redshift. We use a magnitude-limited sample of 65,624 galaxies in the redshift range 0<z<1.3 taken from one of the Canada-France-Hawaii Telescope Legacy Survey Deep Fields. We parametrize galaxy morphology according to the Sersic index n. Our n>2 number fractions at z=0.1 agree well with those based on SDSS galaxies. We find that the n>2 galaxy number fraction is constant with redshift in the field. However, for overdense environments this fraction is larger and increases towards lower redshifts, higher densities and higher luminosities. Rest-frame color-magnitude diagrams show that the color distribution is bimodal out to our redshift limit of z~1 with a prominent red-sequence of galaxies at 0.2<z<0.4 and a large blue-peak dominance at 0.8<z<1. We use this bimodality to define a red galaxy fraction (rest-frame u-g>1). For all environments, this fraction increases towards lower redshifts and higher luminosities. The red fraction within cluster-like regions changes 60% faster with redshift as compared to the field for M_g<-19.5. Using, for the first time, observations across many cluster-field interfaces distributed over a single, large volume, we trace the large-scale morphology-density relation and the Butcher-Oemler effect over a period of almost 8 Gyr.Comment: Accepted by Ap

Suzaku and Chandra observations of the galaxy cluster RXC J1053.7+5453 with a radio relic

We present the results of Suzaku and Chandra observations of the galaxy cluster RXC J1053.7+5453 ($z=0.0704$), which contains a radio relic. The radio relic is located at the distance of $\sim 540$ kpc from the X-ray peak toward the west. We measured the temperature of this cluster for the first time. The resultant temperature in the center is $\sim 1.3$ keV, which is lower than the value expected from the X-ray luminosity - temperature and the velocity dispersion - temperature relation. Though we did not find a significant temperature jump at the outer edge of the relic, our results suggest that the temperature decreases outward across the relic. Assuming the existence of the shock at the relic, its Mach number becomes $M \simeq 1.4$. A possible spatial variation of Mach number along the relic is suggested. Additionally, a sharp surface brightness edge is found at the distance of $\sim 160$ kpc from the X-ray peak toward the west in the Chandra image. We performed X-ray spectral and surface brightness analyses around the edge with Suzaku and Chandra data, respectively. The obtained surface brightness and temperature profiles suggest that this edge is not a shock but likely a cold front. Alternatively, it cannot be ruled out that thermal pressure is really discontinuous across the edge. In this case, if the pressure across the surface brightness edge is in equilibrium, other forms of pressure sources, such as cosmic-rays, are necessary. We searched for the non-thermal inverse Compton component in the relic region. Assuming the photon index $\Gamma = 2.0$, the resultant upper limit of the flux is $1.9 \times 10^{-14} {\rm erg \ s^{-1} \ cm^{-2}}$ for $4.50 \times 10^{-3} {\rm \ deg^{2}}$ area in the 0.3-10 keV band, which implies that the lower limit of magnetic field strength becomes $ 0.7 {\rm \ \mu G}$.Comment: 13page, 8 figures, accepted for publication in PASJ. arXiv admin note: text overlap with arXiv:1508.0584

Evidence for Pop III-like stellar populations in the most luminous Lyman-α emitters at the epoch of re-ionisation:spectroscopic confirmation

Faint Lyman-$\alpha$ (Ly$\alpha$) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg$^2$) Ly$\alpha$ survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Ly$\alpha$ candidates in the COSMOS field: `MASOSA' and `CR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (Keck), to confirm both candidates beyond any doubt. We find redshifts of z=6.541 and z=6.604 for MASOSA and CR7, respectively. MASOSA has a strong detection in Ly$\alpha$ with a line width of $386\pm30$ km/s (FWHM) and with high EW$_0$ (>200 \AA), but it is undetected in the continuum. CR7, with an observed Ly$\alpha$ luminosity of $10^{43.93\pm0.05}$erg/s is the most luminous Ly$\alpha$ emitter ever found at z>6. CR7 reveals a narrow Ly$\alpha$ line with $266\pm15$ km/s FWHM, being detected in the NIR (rest-frame UV, with $\beta=-2.3\pm0.1$) with an excess in $J$, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640$\AA$ emission line ($6\sigma$) which explains the excess seen in the $J$ band photometry (EW$_0$~80 \AA). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra, nor any signatures of Wolf-Rayet (WR) stars. We find that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission, and a more normal stellar population which dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Ly$\alpha$ and HeII emission, and two red components (~5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation

FR II radio galaxies at low frequencies – II. Spectral ageing and source dynamics

In this paper, the second in a series investigating FR II radio galaxies at low frequencies, we use LOFAR and VLA observations between 117 and 456 MHz in addition to archival data to determine the dynamics and energetics of two radio galaxies, 3C452 and 3C223, through fitting of spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C223, and revised energetics based on these values. We find spectral ages of $77.05^{+9.22}_{-8.74}$ and $84.96^{+15.02}_{-13.83}$ Myr for 3C452 and 3C223 respectively suggesting a characteristic advance speed for the lobes of around one per cent the speed of light. For 3C452 we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values even when considered on well resolved scales at low frequencies, but find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is due to the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms and, as such, hotspots should not be used in the determination of the underlying initial electron energy distribution

Radio source-component association for the LOFAR Two-metre Sky Survey with region-based convolutional neural networks

Large scale structure and cosmolog