We report on a study of the surface density of Extremely Red Objects (EROs)
in the fields of 13 radio-loud quasars at 1.8 < z < 3.0 covering a total area
of 61.7 sqr arcmin. There is a large variation in the ERO surface density from
field to field, and as many as 30--40 % of the fields have roughly 4--5 times
more EROs than what is expected from a random distribution. The average surface
density exceeds the value found in large random-field surveys by a factor of
2--3, a result which is significant at the >3 sigma level. Hence, it appears
that the quasar lines of sight are biassed towards regions of high ERO density.
This might be caused by clusters or groups of galaxies physically associated
with the quasars. However, an equally likely possibility is that the observed
ERO excess is part of overdensities in the ERO population along the line of
sight to the quasars. In this case, the non-randomness of quasar fields with
respect to EROs may be explained in terms of gravitational lensing.Comment: 6 pages, 2 figures, to appear in "Radio galaxies: past, present and
  future", eds. M. Jarvis et al., Leiden, November 200

Barthel

Bertin

Cimatti

Daddi

Elston

G. Neugebauer

Hall

Hewitt

L. Armus

M. Wold

M.D. Lehnert

Mannucci

McCarthy

Roche

Scodeggio

Smail

T.H. Jarrett

Thompson

English

arXiv

Crossref

The surface density of Extremely Red Objects in high-z quasar fields

We report on a study of the surface density of Extremely Red Objects (EROs) in the fields of 13 quasars at 1.83σ level. Hence, it appears that the quasar lines of sight are biased towards regions of high ERO density. This might be caused by clusters or groups of galaxies physically associated with the quasars. However, an equally likely possibility is that the observed ERO excess is part of overdensities in the ERO population along the line of sight to the quasars. In this case, the non-randomness of quasar fields with respect to EROs may be explained in terms of gravitational lensing

Wold, M.

Armus, L.

Neugebauer, G.

Jarrett, T. H.

Lehnert, M. D.

Caltech Authors - Main

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The surface density of Extremely Red Objects
in high-z quasar fields
M. Wold a L. Armus b G. Neugebauer c T.H. Jarrett d
M.D. Lehnert e
aJPL Postdoctoral Researcher, SIRTF Science Center/Caltech
bSIRTF Science Center/Caltech
cCaltech
dIPAC/Caltech
eMax-Planck-Institut fu¨r Astrophysik
Abstract
We report on a study of the surface density of Extremely Red Objects (EROs)
in the fields of 13 radio-loud quasars at 1.8 < z < 3.0 covering a total area of 61.7
arcmin2. There is a large variation in the ERO surface density from field to field, and
as many as 30–40 % of the fields have roughly 4–5 times more EROs than what is
expected from a random distribution. The average surface density exceeds the value
found in large random-field surveys by a factor of 2–3, a result which is significant at
the > 3σ level. Hence, it appears that the quasar lines of sight are biassed towards
regions of high ERO density. This might be caused by clusters or groups of galaxies
physically associated with the quasars. However, an equally likely possibility is that
the observed ERO excess is part of overdensities in the ERO population along the
line of sight to the quasars. In this case, the non-randomness of quasar fields with
respect to EROs may be explained in terms of gravitational lensing.
Key words: galaxies: clusters – quasars: gravitational lensing
1 Introduction
EROs are galaxies with very red optical to near-infrared colours. Several def-
initions of EROs are in use, but the among the most common are R−K ≥ 5
and I −K ≥ 4. The colour criterion selects z ≈ 1–2 early-type galaxies with
passively evolving stellar populations characterized by strong 4000 A˚ breaks in
their spectral energy distributions, but also dusty starbursting galaxies which
Preprint submitted to Elsevier Science 17 December 2013
have their UV-flux attenuated by internal dust. The first EROs were discov-
ered in the deep near-infrared surveys by Elston, Rieke & Rieke (6), and since
then there have been several surveys aimed at studying their spectroscopic
(4), photometric (13; 14; 10) and clustering properties (5; 12).
Here, we have searched for EROs in the fields of z = 1.8–3.0 quasars. It has
been suggested that the fields of high-redshift radio galaxies and quasars often
have an excess of EROs (11; 9; 3; 15; 7). E.g. Cimatti et al. (3) find an excess
of R−K ≥ 6 EROs in 14 radio galaxy and quasars fields at 1.5 < z < 2.0 and
Hall et al. (7) find that EROs are about 3 times more numerous in the fields of
z ≈ 1.5 quasars. Several possibilities for the ERO excess exist, but the favored
explanation seems to be that the EROs are early-type galaxies associated with
the quasars, perhaps signifying a forming cluster or a galaxy group at the
quasar redshift. Another possibility is that the ERO excess traces foreground
clusters or overdensities in the ERO population which gravitationally magnify
the quasars, boosting them into flux-limited samples. In either case, the ERO
excess may represent clusters in formation at cosmologically significant epochs.
Care must be taken, however, because the angular distribution of EROs on
the sky is characterized by overdensities and large voids (5; 12). This causes
a strong field-to-field variation in the ERO surface density, especially if the
fields are small.
2 The quasar sample
The quasar fields presented here are part of a larger ongoing survey to study
the host galaxies (with the NIRC 1 on Keck I) and environments of radio-
loud and radio-quiet quasars at 1.8 < z < 3.0. The full sample consists of 40
quasars from the catalogs by Barthel et al. (1) and Hewitt & Burbidge (8), and
are nearly evenly distributed between radio-loud and radio-quiet systems with
comparable V magnitudes and redshifts. In order to study the environments
of these quasars, we have obtained deep R and Ks images with the COSMIC
2
and the PFIRCAM 3 instruments on the Palomar 200 inch telescope. The R-
band imaging is completed, and a subset of 13 radio-loud quasar fields have
been imaged in Ks. The ERO samples are drawn from an area of 2.1 ×2.1
arcmin2 around each quasar (the region of overlap of the R and Ks imaging).
The 5σ detection limits in the R- and the Ks-filters are ≈ 25 and ≈ 20.5,
respectively, and this has allowed us to search for EROs in the quasar fields
using the R −K ≥ 5 ERO definition.
1 Near Infrared Camera
2 Carnegie Observatories Spectroscopic Multislit and Imaging Camera
3 Prime Focus Infrared Camera
2
3 Results
The colour-magnitude diagram of objects detected in the quasar fields is shown
in Fig. 1. In total, there are 160 objects with R − K ≥ 5.0. Using the sex-
tractor (2) star-galaxy classifier, and treating all objects at Ks < 18 with a
star-galaxy class greater than 0.85 as stars, we find that six of the 160 EROs
are possible stars. Excluding these yields a surface density of 1.26 EROs per
arcmin2 at Ks ≤ 19, and 2.35 arcmin
−2 at Ks ≤ 20. This surface density is
two–three times higher than what is found in wide-field random surveys, typ-
ically 0.5 at Ks ≤ 19 and 1.4 arcmin
−2 at K ≤ 20 (5; 12). Although there is a
large variation in the ERO surface density from field to field, approximately
30–40 % of the quasar fields have notably large overdensities, a relatively large
fraction of the sample in studies like this.
At the resolution of our images (≈ 1 arcsec), it is difficult to distinguish
between stars and extended objects, and sextractor has problems with
star-galaxy separation at faint magnitudes. We therefore performed the star-
galaxy separation only atKs < 18. The contamination of faint red stars among
EROs is uncertain, but in our case it is likely to be small since the quasar fields
lie at high galactic latitudes (> 35 deg, except three which lie between 21 and
27 deg). For completeness, we also carried out the analysis using star-galaxy
separation regardless of Ks flux. This resulted in ≈ 20 % (as compared to
4 %) of the EROs being classified as stars. This of course lowers the ERO
counts, but our conclusions remain unaltered. The surface densities using this
approach are 1.08 at Ks ≤ 19 and 1.93 arcmin
−2 at Ks ≤ 20. We note that
Mannucci et al. (10) find ≈ 10 % stars at K ′ < 20 based on R, J and K ′
imaging.
In Fig. 2, we compare the average cumulative surface density of EROs in our
quasar fields with random field surveys from the literature. The field counts are
represented by the surveys of Daddi et al. (5), Roche et al. (12) Cimatti et al.
(4) and Scodeggio & Silva (13), and they are seen to agree well with each other,
except the counts in the Chandra Deep Field (CDF) by Scodeggio & Silva. This
discrepancy is most likely caused by the inhomogeneous distribution of EROs
on the sky, given that the area of the CDF is only 43 arcmin2. The counts in our
quasar fields are seen to be consistently higher than the random field counts
at all flux levels. This is not what we expect if we are sampling the random
field ERO population with our 13 widely separated fields. The average ERO
surface density from several small, widely separated fields should in principle
be similar to the surface density in a large, contiguous field. Hence, it appears
that the quasar fields are biassed toward regions of high ERO density.
Using the average of Daddi et al’s and Roche et al’s counts, we predict that
our quasar fields should contain ≈ 27 EROs at Ks ≤ 19, whereas the ob-
3
Fig. 1. Colour-magnitude diagram of objects in the 13 quasar fields. The open
circles are ≥ 5σ detections in Ks, and the triangles show objects with a detection
significance < 3σ in R, where a lower limit to their R−Ks colour is given assuming
a 3σ detection in R. The asterisks are objects classified as stars. The horizontal line
across the plot shows the ERO definition employed in this work.
served number is 79. There is thus a clear excess of EROs in the quasar fields,
but given that the EROs cluster strongly and therefore that the field-to-field
variation is large, is the excess we observe significant?
In order to answer this question, we calculate (a) the expected number of
EROs over the total area of our fields from the literature surveys, Nfield, and
(b), the expected variance, σ2, of the counts given that the ERO distribution
is non-random, i.e. σ2 = Nfield (1 +NfieldAωC) (5), where C is a factor that
depends on the area of the field and Aω is the amplitude of the ERO angular
two-point correlation function as found by Daddi et al. (5) and Roche et al.
(12). The significance of the excess in the quasar fields is thereafter evaluated
as (Nqso−Nfield)/σ, where Nqso is the observed number of EROs in the quasar
fields. The result of this calculation shows that the significance of the excess
at Ks ≤ 18, 19 and 20 is 3.1,4.6 and 2.3σ, respectively. The ERO excess in the
quasar fields therefore appears to be significant, even after taking into account
the large fluctuations in the ERO surface density.
4 Discussion
Our analysis strongly suggests that the quasar lines of sight are biassed to-
ward regions of high ERO density, and our result is consistent with that of
Cimatti et al. (3) and Hall et al. (7). At the moment it is impossible to tell
whether the EROs are at the same redshift as the quasars or if they are in
4
Fig. 2. Cumulative ERO surface density in our quasar fields (open circles) compared
with various literature samples of EROs in random fields. Note that a comparison
has only been made with surveys defining EROs as R−K ≥ 5 or R−Ks ≥ 5. The
error bars are Poissonian.
the foreground. In order to address this, either photometric or spectroscopic
redshifts are needed. We therefore offer two different explanations for the ob-
served excess and discuss each of them below:
The excess might be caused by galaxies, both early types and dusty starbursts,
at the quasar redshifts. In this case, a major part of the EROs could be the
early-type population of forming(?) clusters or groups of galaxies physically
associated with the quasars. The evidence in favor of this consists of observa-
tions finding that luminous quasars and radio galaxies often reside in regions
of high galaxy density. Another possibility is that the quasars are markers of
large-scale structure traced by the EROs, such as walls or sheets of galaxies, as
suggested by Thompson, Aftreth & Soifer (15). Our current Ks-band images
are too small to determine whether the EROs cluster around the quasars or if
they are just part of a larger-scale overdensity, but we are currently awaiting
wide-field (9×9 arcmin2) near-infrared data to properly address this issue.
There is no evidence for a significant excess in the R- and Ks-band counts
in our fields, which we might expect if there are relatively rich clusters in
the fields. However, this does not exclude the possibility that the excess is
caused by clusters or groups of galaxies because most cluster members would
simply be swamped in the large numbers of foreground sources. By selection
on R −Ks colour we can find the ellipticals at z ≈ 1–2 preferentially against
much of the confusing foreground galaxies.
Another interesting possibility is that the excess is caused by EROs foreground
to the quasars, either associated with clusters or groups of galaxies, or just
5
overdensities (‘clumps’) in the ERO population. In this case, lensing magnifi-
cation bias is likely to be important for the quasars. By pointing the telescope
to a distant quasar, one is more likely to pick out lines of sight where there
are mass concentrations along the line of sight which gravitationally amplify
the quasar light.
5 Future and ongoing work
In order to address the question of how extended the ERO excess is, we have
started an imaging programme in J and Ks using the new WIRC
4 instrument
on the Palomar 200 inch. The WIRC has a field of view of 9×9 arcmin2 which
matches the size of our wide-field R-band images. We are also planning to
obtain spectra and morphologies of the EROs. This will allow us to determine
redshifts, spectral types and structural parameters of the EROs, thereby open-
ing up for an investigation of the Fundamental Plane of early-type galaxies at
cosmologically significant epochs.
References
[1] Barthel P.D., Miley G.K., Schilizzi R.T., Lonsdale C.J., A&AS 73 (1988)
515.
[2] Bertin E., Arnouts S., A&AS 117 (1996) 393.
[3] Cimatti A., Villani D., Pozzetti L., di Serego Alighieri S., MNRAS 318
(2000) 453.
[4] Cimatti A., et al., A&A 381 (2002) L68.
[5] Daddi R., Cimatti A., Pozzetti L., Hoekstra H., Ro¨ttgering H.J.A., Ren-
zini A., Zamorani G., Mannucci F., A&A 361 (2000) 535.
[6] Elston R., Rieke M.J., Rieke G.H., ApJ 341 (1988) 80.
[7] Hall P.B., et al., AJ 121 (2001) 1840.
[8] Hewitt A., Burbidge G., ApJS 87 (1993) 451.
[9] Hu E.M., Ridgway S.E., AJ 107 (1994) 1303.
[10] Mannucci F., et al., MNRAS 329 (2002) L57.
[11] McCarthy P.J., Persson S.E., West S.C., ApJ 386 (1992) 52.
[12] Roche N.D., Almaini O., Dunlop J., Ivison R.J., Willott C.J., MNRAS
337 (2002) 1282.
[13] Scodeggio M., Silva D.R., A&A 359 (2000) 953.
[14] Smail I., Owen F.N., Morrison G.E., Keel W.C., Ivison R.J., Ledlow
M.J., ApJ 581 (2002) 844.
[15] Thompson D., Aftreth O., Soifer B.T., ApJ 120 (2000) 2331.
4 Wide-field IR Camera
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