Transcript Search For z~5 Galaxies Laura Douglas¹, Malcolm Bremer¹, Matt Lehnert² 1.HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL,

Search For z~5 Galaxies
Laura Douglas¹, Malcolm Bremer¹, Matt Lehnert²
1.HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K.
2. Max-Planck-Institut für extraterrestrische Physik, Glessenbachstraβe, 85748 Garching bei München
The data used were obtained by the EDisCS project (P.I. S. White). Deep optical
photometry was taken using FORS2 at the VLT in the V-, R- and I-bands (2 hours in
each, with typical detection limits of AB>27), with complementary deep J and Ks
imaging from the NTT and I-band ACS images from the HST. The data were
originally obtained to identify clusters at 0.4<z<0.8. In addition to standard reduction
techniques the images were corrected for galactic extinction and the lensing effects
of the observed clusters, using the lensing maps provided by Clowe et. al. (2004).
An I-band cluster image and its corresponding lensing map can be seen in Fig 1.
0.5mag
Figure 2: Colour-colour plot where
the diamonds are EROs and the
triangles are identified stars. The
arrows depict the colour limits of
the candidates as they are all
detected in the I-band but not in the
R-band, to a limit of 27.8, and the
Ks-band, to a limit of 22.3. Two of
the unresolved objects, previously
identified as stars, have colours
similar
to
the
high-redshift
candidates, suggesting they could
be quasars.
Colour-Colour Plot
IAB-KAB
0mag
Examples of good candidates and contaminants can be seen in Figs 3 and 4. After
removing the majority of contaminants we have identified 150 excellent candidates
in the first 430 arcmin2. The number of candidates in each field ranges from 5 to 47.
This significant difference does not appear to be correlated with the lensing
properties of the different clusters, but rather a consequence of cosmic variance
which demonstrates the importance of having a large survey area.
HST
V
R
I
J
Ks
Figure 1: I-band image of cluster CL1054-1245 and its corresponding lensing map. The lensing
map shows the number of magnitudes correction that should be applied as a function of position for
z=5 objects.
The high-redshift galaxies were identified using the dropout technique pioneered by
Steidel and collaborators. The technique uses the fact that emission from these high
redshift sources at wavelengths shortward of Lyman alpha is absorbed by the
intervening neutral hydrogen in the IGM. This break in the spectrum can be
observed using filters shortward and longward of the Lyman break. In practice this
means we select objects with IAB<26.3 and RAB-IAB>1.3. Objects with a mAB=26 have
a MAB(1700 Å)>-21.
Object Selection
A object catalogue was prepared, using Sextractor, from the reduced EDisCS
images. Apertures were defined by the I-band image and were then applied to the V, R-, J- and Ks-band images. High-redshift candidates were selected using the
colour cuts above, determined from modelling of expected colours of high-redshift
galaxies, and their half-light radii from the high resolution HST images.
Once this sample had been identified, photometric and morphological cuts were
used to remove any contaminants. High-redshift galaxies are not the only sources to
be identified by the colour cuts used, so the IR and HST data provided possible J- or
Ks-band detections and morphological information that could identify EROs
(Extremely Red Objects, typically galaxies at z~1) and cool stars such as M-type
dwarfs, the main contaminants in a sample like this. Despite the fact that the IR data
is not deep enough to identify high-redshift galaxies it is deep enough to identify
EROs and cool sub stellar objects. Also, the HST data can identify stars as they are
unresolved and EROs which have half-light radii >0.5” whereas true high-redshift
galaxies are resolved but with small half-light radii, 0.1”-0.2” (Bremer et. al., 2004,
MNRAS, 347,L7).
Results
•Ks-Band Stacked Images - Average Colours of z=5 Galaxies
The I-K colour of a galaxy is an important value as it shows how long the galaxy has
had active star formation: the redder the colour, the longer the star formation
episode. To obtain an average I-K colour for the candidate galaxies we stacked the
Ks-band data to see if there was a combined detection. By combining up to
13 areas of the same field the image was over 1 magnitude
deeper than an individual frame. Fig 5 shows a nondetection in the stacked image KAB>25, implying that the
average colour of these objects is I-K<1. This rules out
strong star formation in these objects at z>10, therefore the
universe could not have been ionised at these redshifts. If
the high-redshift candidates were just below the detection
limit of the Ks-band with a range of I-K colours the stacked
image would give us a detection, so the lack of detection
Figure 5: Example of
tells us that all the candidates are well below the detection
stacked images.
limit.
•Number Counts and the Bright-end Slope of the Luminosity Function
Lehnert & Bremer (2003, ApJ, 593,630) claimed that the number of z>5 sources
selected in a similar manner to this work is less than that expected from the z=3-4
luminosity function of Lyman break galaxies, at least at IAB<26.3. Although their data
was taken with the same instrument as ours their conclusion was drawn from only
10% of the area of sky of this most recent survey. Nevertheless, our average
number counts over 10 fields, also implies a lack of bright sources compared to that
expected if there was no evolution in the luminosity function between z=3 and z=5
(Fig 6). A caveat is that we removed contaminants from the brighter bins using the
IR data but not from the fainter bins. Although Lehnert and Bremer (2003) found no
contaminants in their fainter bins we clearly need to confirm this assumption with
spectroscopy.
Number Counts
Figure 3: Images of objects in the V-, R-, I-, J- and Ks-bands with composite optical image. Top
object is an example of a good high-redshift candidate, the middle object is an example of a likely M
type dwarf and the bottom object is an example of a likely ERO.
Our work, and that of others, on the Hubble Ultra Deep Field (see presentation by
M. Bremer) have shown that in addition to compact objects a significant fraction of
high-redshift candidates are part of a double system or exhibit tail-like features. Two
examples of this morphology have been found in our sample (Fig 4). The greater
resolution of the HST images reveals a double object, in which each member shows
similar colours, and an object with a tail of similar colour to the main detection.
HST
V
R
I
J
Ks
Number (0.5 Mag bin)-1
Availability of deeper images and more sensitive equipment has lead to
identification of ever increasing numbers of high-redshift galaxies. Until recently the
size of candidate samples has been small, limiting the reliability of the statistics of
these sources. To remedy this we have begun a survey of 800 arcmin2 of sky over
20 fields using extremely deep multicolour imaging.
A break in colours between candidate high-redshift galaxies and the redder EROs is
shown in Fig 2. The colours of stars are not such a useful tool but with the added
half-light radii information from the HST these can still be easily identified.
RAB-IAB
Introduction
IAB
Figure 6: Number count plot where the diamonds are the simulated data assuming no evolution
from z=3-4 to z=5 and the crosses are our new data.
Figure 4: Images of objects in the V-, R-, I-, J- and Ks-bands with composite optical image. The top
object is a possible double object and the bottom object shows a tail-like feature.
Even without the removal of contaminants there would still be a lack of sources of
ionising photons. This paucity implies that the unobscured star formation density
and the UV density provided by the brightest galaxies decreases by a factor of ~3,
which in turn is insufficient to ionise the universe implying that the majority of
ionising photons originated from fainter sources. (Lehnert & Bremer, 2003).