The Galaxy Environment of Quasars in the z~1.3 Clowes-Campusano Large Quasar Group

C.P. Haines (UCLan), R.G. Clowes (UCLan),
L.E.Campusano (U. de Chile)

A poster presented at the ``New Era of Wide Field Astronomy'' conference at the University of Central Lancashire, UK, in August 2000.
























Fig. 1. - The Clowes-Campusano LQG with each quasar labelled by its redshift. The 30x30 arcmin2 region targetted for BTC imaging is indicated by the box, and the quasars for which we have K imaging are indicated by green circles. The boundaries of the plot match the boundaries of the AQD survey of ESO/SERC field 927 (Clowes & Campusano 1991,1994; Clowes, Campusano & Graham 1999). Note that this plot and the other spatial plots have east to the right.



ObservedExpected
MagnitudeTotalI-K>3.75TotalI-K>3.75
15<K<16110.5330
16<K<17312.1860
17<K<181354.2040.050
18<K<19231611.361.939
19<K<20392321.811.896


Table 1. - A comparison of total and extremely-red (I-K>3.75) galaxy counts in the K image centred on the z=1.226 quasar, binned by magnitude, and those expected for the same-sized region in the field (The Hawaii K-band survey - Songaila et al. 1994). A 3.5 sigma excess of K<20 galaxies is observed, and it is clear that this excess is due to the I-K>3.75 galaxies which make up more than half the total observed K<20 galaxies, but are comparatively rare in the field.



Fig. 2. - I-K against V-I colour-colour diagram of all galaxies in field of the z=1.226 quasar J1046.9+0541. For comparison, model tracks are shown for an instantaneous burst at zformation=4.5 (red solid curve), which represents the colour evolution of elliptical galaxies, as well as exponentially-decaying star-formation rate models with time-scales of 1 Gyr (dashed green curve) and 5 Gyr (dot-dashed blue curve). Each track shows the colour evolution from z=1.5 to z=0 with crosses at 0.1 redshift intervals. The four galaxies whose redshift probability distributions are shown in Figures 4a-d are indicated by labelled yellow diamonds. There are 15-18 galaxies (shown as red squares and hereafter described as red galaxies) whose extremely-red colours (I-K~4.3,V-I~2-3) are well described by the burst model at z~1.2. These are the same galaxies which make up the C-M relations of Figure 3 and indicate a population of massive ellipticals at the quasar redshift. There are also an equal number of galaxies (shown as green circles and hereafter described as (red outliers) with the same extremely-red I-K colour indicative of high redshift galaxies with predominantly old stellar populations, but much bluer optical V-I colours, indicating recent star-formation. Such galaxies appear relatively common in other high redshift clusters (eg. Tanaka et al. 2000), suggesting that the Butcher-Oemler effect increases with strength to z~1.2.




Fig. 3. - Colour-magnitude diagrams of galaxies in the z=1.226 quasar field. The solid symbols represent those galaxies detected in all three bands. The half-filled symbols represent those galaxies detected in I and K only. The four galaxies whose redshift probability distributions are shown in Figures 4a-d are indicated by yellow diamonds. The blue and green dash-dotted lines indicate the 50% completeness levels for each filter in the centre of the image, with the dotted line indicating the completeness level for the edge of the K image. Galaxies whose colours are well described by the burst model at z~1.2 as shown in Figure 2 have square red symbols. These galaxies form fingers in the C-M diagrams at I-K~4.3 and V-K~6.9 comparable to the C-M relations observed in other z~1.2 clusters, indicating a population of 15-18 massive ellipticals at the quasar redshift.



Fig. 4. - Redshift probability distributions for galaxies in the fields of the z=1.226 (galaxies a-d) and z=1.306 (galaxies e-h) quasars, based on their VIK colours. The redshift probability distributions are produced by the HYPERZ code of Bolzonella, Miralles & Pello (2000), which uses the Bruzual \& Charlot evolutionary code (GISSEL98, Bruzual & Charlot 1993) to build synthetic template galaxies, with 51 different ages for the stellar populations, and 8 star-formation histories, roughly matching the observed properties of local galaxies from E to Im type: an instantaneous burst, a constant star-forming system, and six exponentially decaying SFRs with time-scales from 1 to 30 Gyr. The models assume solar metallicity and a Miller & Scalo IMF, and internal reddening is considered using the Calzetti et al. (2000) model with AV varying between 0 and 0.5 mag. The best fitting model is described by its redshift zbest and star-formation history. Distributions (a) and (b) correspond to two galaxies which help make up the observed C-M relations of Figure 3. Galaxy (c) has a similarly red I-K colour to (a) and (b), but a much bluer V-I colour, indicating some recent star-formation. Galaxy (d) does not pass our I-K>3.75 selection criterion, but is best fit by a disk-galaxy model at the quasar redshift. Galaxy (e) is the best candidate within the z=1.306 quasar field for being a massive elliptical at the quasar redshift, and galaxy (f) appears similar to the `red-outliers' common in the z=1.226 quasar field. Galaxies (g) and (h) are both members of a dense cluster of red galaxies (figure 11) that lies 3 arcmin from the quasar, that appeared from the optical images to be the most likely candidate to be a cluster associated with the quasar. Galaxy (g), typical of the yellow sources in the core, is tightly constrained as being an elliptical at z~0.9, and is inconsistent with being at the quasar redshift. Galaxy (h) is one of the blue galaxies in the cluster core, and is consistent with being at the same redshift as (g).



Fig. 5. - Spatial distribution of galaxies in the field of quasar J1046.9+0541 (shown as green triangle). The different symbols indicate the galaxies' colours, with circles (squares) indicating those galaxies detected (not detected) in I. Solid red symbols indicate the red (I-K>3.75, V-I>2.00) galaxies which could be early-type galaxies at the quasar redshift. Solid green symbols indicate the red-outlier galaxies with the red I-K colours and blue V-I colours of high-redshift galaxies with some recent star-formation (note that the I-K=3.5 cut-off is not followed rigidly to include those blue star-forming galaxies likely to be at the quasar redshift). The blue star-shaped symbols indicate blue (V-I<1.00) galaxies which are probably undergoing significant star-formation. The size of the symbols (except stars) indicate the Ktotal magnitude. The red galaxies are concentrated in two compact groups, 50 arcsec to the north and south-east of the quasar. In contrast, there is a concentration of blue star-forming galaxies within 30 arcsec of the quasar, which appears to be extended to the north-east, forming a band of enhanced star-formation which bisects the two groups of red galaxies. The blue galaxies avoid the red groups of galaxies: a 3-sigma anti-correlation is observed within 20''. One explanation for the relative distribution of red and blue galaxies, is that the two groups of red galaxies are in the early stages of merging, and that this merger has triggered the band of enhanced star-formation and also the quasar.



Fig. 6. - VIK three-colour image of the 2.25x2.25 arcmin2 field centred on the z=1.226 quasar J1046.9+0541 (appears as central bright blue source). The red galaxies likely to be massive ellipticals at the quasar redshift can be seen as orange/red sources, and are concentrated towards the top-centre and bottom-right of the image. The band of blue star-forming galaxies is visible in the form of blue/violet sources near the quasar and in the right-centre of the image. A particularly red source, in the bottom-centre of the image appears to be an ERO with I-K>5.



Fig. 7. - Density map of red galaxies with V-I>2.25,I<23.5 (V-I>2.00,I-K>3.75 in K image) in the 7x7 arcmin2 field centred on the z=1.226 quasar J1046.9+0541 (shown as red/black circle). The region covered by the K image is indicated by the yellow box, and the `red' galaxies are marked by black/white circles, whose diameters indicate their I magnitudes. The colour selection criteria are chosen to maximize the fractional contribution of high-redshift (1<z<1.5) massive ellipticals, so that a density map of these galaxies indicates the likely extent of clustering associated with the quasar. The quasar does not appear to lie within any of the density peaks, but is found midway between two concentrations of {\em red} galaxies, one 1 arcmin to the north, and a second 1 arcmin to the south-east which appears to extend some 2-3 arcmin to the north and east. Two further groupings of 6-8 red galaxies are apparent, 3 arcmin to the south of the quasar, and 2 arcmin apart. The structure as a whole is suggestive of a cluster in the early stages of formation through the merger of subclusters.



Fig. 8. - Density distributions of red galaxies with V-I>2.25, I<23.50 in 7'x7' fields centred on the 4 z~1.3 quasars (3 LQG and 1 background - green triangles) in the BTC image. The red boxes indicate the regions covered by K imaging. The selection criteria are chosen to pick out z>0.5 elliptical galaxies (see Figure 2), and so peaks in the density distribution should mark out z>0.5 clusters. Significant clustering is observed around the z=1.226 quasar, with the two groups of red galaxies of Figure 5 appear extended beyond the K image, the south-eastern group extending 3' to the north and east. Two further groups of 6--10 galaxies lie 3' to the south of the quasar, and the structure as a whole is suggestive of a cluster in the early stages of formation through the merger of subclusters. A dense cluster is apparent 3' to the west of the z=1.306 quasar. K imaging obtained to estimate the redshift of this cluster indicates that it is at z~0.9 (Figures 4g,h and 9). Although not at the quasar redshift, it does suggest that these density maps are suitable for locating high redshift clusters. The third LQG quasar (c) does not appear to have any associated clustering, but the z=1.426 quasar is located on the edge of a high density region, with a second cluster 3' further to the north, suggesting that this quasar is located in a rich environment. The high density regions of figures (a),(b) and (d) are 3 of the 4 most significant across the whole BTC field. Quasars (a) and (d) are both found on the edge of the clusters, rather than in the core, in agreement with other observations at these redshifts (e.g. Sanchez & Gonzalez-Serrano 1999).



Fig. 9. - I-K against V-I colour-colour diagram of all galaxies in field of the z=1.306 quasar J1047.6+0525. Note that the area covered by this field is almost twice that of the Figure 2. The model tracks and symbols are the same as in Figure 2, except for the addition of the solid yellow/brown circles which indicate those galaxies whose colours are well described by the burst model at z~0.8. The four galaxies whose redshift probability distribution are shown in Figures 4e-h are indicated by labelled yellow diamonds. In comparison to Figure 2 there are far fewer galaxies well described by the burst model at z~1.2, indicating that this quasar's environment is much poorer, and although there remain a reasonable number of red-outlier galaxies, they are not concentrated significantly towards the quasar. In contrast to the rarity of galaxies described by the burst model at z~1.2, there is a large excess of galaxies (shown as yellow/brown circles) well described by the burst model at z~0.8, and as most of these have K<19 (30 of 44), this would indicate a likely cluster at this redshift.



Fig. 10. - Spatial distribution of galaxies in the field of the z=1.306 quasar J1047.6+0525 (green triangle). The symbols are the same as in Figure 5, except for the addition of the solid yellow/brown circles, which indicate those galaxies with the colours expected of elliptical galaxies at z~0.8. These galaxies, as well as some blue (V-I<1.00) galaxies, are concentrated in a compact group in the south-western corner of the image, indicating a compact, dense cluster at z~0.8. In contrast to Figure 5, there appear few galaxies likely to be associated with the quasar, indicating a poorer environment.



Fig. 11. - VIK 3-colour image of the dense clump of red galaxies apparent in Figure 9. Those galaxies which have the colours of elliptical galaxies at z~0.8 (e.g., Figure 4g) appear as compact, orange/yellow sources, in comparison to the redder, clearly elliptical source to the far right. There also appears to be significant star-formation occuring in the cluster in the form of several blue galaxies (e.g., Figure 4h).