SSA22 Protocluster Research Articles

TheChandraDeep Protocluster Survey: point-source catalogues for a 400-ks observation of thez= 3.09 protocluster in SSA22

We present X-ray point-source catalogues for a deep ≈400-ks Chandra ACIS-I (Advanced CCD Imaging Spectrometer) exposure of the SSA22 field. The observations are centred on a z= 3.09 protocluster, which is populated by Lyman break galaxies (LBGs), Lyα emitters (LAEs) and extended Lyα-emitting blobs (LABs). The survey reaches ultimate (3 count) sensitivity limits of ≈5.7 × 10−17 and ≈3.0 × 10−16 erg cm−2 s−1 for the 0.5–2 and 2–8 keV bands, respectively (corresponding to L2-10 keV≈5.7 × 1042 erg s−1 and L10-30 keV≈2.0 × 1043 erg s−1 at z= 3.09, respectively, for an assumed photon index of Γ= 1.4). These limits make SSA22 the fourth deepest extragalactic Chandra survey yet conducted, and the only one focused on a known high-redshift structure. In total, we detect 297 X-ray point sources and identify one obvious bright extended X-ray source over a ≈330 arcmin2 region. In addition to our X-ray catalogues, we provide all available optical spectroscopic redshifts and near-infrared and mid-infrared photometry available for our sources. The basic X-ray and infrared properties of our Chandra sources indicate a variety of source types, although absorbed active galactic nuclei (AGN) appear to dominate. In total, we have identified 12 X-ray sources (either via optical spectroscopic redshifts or LAE selection) at z= 3.06 – 3.12 that are likely to be associated with the SSA22 protocluster. These sources have X-ray and multiwavelength properties that suggest they are powered by AGN with 0.5–8 keV luminosities in the range of ≈1043–1045 erg s−1. We have analysed the AGN fraction of sources in the protocluster as a function of local LAE source density and find suggestive evidence for a correlation between AGN fraction and local LAE source density (at the ≈96 per cent confidence level), implying that supermassive black hole growth at z≈ 3 is strongest in the highest density regions.

THECHANDRADEEP PROTOCLUSTER SURVEY: Lyα BLOBS ARE POWERED BY HEATING, NOT COOLING

We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 protocluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying a large fraction of active galactic nuclei (AGN) in LABs, fAGN = 17+12−7% down to L2–32 keV ∼ 1044 erg s−1. All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of NH > 1023 cm−2. The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared (mid-IR) emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photoionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that "heating" by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence for extended X-ray emission, and a stack equivalent to a ≳9 Ms exposure of an average LAB also yields no statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies there is no hot (T ≳ 107 K) gas component in these halos, and also rules out inverse Compton scattering of cosmic microwave background photons, or local far-IR photons, as a viable power source for LABs.

EXTENDED Lyα NEBULAE ATz≃ 2.3: AN EXTREMELY RARE AND STRONGLY CLUSTERED POPULATION?

To obtain an unbiased sample of bright LyA blobs [L(LyA) > 10^43 ergs/s], we have undertaken a blind, wide-field, narrow-band imaging survey in the NOAO Deep Wide Field Survey Bootes field with the Steward Bok-2.3m telescope. After searching over 4.82 sq. degrees at z=2.3, we discover four LyA blobs with L(LyA) = 1.6-5.3 x 10^43 ergs/s, isophotal areas of 28-57 sq. arcsec, and broad LyA line profiles (FWHM = 900-1250 km/s). In contrast with the extended Lyman alpha halos associated with high-z radio galaxies, none of our four blobs are radio-loud. The X-ray luminosities and optical spectra of these blobs are diverse. Two blobs (3 and 4) are X-ray-detected with L_X(2-7 keV) = 2-4 x 10^44 ergs/s and have broad optical emission lines (C IV) characteristic of AGN, implying that 50% of our sample blobs are associated with strong AGN. The other 50% of blobs (1 and 2) are not X-ray or optically-detected as AGN down to similar limits. The number density of the four blobs is ~3 x 10^{-6} Mpc^{-3}, comparable to that of galaxy clusters at similar redshifts and 3x lower than that found in the SSA22 proto-cluster at z=3.1, even after accounting for the over-density of that region. The two X-ray undetected blobs are separated by only 70" (550 kpc) and have almost identical redshifts (< 360 kpc along the line-of-sight), suggesting that they are part of the same system. Given the rarity of the blobs and our discovery of a close pair, we speculate that blobs occupy the highest density regions and thus may be precursors of today's rich cluster galaxies.

DETECTIONS OF LYMAN CONTINUUM FROM STAR-FORMING GALAXIES ATz∼ 3 THROUGH SUBARU/SUPRIME-CAM NARROW-BAND IMAGING

Knowing the amount of ionizing photons from young star-forming galaxies is of particular importance to understanding the reionization process. Here we report initial results of Subaru/Suprime-Cam deep imaging observation of the SSA22 proto-cluster region at z=3.09, using a special narrow-band filter to optimally trace Lyman continuum (LyC) from galaxies at z~3. The unique wide field-of-view of Suprime-Cam enabled us to search for ionizing photons from 198 galaxies (73 Lyman break galaxies (LBGs) and 125 Ly-alpha emitters (LAEs)) with spectroscopically measured redshifts z~3.1. We detected LyC from 7 LBGs, as well as from 10 LAE candidates. Some of the detected galaxies show significant spatial offsets of LyC from non-ionizing UV emission. For some LBGs the observed non-ionizing UV to LyC flux density ratios are smaller than values expected from population synthesis models with a standard Salpeter initial mass function (IMF) with moderate dust attenuation (which is suggested from the observed UV slopes), even if we assume very transparent IGM along the sightlines of these objects. This implies an intrinsically bluer spectral energy distribution, e.g, that produced by a top-heavy IMF, for these LBGs. The observed flux desity ratios of non-ionizing UV to LyC of 7 detected LBGs range from 2.4 to 23.8 and the median is 6.6. The observed flux density ratios of the detected LAEs are even smaller than LBGs, if they are truly at z~3.1. We find that the median value of the flux density ratio for the deteced LBGs suggest that their escape fractions is likely to be higher than 4%, if the LyC escape is isotropic. The results imply that some of the LBGs in the proto-cluster at z~3 have the escape fraction significantly higher than that of galaxies (in a general field) at z~1 studied previously.

THECHANDRADEEP PROTOCLUSTER SURVEY: EVIDENCE FOR AN ENHANCEMENT OF AGN ACTIVITY IN THE SSA22 PROTOCLUSTER ATz= 3.09

We present results from a new ultra-deep 400 ks Chandra observation of the SSA22 protocluster at z = 3.09. We have studied the X-ray properties of 234 z ~ 3 Lyman break galaxies (LBGs; protocluster and field) and 158 z = 3.09 Ly-alpha emitters (LAEs) in SSA22 to measure the influence of the high-density protocluster environment on the accretion activity of supermassive black holes (SMBHs) in these UV-selected star forming populations. We detect individually X-ray emission from active galactic nuclei (AGNs) in six LBGs and five LAEs; due to small overlap between the LBG and LAE source population, ten of these sources are unique. At least six and potentially eight of these sources are members of the protocluster. These sources have rest-frame 8-32 keV luminosities in the range of L_8-32 keV = (3-50) X 10^{43} ergs/s and an average observed-frame 2-8 keV to 0.5-2 keV band-ratio of ~0.8 (mean effective photon index of Gamma_eff = 1.1), suggesting significant absorption columns of N_H > 10^{22}-10^{24} cm^{-2}. We find that the fraction of LBGs and LAEs in the z = 3.09 protocluster harboring an AGN with L_8-32 keV > 3 X 10^{43} ergs/s is 9.5^{+12.7}_{-6.1}% and 5.1^{+6.8}_{-3.3}%, respectively. These AGN fractions are somewhat larger (by a mean factor of 6.1^{+10.3}_{-3.6}; significant at the 95% confidence level) than z ~ 3 sources found in lower-density "field" environments. Theoretical models imply that these results may be due to the presence of more actively growing and/or massive SMBHs in LBGs and LAEs within the protocluster compared to the field. Such a result is expected in a scenario where enhanced merger activity in the protocluster drives accelerated galaxy and SMBH growth at z > 2-3. (abridged)

The absorbing haloes of high‐redshift radio galaxies and Lyα blobs

AbstractResults are summarized from a program of VLT‐UVES echelle spectroscopy on the HI Lyα absorption in z &gt; 2 radio galaxies. The nature and incidence of this absorption are reviewed, along with scenarios for its origin. Comparison is made with integral field spectroscopy of the two radio‐quiet 100 kpc‐scale Lyα haloes in the z = 3.09 SSA22 protocluster–the Steidel Lyα blobs. The emphasis is on using such absorption to study the injection of energy and metals from radio jets and starburst superwinds, processes considered vital to the formation of the most massive galaxies. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)