Radio Galaxy 53W002 Research Articles

A Search for Massive Galaxy Population in a Protocluster of LAEs at z = 2.39 near the Radio Galaxy 53W002

We searched for massive galaxy population in the known large-scale high-density structure of Lyα emitters (LAEs) at z = 2.39 near the radio galaxy 53W002 by using imaging data from bands taken with Suprime-Cam and MOIRCS on the Subaru telescope. We selected 62 protocluster member candidates by their JHK s -band colors and spectral energy distribution (SED) fitting analysis (JHK s -selected galaxies) in our survey field of and compared their physical properties estimated from the SED fitting with a comparison sample in the COSMOS field. We found significant number density excesses for the JHK s -selected galaxies in the 53W002 field at K s < 22.25, J − K s > 2, or V − K s > 4. In particular, the number density of the JHK s -selected galaxies with K s < 22.25 and J − K s > 2 in the 53W002 field is nine times higher than the comparison sample. Most of those with K s < 22.25 and J − K s > 2 are massive galaxies with M s > 1011 M ⊙, and their specific star formation rates (sSFRs) of 10−11–10−10 yr−1 suggest that the star formation has not yet stopped completely. We also found a density excess of quiescent galaxies with M s = 5 × 1010 M ⊙ to 1011 M ⊙ and sSFR < 10−11 yr−1, as well as that of low-mass galaxies with M s = 109.75–1010 M ⊙ and various sSFRs. The massive galaxies with M s > 1011 M ⊙ are not located at the density peaks of LAEs but widely distributed along a similar direction to the structure of LAEs over ∼15–20 comoving Mpc. On the other hand, the quiescent galaxies with sSFR < 10−11 yr−1 clearly avoid the structure of LAEs. Our results suggest that massive galaxies also exist in this protocluster discovered by the moderate overdensity of LAEs and their star formation activities depend on location in the protocluster.

CHARACTERIZATION OF THE DISTRIBUTION OF THE Lyα EMITTERS IN THE 53W002 FIELD ATz= 2.4

We present the results of our wide-field narrow band imaging of the field around the radio galaxy 53W002 at z = 2.390 with Subaru/Suprime-Cam. A custom made filter NB413 centered at 4140 \AA\ with the width of 83 \AA\ is used to observe the 31' x 24' area around the radio galaxy. We detected 204 Ly\alpha emitters (LAEs) at z = 2.4 with a rest frame equivalent width larger than 25 \AA\ to the depth of 26 AB mag (in NB413). The entire LAE population in the 53W002 field has an average number density and distributions of equivalent width and size that are similar to those of other fields at z ~ 2. We identify a significant high density region (53W002F-HDR) that spreads over ~ 5' x 4' near 53W002 where the LAE number density is nearly four times as large as the average of the entire field. Using the probability distribution function of density fluctuation, we evaluate the rareness probability of the 53W002F-HDR to be 0.9^{+2.4}_{-0.62}%, which corresponds to a moderately rich structure. No notable environmental dependency at the comoving scale of 10 Mpc is found for the distributions of the Ly\alpha equivalent width and luminosity in the field. We also detected 4 Ly\alpha blobs (LABs), one of which is newly discovered. They are all found to be located in the rims of high density regions. The biased location and unique morphologies in Ly\alpha suggest that galaxy interaction play a key role in their formation.

The [ITAL]Hubble Space Telescope[/ITAL] WFPC2 [ITAL]B[/ITAL]-Band Parallel Survey: A Study of Galaxy Morphology for Magnitudes 18 ≤ [ITAL]B[/ITAL] ≤ 27

We present the results of the Hubble Space Telescope B-Band Parallel Survey (BBPS). It covers 0.0370 deg2 and consists of 31 shallow (four- to six-orbit), randomly selected high-latitude HST WFPC2 parallel fields with images taken in both the B (F450W) and I (F814W) filters. The goal of this survey is to morphologically classify the galaxies in a homogeneous manner and study galaxy properties as a function of type and B-band magnitude for 18 mag ≲ bJ ≲ 23.5 mag. The full sample contains 1800 galaxies, 370 of which are brighter than the formal statistical completeness limit of bJ ≲ 23.5 mag. The galaxies are selected from the B-band images and classified using an artificial neural network (ANN) galaxy classifier on the higher signal-to-noise ratio I-band images. These provide (more) reliable types for I ≲ 24 mag (or bJ ≲ 26 mag), since these I-band classifications are less subject to the uncertain redshifted rest-frame UV morphology. The ANN classification depends on the shape of the surface brightness profile, but not on color. These results are combined with similar (deeper) studies in the Hubble Deep Field and the deep WFPC2 field surrounding the radio galaxy 53W002, for which galaxies have been classified to bJ ≲ 27 mag. The galaxy counts for the combined B-band–selected samples show adequate statistics for a range 19 mag ≲ bJ ≲ 27 mag and are in good agreement with other studies in the flux range where they overlap, while showing improved statistics at the bright end. The galaxies are subdivided into three morphological classes: early types (E/S0), mid types (Sabc), and late types (Sd/Irr), and the B-band counts are presented for each class, as well as the total counts. The faint end of the counts is dominated by the irregular galaxies, which have a steep count slope of d log N/dm ≈ 0.4. These type-dependent counts are compared with models based on local luminosity functions that include the effects of the cosmological constant, ΩΛ. The whole BBPS sample, along with the two deeper fields, is used to delineate the general trends of effective radius and B-I color as function of both morphological type and apparent magnitude for 18 mag ≲ bJ ≲ 27 mag. These properties are discussed in the context of recent redshift surveys. A possible explanation for the combined results is given in terms of the effects of ΩΛ on the evolution of the merger rate in a hierarchical scenario.

A SCUBA Galaxy in the Protocluster around 53W002 atz = 2.4

We analyze an 850 μm Submillimetre Common-User Bolometer Array (SCUBA) map of the environment of the z = 2.39 radio galaxy 53W002, which has been shown to reside in an overdensity of Lyα-detected galaxies. We identify four luminous submillimeter sources within a 23 (1.2 Mpc at z = 2.39) diameter area around the radio galaxy (which itself is a weak submillimeter source). We employ a 1.4 GHz map to accurately locate the counterpart of one of these sources, SMM J17142+5016, and identify this source with a narrow-line active galactic nucleus with an extended Lyα halo at z = 2.390 that is a member of the structure around 53W002. Hence, SMM J17142+5016 is the first spectroscopically confirmed submillimeter-selected companion to a high-redshift radio galaxy. We discuss the OH-Airglow Suppression Spectrograph JHK spectrum of this galaxy and in addition present five new constraints on its spectral energy distribution longward of 1 μm, using these to estimate its bolometric luminosity as ~8 × 1012 L☉, or a star formation rate of ~103 M☉ yr-1 if young stars provide the bulk of the luminosity. This result provides direct support for the statistical detection of overdensities of SCUBA galaxies around high-redshift radio galaxies and confirms theoretical predictions that SCUBA galaxies, as the progenitors of massive elliptical galaxies, should be strongly clustered in the highest density regions of the distant universe.

Active Nuclei and Star-forming Objects at [ITAL][CLC]z[/CLC][/ITAL] ] 2: Metallicities, Winds, and Formation Histories

We present near-infrared observations of the active galactic nuclei (AGNs) and star-forming objects in the field of the radio galaxy 53W002 at z = 2.39. The star-forming objects are of special interest as candidate protogalactic objects. The 1.1–2.2 μm passbands sample the emitted-optical range at this redshift, providing new diagnostics of the structure, metal abundance, and age of the members of this grouping originally selected through Lyα emission. The star-forming objects are uniformly very blue in continuum slope, which fits with the strong Lyα emission in indicating metal abundances that are less than half solar; some are as blue as the most metal-poor local objects. They fall in a range of luminosity and metallicity that is not populated by local objects, indicating a shorter star-forming history at this early epoch. The best local analogs, such as Mrk 66 and 357, either have several times lower luminosity at comparable [O/H] or significantly higher [O/H] for comparable luminosity. Spectroscopy from the Near-Infrared Camera and Multi-Object Spectrometer yields detections of [O III] emission for two objects and interesting [O III] and Hβ limits for the rest, augmented by Hα limits from Infrared Telescope Facility imaging. These data are satisfied by model stellar populations that have been forming stars for the last 2–5 × 106 yr before z = 2.39. We do not see evidence for older preexisting stellar populations, either in the broadband colors or as redder halos in which the star-forming regions are imbedded. These results suggest that the compact star-forming objects we see at z = 2.0–2.5 are indeed early stages in the building of galaxies rather than transient star-forming events in larger pre-existing dynamical systems. The results also allow an alternative scheme, in which these are low-mass systems that are blowing winds rather than self-enriching, in which case they should fade rapidly with cosmic epoch. For the three prominent AGNs at z = 2.39, Hα and [O III] emission were measured. Unlike the fainter star-forming objects, their line ratios (specifically Lyα/Hα) show metallicities just as high as in nearby systems. If the AGNs occur in those systems that started with the highest density and began active star formation before the less massive surrounding objects, they will have higher metallicity (as we see in their emitted-ultraviolet line ratios). The "ionization cones" seen prominently in Lyα also appear in [O III] and Hα, with a role for continuum reflection in some cases as well. The contrast between the AGNs and fainter star-forming objects can be broadly accommodated in a hierarchical formation picture, although there are still important unknowns as to the fate of the star-forming objects.

Subaru Deep Near-Infrared Imaging of the Field of a Possible Proto-Cluster Near the Radio Galaxy 53W002 at $z=2.4$

Abstract We present the results of deep $K^\prime$ - and $J$-band imaging of the field of a proto-cluster region near the radio galaxy 53W002 at $z=2.390$ with the Subaru Telescope. The data were analyzed together with deep optical and near-infrared (NIR) images taken with the Hubble Space Telescope to investigate the properties of ten optically compact emission-line galaxies and candidates. Excluding the three objects which may contain active galactic nuclei, many are faint or undetected at $K^\prime$ ($\lambda_\mathrm{rest} \approx 6000 \,{Å}$), and are therefore revealed to be intrinsically small starbursting objects. On the other hand, we detect few objects with colors and magnitudes expected for quiescent massive galaxies at $z=2.4$ in the field.

JHK Spectra of the z = 2.39 Radio Galaxy 53W002

Abstract We present low-resolution, near-IR JHK spectra of the weak $z=2.39$ radio galaxy 53W002, obtained with the OH-airglow Suppressor spectrograph (OHS) and Cooled Infrared Spectrograph and Camera for OHS (CISCO) on the Subaru Telescope. They cover rest-frame wavelengths of 3400–7200$Å$, and the emission lines of [O II]$\lambda$3727, H$\beta$, [O III]$\lambda\lambda$4959, 5007, H$\alpha$, [N II]$\lambda\lambda$6548, 6583 and [S II]$\lambda\lambda$6716, 6731 were detected. Using the H$\alpha$/H$\beta$ line ratio, we find an extinction of $E(B\hbox{--}V)=0.14$. The emission-line ratios are reproduced by a cloud of electron density $n_{\mathrm{e}}=1\times 10^{3-4}\,\mathrm{cm}^{-3}$ with solar metallicity, ionized by an $\alpha=-0.7$ power-law continuum with ionizing parameter $U=1\times 10^{-3}$. In addition to these emission lines, we make the first spectroscopic confirmation of the Balmer discontinuity in a high-$z$ radio galaxy. Together with rest-frame UV photometry from the literature, we show that at least 1/3 of the present stellar mass was formed in the current starburst. The stellar mass was estimated to be $(1\hbox{--}1.4)\times10^{11}\,{{{M}_{\odot}}}$ by one-component model fitting, which is smaller than that of typical $z\sim 1$ B2/6C radio galaxies. We suggest that 53W002 is currently assembling a large part of its stellar mass through merger events with the surrounding sub-galactic clumps, some of which can be identified with the L$\alpha$ emitters detected in narrow-band imaging. After a few such events over the next few Gyr, 53W002 will evolve into a massive elliptical galaxy.

New CO and Millimeter Continuum Observations of the z = 2.394 Radio Galaxy 53W002.

The z=2.39 radio galaxy 53W002 lies in a cluster of Lyalpha emission-line objects and may itself be undergoing a major burst of star formation. CO (3-2) emission, at 102 GHz, was detected from 53W002 in 1997 by Scoville et al., who also reported a possible 30 kpc extension and velocity gradient suggesting a rotating gaseous disk. In this Letter, we present new interferometric CO (3-2) observations that confirm the previous line detection with improved signal-to-noise ratio, but show no evidence for source extension or velocity gradient. The compact nature of the CO source and the molecular mass found in this object are similar to luminous infrared galaxies and other active galactic nuclei previously studied.

Evidence for Large-Scale Structure at [CLC][ITAL]z[/ITAL][/CLC] ≈ 2.4 from L[CLC]y[/CLC]α Imaging

The history of large-scale structure depends on cosmological parameters and on how merging unfolds among both galaxies and groups. There has been recent evidence for clustering among Lyα emitters, Lyman-break galaxies, and Lyman absorbers. We present deep, wide-field medium-band imaging in redshifted Lyα of fields surrounding regions selected to have HST detections of faint Lyα emitters, over a range of surface densities, to characterize the large-scale environment. The radio galaxy 53W002 was previously found, using HST, to be part of a rich grouping at z = 2.39, including ≈5 spectroscopically confirmed, compact, lower luminosity star-forming objects. Our new data show this to be part of a larger structure traced by bright active nuclei, all contained within a projected span of 68 (3.2 Mpc). Of the 14 candidate emitters, six have been spectroscopically confirmed as active nuclei in the range z = 2.390 ± 0.008. Various statistical tests give a significance of 95%–99% for the reality of this structure on the sky. Our data thus strengthen the evidence for clustering at these redshifts. The grouping around 53W002 is more extended than a relaxed King model, at the 90% confidence level. This may be evidence either for a configuration that has yet to decouple fully from the Hubble expansion or for multiple subgroupings that will themselves at some point form a more compact, relaxed structure. The redshift range for measured members is comparable to the Hubble flow across the structure, which may imply that the structure is seen near turnaround and suggests that its mass cannot be derived from the velocity dispersion. We surveyed two additional 14' fields at z = 2.4, each centered on an HST WFPC2 field that has been searched for faint Lyα emitters, as well as on three contiguous fields near 53W002 for objects at z ~ 2.55. Only a single emitter consistent with showing Lyα at z ≈ 2.4 appears in these fields, to a somewhat brighter flux limit, while a total of six candidate emitters appear in the three fields at z ~ 2.6. Comparison with the (deeper but narrower) WFPC2 surveys suggests that the surface-density contrast from field to field is larger for brighter objects. From this survey alone, groupings such as the 53W002 cluster must have an area-covering fraction less than 0.04 in this redshift range. Three of the active galactic nuclei in the structure at z = 2.4 show extended emission-line structure, with equivalent widths suggesting in situ photoionization as far as 50 kpc from the nuclei. Such objects may populate deep surveys as diffuse Lyα-emission clouds when their cores are sufficiently obscured.

Young and Old Galaxies at High Redshift

We review the results from recent deep HST imaging on faint galaxies. Artificial Neural Networks (ANNs) were used to classify faint galaxies based on surface brightness and light profiles. We use available UV + ground-based U-band images of nearby galaxies to address the effects from the uncertain redshifted UV-morphology on the classifications, resulting in ‘restframe ANN’ classifiers. These distinguish quite consistently between E/SO’s, Sabc’s, and Sd/Irr for B ≲ 27 mag. A Fourier-based method is used to quantify higher order morphological features and asymmetries in faint galaxies. We review the faint blue galaxy population as classified with these restframe ANN’s. The median scale-length at B ≲ 27 mag is r hl ≃ 0″.25–0″.3(∼ 1–2kpc at z≃1–2). Although E/SO’s are the reddest and Sd/lrr’s are the bluest galaxies, early and late types are not wel separated in color-magnitude diagrams to B ≲ 28 mag. We discuss the B-band galaxy counts as a function of type for 18.5 ≲ B ≲ 28 mag. We briefly review deep medium-band imaging with HST/WFPC2 in the filter F410M (Lyα at z ≲ 2.4) which yielded 18 faint, compact objects surrounding the radio galaxy 53W002 at z ≲2.39, as well as another 30 in random HST parallel fields, plus ground-based images over a larger area. Deep HST/PC images of 53W002 at ∼ 0″.06 FWHM resolution in BVI + redshifted Lyα suggest that both reflected AGN continuum-light shining through a cone and jet-induced star-formation play a role in its ‘alignment effect’. We discuss the formation and evolution of 53W002 in the context of its many surrounding sub-galactic sized objects. We discuss the nature of a μJy radio source that remained essentially unidentified in HDF flanking fields. The object was identified in deep HST/NICMOS J- and H-band images, and a single emission line at 6595 A was detected, most likely Lyα at z = 4.42. This faint (H = 23.9 mag), compact (re ≃ 0″.2), red (I - K = 2.0 mag) object is most likely a dusty, star-forming young galaxy with an embedded active nucleus. Finally, HST/NICMOS images are presented of some of the oldest known red galaxies at z ≃ 1.5. Both galaxies are dominated by a r 1/4-profile with 5 kpc scale-length, and are amongst the oldest known relaxed systems at that epoch.

Compact Lyα-emitting Candidates at [ITAL]z[/ITAL] ≃ 2.4 in Deep Medium-Band [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL] WFPC2 Images

Medium-band imaging with Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) in the F410M filter has previously revealed a population of compact Lyα emission objects around the radio galaxy 53W002 at z 2.4. We report detections of similar objects at z 2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyα candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z 2.4 subgalactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.

Deep [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Galaxy and Pair Counts as Tests of Merger History

We consider the use of counts of faint galaxy pairs as tests of the role of merging in galaxy evolution, using both simulations and Hubble Space Telescope (HST) data in comparison with models incorporating a range in the evolution of merger rate. Artificial data, including simulation of the surface-brightness selection effects implicit in HST imagery, indicate that simple pairing statistics are robust enough for this test. However, the differences between pair fractions for various plausible scenarios of merger-rate evolution are small enough to be easily hidden by statistical errors or environmental differences. With a Poisson-based algorithm, we identify pairs with physical distances between components 60 h-1 kpc. Deep HST WFPC2 images of the field around the radio galaxy 53W002 at z = 2.4 in F814W (I) and F606W (V) show that 12%?20% (~10 pairs) of galaxies at mI 24 or mV 24.6 are paired. This supports a mild enhancement of pair fraction toward high redshifts (1?2), as concluded in other recent studies of faint galaxy pairs. The magnitude limits eliminate most possible members of the grouping associated with 53W002 itself. We compare the counts of galaxies and galaxy pairs in the 53W002 field and other recent studies to a detailed model incorporating overall traditional galaxy spectral evolution and various merger rates, in conjunction with our previous evaluation of the local merger rate. The data support evolution of the merger rate, such that the comoving density of merging systems varies as (1 + z)?, with 0 ? ? ? 2. This is consistent with the constraint ? < 3, beyond which local elliptical galaxies (E/S0 types) are overproduced from merging, and includes the case ? ? 2 required by our model to support the merger hypothesis, whereby almost all elliptical galaxies are merger remnants. The ? ? 2 case also expects a large fraction of early galaxies undergoing merging. Therefore, we support the merger-rate evolution required to support the merger hypothesis and the expectation that early merging is important in the overall evolution of galaxies. Assuming merger remnants to be elliptical galaxies subsequently undergoing passive evolution, our model can reproduce at once counts of faint galaxies and galaxy pairs, as well as the present-day luminosity function of ellipticals, for the Hubble constant H0 ? 50 km s-1 Mpc-1 and a cosmological constant ? = 0. These lines of evidence do not appear to offer compelling arguments for more exotic cosmologies or star formation histories. A preliminary study shows the fraction of paired galaxies in the 53W002 field to be much larger than that in the Hubble Deep Field (5%?6% at most in the HDF), suggesting physical field-to-field fluctuations in pair statistics. Further progress to constrain merging history requires not only larger samples, preferably coupled to at least partial redshift information, but better indications as to which faint objects in multiple systems should be considered as independent galaxies and which are parts of single dynamical units.

Deep HST Imaging of a Galaxy Cluster at z = 2.40

We present a 67-orbit HST/WFPC2 exposure on the weak radio galaxy 53W002 at z=2.390 and its surrounding cluster. Color Plate 1 shows 12 orbits in IF814W &amp; VF606W, and 24 in BF450W. Potential cluster members were identified through 15 orbits in F410M, optimized for narrow-band searches for compact Lyα objects at z≃2.4 (P96), and confirmed through spectroscopy (W91, P96); 16 candidates were found with significant narrow-band emission in F410M: 4 out of 5 had a confirming MMT spectroscopic redshift at z≃2.40 (P96). All are located within 60″ from 53W002, or ∼ 0.24h–1100 Mpc (qo=0.5) at z ≃2.4, the physical scale of a group or small galaxy cluster. One object contains a weak (variable) AGN, another is a merger with two companions. Their underlying young stellar population is very compact, with rh.l. ≃0.2″ (≃ 0.8h–1100 kpc), and considerably fainter than the L∗-value at z∼2.4, implying sub-galactic sized objects. These results may explain why ground-based Lya searches for PG's have been largely unsuccessful. The narrow-line galaxy 53W002 was imaged in the PC at ∼0.07″ FWHM (WK95, see also W94). Its AGN component is ≤ 20±4% of the total continuum, surrounded by an extended r1/4-envelope with rh.l. ≃1.1″ (4.3 kpc), and has an SED of ∼0.3 Gyr in the center to ∼0.5-1.0 Gyr at ∼4 kpc. A one-sided cloud is seen ∼1.8 kpc West, ∼0.3 mag bluer than the SED, aligned with the radio source and its Ly-α cloud, presumably weak scattered AGN light, and/or jet-induced star-formation.

The Serendipitous Discovery of a Group or Cluster of Young Galaxies at [ITAL]z[/ITAL] ≃ 2.40 in Deep [ITAL]Hubble Space Telescope[/ITAL] WFPC2 Images

We report the serendipitous discovery of a group or cluster of young galaxies at z ≃ 2.40 in a 24 orbit Hubble Space Telescope/WFPC2 exposure of the field around the weak radio galaxy 53W002. Potential cluster members were identified on ground-based narrowband redshifted Lyα images and confirmed via spectroscopy.

Deep Hubble Space Telescope imaging of a compact radio galaxy at Z = 2.390

We present deep 4 hr HST/WFC images in V and I of the LBDS radio galaxy 53W002, a weak and compact narrow-line galaxy recently discovered at z = 2.390. After deconvolution, the resolution is 0.2" FWHM. Its I- band structure is quite compact with 30% +/- 10% of its flux in the central &lt;~ 1 kpc, presumably from its AGN. This is surrounded by a clearly extended and rather symmetric envelope with effective radius ~1.1" (4-15 kpc for H_0_ = 50-100,q_0_ = 0-0.5). In V, 53W002 is somewhat elongated at the same sky PA as its Lyα cloud and its VLA radio axis. We compare 52W002's rest-frame UV profile to a properly rescaled image of a nearby early-type galaxy. The much younger starburst in 53W002 has 10-20 times higher central UV surface brightness, but otherwise an r^1/4^-like light profile consistent with that of nearby luminous early- type galaxies. The available data are consistent with 53W002 being a genuinely young galaxy that only started forming stars &lt;~0.5 Gyr before z = 2.390, but has nonetheless (just) managed to develop a rather regular light profile at z = 2.390, and suggest that the dynamical collapse of this galaxy started at about the same time as its first major burst of star formation. For this galaxy, both ~0.5 Gyr time scales are consistent with z_form_ = 2.7-4.2 (for H_0_ = 50-100, q_0_ = 0-0.5).