Issue |
A&A
Volume 497, Number 3, April III 2009
|
|
---|---|---|
Page(s) | 689 - 702 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361/200811429 | |
Published online | 18 February 2009 |
The Building the Bridge survey for z = 3 Ly
emitting galaxies
II. Completion of the survey![[*]](/icons/foot_motif.png)
L. F. Grove1 - J. P. U. Fynbo1 - C. Ledoux2 - M. Limousin1,3 - P. Møller4 - K. K. Nilsson5 - B. Thomsen6
1 - Dark Cosmology Centre, Niels Bohr Institute, University of
Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
2 -
European Southern Observatory, Alonso de Córdova 3107, Casilla
19001, Vitacura, Santiago 19, Chile
3 -
Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS,
57 avenue d'Azereix, 65000 Tarbes, France
4 -
European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching,
Germany
5 -
Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
6 -
Institute of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark
Received 27 November 2008 / Accepted 26 January 2009
Abstract
Context. We have substantial information about the kinematics and abundances of galaxies at
studied in absorption against the light of background QSOs. At the same time we have already studied 1000s of galaxies detected in emission mainly through the Lyman-break selection technique; however, we know very little about how to make the connection between the two data sets.
Aims. We aim at bridging the gap between absorption-selected and emission-selected galaxies at
by probing the faint end of the luminosity function of star-forming galaxies at
.
Methods. Narrow-band surveys for Lyman-
(Ly
)
emitters have proven to be an efficient probe of faint, star-forming galaxies in the high-redshift universe. We performed narrow-band imaging in three fields with intervening QSO absorbers (a damped Ly
absorber and two Lyman-limit systems) using the VLT. We target Ly
at redshifts 2.85, 3.15, and 3.20.
Results. We find a consistent surface density of about 10 Ly-emitters per square arcmin per unit redshift in all three fields down to our detection limit of about
.
The luminosity function is consistent with what has been found by other surveys at similar redshifts. About 85% of the sources are fainter than the canonical limit of R=25.5 for most Lyman-break galaxy surveys. In none of the three fields do we detect the emission counterparts of the QSO absorbers. In particular we do not detect the counterpart of the z=2.85 damped Ly
absorber towards Q2138-4427. This implies that the DLA galaxy is either not a Ly
emitter or is fainter than our flux limit.
Conclusions. Narrow-band surveys for Ly
emitters are excellent for probing the faint end of the luminosity function at
.
There is a very high surface density of this class of objects; yet, we only detect galaxies with Ly
in emission, so the density of galaxies with similar broad band magnitudes will be substantially higher. This is consistent with a very steep slope of the faint end of the luminosity function as has been inferred by other studies. This faint population of galaxies is playing a central role in the early Universe. There is evidence that this popualtion is dominating the intergrated star-formation activity, responsible for the bulk of the ionising photons at
and likely also responsible for the bulk of the enrichment of the intergalactic medium.
Key words: cosmology: observations - galaxies: quasars: individual: BRI 1346 - galaxies: quasars: individual: BRI 1202-0725 - galaxies: quasars: individual: Q 2138-4427 - galaxies: high-redshift
1 Introduction
Strong arguments
(Fynbo et al. 1999; Schaye 2001; Barnes & Haehnelt 2008; Haehnelt et al. 2000; Rauch et al. 2008)
indicate that there is very little overlap between emission selected
galaxies (primarily Lyman-break galaxies, LBGs, Steidel et al. 2003)
and absorption selected galaxies (primarily the damped
Lyman-
absorbers, DLAs Wolfe et al. 2005). The simple
reason for this is that LBG samples are continuum flux limited and
that the current flux limit of
is not deep enough to
reach the level of typical absorption selected galaxies. This is
unfortunate as we then know little about how to combine the detailed
information on abundances and kinematics inferred from observations of
DLAs with the information about colours and luminosities of high-zgalaxies detected in emission.
In 2000 we started a survey aiming at bridging the gap between
emission and absorption selected galaxies. The goal of the survey was
to detect faint
galaxies using narrow-band imaging
selection of Lyman-
(Ly
)
emitters (LAEs) and in this
way bridge the gap between the DLAs and the LBGs. During the 1990ies
and early 2000s it was established that LAEs can be used to select
high-z galaxies (e.g. Møller & Warren 1993) and that this method
easily traces significantly deeper into the luminosity function than
what is possible with spectroscopic samples of LBGs
(e.g. Fynbo et al. 2001; Cowie & Hu 1998). In our survey we targeted the
fields of QSOs with intervening DLAs primarily to be able also to
search for the galaxy counterparts of the DLAs, but also to anchor the
fields to known structures at the targeted redshifts. The first
paper of the survey was published by (Fynbo et al. 2003, hereafter
Paper I), where we presented the results from two of the three
targeted fields. Since then the study of LAEs has progressed
substantially, mainly on two fronts. First, large samples covering a
range of redshifts have been collected using wide-field imagers both
on 4 m and 8 m class telescopes
(e.g. Nilsson et al. 2009; Ouchi et al. 2008; Gronwall et al. 2007). Second, more
detailed studies of the properties have been carried through, most
notably based on LAEs in the GOODS fields
(e.g. Nilsson et al. 2007; Pentericci et al. 2008; Gronwall et al. 2007).
In this final paper from our ``Building the Bridge'' survey we first
present the results from the third field, namely the sample of LAEs
identified in the field of the quasar BRI 1202-0725. Second, we
combine the results of the entire survey covering three fields to
derive the luminosity function of the LAEs at .
We then
discuss our results in the light of the substantial progress that has
been made by a number of groups over the last few years on the study
of LAEs at
.
Throughout this paper, we assume a cosmology with H0=70 km s-1 Mpc-1,
and
.
We also
use magnitudes on the AB system throughout (Oke 1974).
2 The field of BRI 1202-0725
2.1 Imaging
The field of BRI 1202-0725 (
)
was included in this survey due to the presence of
a Lyman-limit system along the line-of-sight towards the quasar at a redshift
of z=3.2 (Storrie-Lombardi et al. 1996). This field was observed in service
mode at the VLT 8.2 m telescope, unit Yepun, during the nights January 30
through February 6 2003 using the FORS2 instrument. The wavelength of the
Ly
transition at the redshift of z=3.20 is
,
which
corresponds to the central wavelength of the 61 Å wide [OIII] VLT filter.
The field was observed in this narrow-band filter (NB) and in the Bessel Vand Special R filters. The transmission curves of the three filters are shown
in Fig. 1. The integration times in the V and Rfilters were set by the criterion that the broad on-band V imaging should
reach about half a magnitude deeper than the narrow-band imaging so as to get a
reliable selection of objects with excess emission in the narrow-band
filter. For the broad off-band R imaging, we aimed at reaching the
significance level at one magnitude deeper than the spectroscopic limit of
R(AB)=25.5 for LBGs (i.e. aiming at R(AB)=26.5 at the
significance level). The total integration times, the seeing (FWHM) of the
combined images and the 5
detection limits for a 3
diameter
aperture are given for each filter in Table 1. For comparison
with previous work we also list the corresponding numbers for the fields of
BRI 1346-0322 and Q 2138-4427 presented in Paper I
.
![]() |
Figure 1: Transmission curves of the narrow- and broad-band filters used for the observations of BRI 1202-0725. |
Open with DEXTER |
Table 1:
Log of imaging observations with FORS1 and 2 of the three
surveyed fields. The
detection limits are computed for a
3
diameter aperture. The data for BRI 1346-0322 and
Q 2138-4427 are reproduced from Paper I.
The images were reduced (de-biased and corrected for CCD pixel-to-pixel variations) using the FORS pipeline (Grosbøl et al. 1999). The individual reduced images in each filter were combined using a code that optimises the signal-to-noise (S/N) ratio for faint, sky-dominated sources (see Møller & Warren 1993, for details on this code).
The broad-band images were calibrated as part of the FORS calibration plan via observations of Landolt standard stars (Landolt 1992). We transformed the zero-points to the AB system using the relations given by Fukugita et al. (1995): V(AB)=V -0.02 and R(AB)=R +0.17. For the calibration of the narrow-band images, we used observations of the spectrophotometric standard stars EG274 and GD71.
2.2 LAE candidate selection
![]() |
Figure 2:
Left panel: colour-colour diagram for simulated
galaxies based on Bruzual & Charlot (1995) galaxy SEDs
(Bruzual & Charlot 2003). The open squares are 0< z <1.5 galaxies with
ages ranging from a few to 15 Gyr and the open triangles are
1.5<z<3.0 galaxies with ages ranging from a few Myr to 1 Gyr. The
dotted box encloses the simulated galaxy colours. The dashed line
indicates colours of objects with a particular broad-band colour and
with an SED in the narrow-band filter ranging from an absorption line
in the upper right part to a strong emission line in the lower left
part of the diagram. Middle panel: colour-colour diagrams for
all objects detected at S/N>5 in either the narrow band or the Vband in the BRI 1202-0725. In the lower left part of the diagram
there is a large number of objects with excess emission in the narrow
filter. Candidate LAEs are those objects with a 1 |
Open with DEXTER |
The selection of LAE candidates is based on the ``narrow minus on-band
broad'' versus ``narrow minus off-band broad'' colour/colour plot
technique
(Møller & Warren 1993; Fynbo et al. 1999,2003,2000,2002). The
object identification and photometry was carried out in SExtractor
(Bertin & Arnouts 1996) using the dual-image mode having a detection image
and measuring the photometric properties on the individual images. For
the detection we used a weighted sum of the V-band (20%) and
narrow-band (80%) images to secure an optimal detection of objects
with excess in the narrow filter. Before object detection we convolved
the detection image with a Gaussian filter function having a FWHM
equal to that of point sources. We used a detection threshold of 1.1
times the background sky-noise and a minimum area of 5 connected
pixels in the filtered image. For total magnitudes we use the
SExtractor MAG_AUTO and to compute colours we use the isophotal
magnitudes (MAG_ISO). In our final catalogue, we include only
objects with total S/N>5 in the isophotal aperture in either the
narrow- or the V-band images. In total, we detect 3202 such objects
within the
arcsec2 field around BRI 1202-0725. The
error bars on the colour indices are derived using the maximum
likelihood method of Fynbo et al. (2002). For the selection of LAE
candidates we restricted the sample to S/N>5 in the narrow-band
filter as described below.
In Fig. 2 we show the colour-colour diagram used for
the final selection of LAE candidates. In order to constrain where
objects with no special spectral features in the narrow filter are in
the diagram, we calculated colours based on synthetic galaxy SEDs
taken from the Bruzual & Charlot (1995) models (Bruzual & Charlot 2003). We
have used simple single-burst models with ages ranging from a few Myr
to 15 Gyr and with redshifts from 0 to 1.5 (open squares in
Fig. 2) and models with ages ranging from a few Myr to
1 Gyr with redshifts from 1.5 to 3.0 (open triangles). For the colours
of high-redshift galaxies, we included the effect of Lyblanketing (Møller & Jakobsen 1990; Madau 1995), but for none of the models
the effects of dust are included. Figure 2 shows the
N(AB)-V(AB) versus N(AB)-R(AB) colour diagram for the simulated
galaxy colours (left panel) and for the observed sources in the target
field (middle and right panels). The dashed line indicates where
objects with a particular broad-band colour (corresponding to a 100 Myr old galaxy at z=3.20 and either absorption (upper right) or
emission (lower left) in the narrow filter will fall.
In the middle panel, we show the colour-colour diagram for all of the
objects detected in the field. The large, dense group of points
correspond to the normal field galaxy population without special
properties in the narrow-band filter, in the following refered to as
continuum objects. Unfortunately, the observed distribution of the
continuum objects is oriented along the same direction as that
expected for emission line objects (indicated with the dashed
line). This is due to the fact that the central wavelength of the
narrow-band filter is bluewards of the central wavelength of the
on-band broad filter (V). For this reason we for this field
choose a rather conservative criterion for selection of candidates,
namely
.
After weeding out a few spurious
sources (related to bright stars) we detect 25 such
objects in the BRI 1202-0725 field which we consider as LAE
candidates in the following. The colours of the candidates are shown
in the right panel of the figure.
![]() |
Figure 3:
The
|
Open with DEXTER |
A contour image of the combined narrow-band image of the
arcsec2 field surrounding the QSO BRI 1202-0725 is
shown in Fig. 3. The QSO is identified by a
``
'' at the field centre and the positions of selected LAE
candidates are shown with boxes.
![]() |
Figure 4:
Our equivalent width selection criterion is illustrated by
plotting the line equivalent width against the Ly |
Open with DEXTER |
In the selection of LAE candidates we apply a colour selection which
translates into the equivalent width (EW) selection criterion
illustrated in Fig. 4 given in observed
quantities. The selection criterion translates into candidates having
observed Å (or rest frame
Å).
2.3 Multi-object spectroscopy
The above selection of LAE candidates only selects for excess emission
in the narrow-band filter. This is likely to originate from the
Ly
at
(Fynbo et al. 2003), however, interlopers caused
by other emission lines in galaxies at lower redshifts is also
possible. Therefore, follow-up spectroscopy is necessary for
confirming the Ly
origin of the excess emission.
Follow-up multi-object spectroscopy (MOS) was carried out in visitor
mode on March 21-23, 2004, with FORS2 installed at the VLT telescope,
unit Yepun. The mask preparation was done using the FORS Instrumental
Mask Simulator. The field of BRI 1202-0725 was covered by 3 masks. It
was possible to fit all candidates into the slits. The spectra were
obtained with the G600B grism covering the wavelength range from 3600 Å to 6000 Å at a resolving power of 900. For possible
identification of other emission lines at the same redshift we also
obtained spectra with the G600R grism covering the wavelength range
from 5000 Å to 7500 Å. The detector pixels were binned for all observations through the masks. In
Table 2 we give the main characteristics of the
spectroscopic observations.
Table 2: Log of spectroscopic observations with FORS2.
The MOS data were reduced and combined as described in
Fynbo et al. (2001). The accuracy in the wavelength calibration is about
0.1 pixel for a spectral resolution of R=900, which translates
to
.
Average object extraction was performed within a
variable window size matching the spatial extension of the emission
line. Therefore, the flux should be conserved.
![]() |
Figure 5:
|
Open with DEXTER |
Of the 25 targeted LAE candidates, 18 are confirmed emission line
objects. For the seven remaining candidates no emission line was
identified in the spectrum. The spectra for the 18 confirmed
candidates are shown in Fig. 5. We consider a
candidate confirmed if there is an emission line detected with at
least
significance at the correct position in the slitlet
within the wavelength range corresponding to the filter
transmission. All of the confirmed emission line candidates are
most likely to be Ly
based on the absence of other emission
lines at longer wavelengths (mainly NeIII , H
AND
O[III] , the positions of which are all covered by our
spectroscopy). The limits on the flux ratios we infer are similar to
those derived in Fynbo et al. (2001) and based on their analysis
contamination from OII emitters is very unlikely. The overall
efficiency for detection and confirmation of LAEs is therefore
#(confirmed LAEs)/#(observed LAEs)=18/25=72%. This is the same as
found for Q 2138-4427 and somewhat less than the results of
BRI 1346-0322 (Paper I).
The redshift distribution for the LAE sample in the field of BRI 1202-0725 is shown in Fig. 6 and compared with the filtercurve. It can be seen that the targets fill out the volume probed by the filter. The mean redshift of the LAEs is 3.203 with a standard deviation of 0.013.
We have investigated the properties of the non-confirmed candidates and find that they are in the red end of the broad-band colours of the candidate sample and unresolved. However, their low line fluxes prevent them from being confirmed by the present spectroscopic observations. This analysis was carried out for all three fields and we found similar properties in all cases.
3 The LAE population
In the following we combine the results of the entire Building the
Bridge survey to characterise the population of LAEs. The survey
constitutes observations of three fields centered on the quasars
BRI 1202-0725, BRI 1346-0322 and Q 2138-4427 searching for
Ly
emitters at redshifts
z=3.20, 3.15 and 2.85,
respectively. These redshifts are targeted due the presence of high
column density absorption systems along the line of sight towards the
quasars. In the fields 25, 26 and 36 emission line objects were
identified through the narrow-band technique. In each of the fields of
BRI 1346-0322 and Q 2138-4427 the emission from two emitters were
identified with other emission lines than Ly
(, in three
cases corresponding to [OII) and the fourth case CIV,
][]fynbo2003. Therefore, these four objects have been excluded from
the current analysis. In the following analysis we consider 18, 18
and 23 spectroscopically confirmed LAEs as the confirmed sample. The
entire photometric sample includes 7, 6 and 11 additional
objects. These have not been confirmed spectroscopically. For the
fields of BRI 1202-0725 and Q 2138-4427 all candidates were
observed, so the non-confirmed systems were lacking an emission line
at our sensitivity. For the field BRI 1346-0322 three candidates
were not observed leaving three as spectroscopically not confirmed
candidates.
Table 3: Overview of LAE samples from the three fields included in the present survey.
The LAE population is characterised in terms of magnitudes, colours
and derived entities like Ly
flux, luminosity, EWs and star
formation rates (SFRs). For total magnitudes we use the SExtractor
MAG_AUTO which are used to compute the Ly
flux, luminosity
and SFR. The colour indices are computed based on isophotal magnitudes
(MAG_ISO) and are used to estimate the EW. For all magnitudes the
same detection area is used in all bands. Details of the computation
of the derived properties can be found in Fynbo et al. (2002). Finally,
in the next section we derive the luminosity function of LAEs at
and compare this with the recent measurements by
Ouchi et al. (2008); Gronwall et al. (2007) and
Rauch et al. (2008). Table 3 gives an overview of the
LAE samples of the three fields. From the table it can be seen that
the survey covers LAEs with luminosities in Ly
down to a few
times 1041 erg s-1, making it one of the deepest surveys
of Ly
emitters at
.
Table 4
summarises the main characteristics of our sample and the three fields
used for comparison.
![]() |
Figure 6: Redshift distribution of LAEs in the field of BRI 1202-0725 relative to the filter transmission curve. The redshifts of LAEs fill out the volume probed by the filter. |
Open with DEXTER |
Table 4: Summary of the main properties of our and the comparison samples.
3.1 Characteristics of spectroscopically confirmed LAEs
In
Tables A.1-A.3
we give the measured and derived properties for the confirmed
LAEs. The magnitudes in these tables are total magnitudes, and the
lower limits correspond to
significance levels for the
SExtractor MAG_AUTO apertures. These translate into the limits given
for the fluxes and luminosities. The EWs are derived from the colour
indices based on the isophotal magnitudes. In this case the limits are
caused by a less significant detection in the on-band broad (B or
V) and the values correspond to using the
levels for these
bands.
From Table 3 it can immediately be seen that the total number of objects is comparable among the three fields. The detected number of LAEs translates into surface densities of 8, 8 and 10 per square arcmin per unit redshift, respectively for the fields of BRI 1202-0275, BRI 1346-0322 and Q 2138-4427, consistent with the almost similar flux limits in the three fields. From the table it can also be seen that the other properties are similar between the fields and between the confirmed LAEs and non-confirmed candidates.
In Fig. 7 we show the magnitude distribution for the
LAEs. The lines correspond to candidates with measured magnitudes
while the triangles indicate systems with
upper limits. The
observed magnitudes are translated to correspond to a redshift z=3by correcting by the difference in the distance modulus between the
observed redshift and z=3. For the R-band we do not apply any
K-correction since it is a negligible effect over such small redshift
changes. The error bars are standard deviations among the fields and
indicate that even though the results in the three fields are
consistent, small numbers and cosmic variance leads to significant
differences between the fields. For the narrow-band it is clear that
all the bright objects have been confirmed to correspond to Ly
emitters while the non-confirmed cases are in general found in the
faint end of the distribution.
![]() |
Figure 7:
Total magnitude (MAG_AUTO) distribution of the LAEs in
Narrow- ( top) and R-band ( bottom). The curves are averages between
fields and error bars are the corresponding standard deviations. Open
circles and dotted lines mark the distribution for all photometrically
selected candidates, while filled symbols and solid lines mark the
confirmed Ly |
Open with DEXTER |
From the magnitude distribution of the R-band it can be seen that
all the LAE candidates have very faint magnitudes. For absorption line
systems the magnitude limit of achieving a redshift is
indicated by the arrow in the figure. Therefore, we note that of the
confirmed emitters only 6 out of 59 or
10% are brighter
than this limit. The fraction is similar for the complete candidate
sample. This is consistent with the non-confirmed candidates being
spread over all R-band magnitudes. The faint R-band magnitudes of
the galaxies emphasises their importance for tracing the faint end of
the luminosity function, possibly contributing a significant fraction
of the total star formation (see also Reddy & Steidel 2008).
Figure 8 shows the distribution of rest frame equivalent widths derived from the colour index ``narrow band minus on-band broad'' isophotal magnitudes. We have not corrected the broad-band magnitudes for the narrow-band contribution. Our measurements are compared with those of Gronwall et al. (2007) and Ouchi et al. (2008). It can be seen that the sample of confirmed LAEs for which we could measure the EW reliably is consistent with our survey being deeper than the comparison samples. It can also be seen that the Ouchi et al. sample has a higher fraction of emitters with high EW than the Gronwall et al sample. Our sample it is more consistent with the Ouchi et al. than with the Gronwall et al. sample.
Table 3 also lists the range of SFR derived from the
Ly
luminosities (following Fynbo et al. 2002). The SFRs are
found to be in the range
yr-1 to
yr-1 assuming negligible
extinction.
4 Luminosity function of LAEs at
A widely used diagnostic for describing galaxy samples is their luminosity function (LF). Here, we derive the LF of LAEs at z=3.0combining the data from the present work and from Paper I. The complete survey encompasses three fields with a total of 59 spectroscopically confirmed LAEs. For each field we derive the LF independently to take into account the different narrow-band filters and incompleteness functions. Finally, the individual results are combined to provide the LF at z=3.0.
4.1 Estimating the incompleteness correction
When constructing the sample of LAEs the main source of incompleteness comes from the photometric selection of candidates. The following process is carried out separately for each field to take into account the specific properties of the different data sets. To estimate the detection completeness of the photometric samples we distribute 100 artificial objects in the images and assign broad- and narrow-band magnitudes in a range covering the observed values as well as an additional margin. The magnitudes were assigned based on the off-broad band. In this band magnitudes are distributed uniformly in the interval 5 to 35. For the narrow- and on-broad band magnitudes were assigned based on a uniform colour index (``narrow -off-broad'' and ``on-broad - off-broad'') distribution in the interval minus five to five. This results in uniform magnitude distributions in the narrow- and on-broad bands in the magnitude interval 10 to 30 dropping off and vanishing at magnitudes of zero and 40. The objects are modelled with a two-dimensional Gaussian shape with the size corresponding to the seeing measured in the individual images. Each artificial object is added to each image scaling the Gaussian to the appropriate luminosity depending on the magnitudes computed individually for each band. This process ensures good coverage for both magnitudes and colour indices. The procedure is repeated 1000 times for each field, thus a total of 105 artificial objects are used in the estimate. The images with artificial objects are now treated exactly as the original images and the recovery rate yields the detection completeness. The completeness functions estimated for the three fields in the narrow-band filters are shown in Fig. 9. From that figure it can be seen that the data of the field of BRI 1346-0322 are slightly shallower than the other two fields, which are very similar. These completeness functions are used to correct the measured luminosity functions below. Note that we have not corrected our data for the effects of a non-square band-pass or the photometric error function (see Gronwall et al. 2007).
![]() |
Figure 8:
Rest frame equivalent width distribution for the confirmed
(solid symbols and line) and entire (open symbols and dashed line)
samples ( upper panel). In the lower panel the distribution for the
confirmed sample is compared with that of (Gronwall et al. 2007, dot-dashed
line) and (Ouchi et al. 2008, dashed line). The triangles
indicate cases for which only lower limits (at the |
Open with DEXTER |
![]() |
Figure 9:
Completeness as function of Ly |
Open with DEXTER |
A possible concern is whether the extension of the Ly emitters have a significant effect on the estimated completeness
functions. To test this we have carried out a complementing test where
the original background subtracted images were first dimmed. Then we
corrected the background noise to the original level and tried to
recover the already detected candidate emitters. We dimmed the images
by 0.25, 0.5 and 0.75 mag. The completeness functions estimated
in this way were similar to the ones described above, but suffer from
poor statistics (only 80 sources in total were found in the original
images) and only tracing the magnitudes and colour indices of the
detected sources. This makes the previously described functions more
reliable for correction purposes and are the ones used in the
following.
4.2 Ly
luminosity function
![]() |
Figure 10:
The derived differential Ly |
Open with DEXTER |
We derive the differential luminosity function for the confirmed LAEs with a simple classical approach that has been used in many previous works (e.g. Malhotra & Rhoads 2004; Hu et al. 2004; Ouchi et al. 2008; Ajiki et al. 2003; Ouchi et al. 2003). The volume used to convert the observed number of systems to volume densities is computed based on the field size and having a depth corresponding to the FWHM of the narrow-band filter. Furthermore, we correct the observed luminosity function by the completeness function estimated in the previous section to account for the incompleteness of the sample due to our target selection. The obtained luminosity function is shown in Fig. 10. The error-bars are derived from the errors in the individual fields by standard error propagation.
We fit our derived LF with the Schechter function (Schechter 1976)
using a -minimisation. We carry out three fits in
total. First, we fit the sample of confirmed candidates with
.
Second, we fit the entire photometric
sample in two luminosity intervals:
and
.
The brighter limit corresponds to
the region where all fields are estimated to be close to complete,
while the second limit is where the shallowest field is essentially
not contributing anything (see Fig. 9). The
results of the fits are summarised in Table 5. In
Fig. 10 we include the fit to the confirmed candidates
as the thin solid line in both panels and the fit to the deep sample
of all candidates is included in the lower panel as the thick solid
line. The fit to all candidates with the brighter luminosity limit is
included as the dotted line. It can be seen that despite the small
differences in the best fit parameter values the functions are
consistent.
Table 5: Results of fitting a Schechter function to the LF.
We compare our result with those of Ouchi et al. (2008); Gronwall et al. (2007) and
Rauch et al. (2008). At the faint end we find a good agreement, in
particular if all the non-confirmed candidates are LAEs. At the bright
end we find a marginal excess of objects compared to the other
surveys, even though the numbers are consistent within the error
bars. The (marginal) excess of bright objects may be caused by enhanced
clustering of
bright LBGs around QSO absorber fields (Bouché et al. 2005; Bouché & Lowenthal 2004).
Comparing the density of galaxies brighter than the
limit of Gronwall et al. (2007) of
we find only
a marginal excess of a factor of
1.25.
It is also interesting to compare these
environments with the results of Venemans et al. (2007) studying radio
galaxies. The radio galaxies trace the highest overdensities, likely
representing proto-clusters. The overdensity of LAEs around radio
galaxies are found to be of the order 2-5. Thus the overdensities
found in our fields are, as expected, lower. In the field of
Q 2138-4427 the redshifts are concentrated relative to the width of
the filter function (Paper I, see also Monaco et al. 2005). In this
field the overdensity is only a
factor of 1.1 with respect to the results of Gronwall et al. (2007), thus
we do not consider this good evidence of a proto-cluster.
5 Galaxy counterparts of the QSO absorbers
![]() |
Figure 11:
FORS 1 narrow-band image centered on Q 2138-4427 ( left
panel). North is up and East is to the left. The wavelength at peak
transmission of the (He II) narrow-band filter used corresponds
to Ly |
Open with DEXTER |
One of the goals of our survey was to bridge the gap between emission
and absorption selected objects. The three studied fields are centered
on QSOs with high column density absorption systems at the redshift
for which Ly
falls in the narrow-band filters. In the
fields of BRI 1346-0322 and BRI 1202-0725 the absorbers are
Lyman-limit systems and in the field of Q 2138-4427 it is a DLA.
The observed flux of Q 2138-4427 is reduced by approximately 1.5
magnitude in the narrow-band image as shown in Fig. 3 of Paper I. This
is due to the strong DLA line that absorbs the QSO light making it
much easier to search for any faint emission close to the QSO in the
narrow band than in the broad-band images. Moreover, if the galaxy
counterpart of the
DLA system has Ly
emission, it would be relatively brighter in the narrow-band than in
the broad-band images. To subtract the image of the QSO in the
narrow-band image, we used PSF-subtraction. We modelled the PSF of the
QSO from ten bright, yet unsaturated, stars present in the field. The
extension ALLSTAR of the DAOPHOT-II software
(Stetson 1994,1987) was used to perform the final PSF
model fitting and subtraction. After careful PSF subtraction, we
detected an extended source at an impact parameter of
(corresponding to 12 kpc at z=2.85) from the QSO line-of-sight at a
position angle of
(see Fig. 11). The proximity
of this source to the QSO line-of-sight makes it a good candidate for
the DLA galaxy counterpart.
To establish whether the source seen in Fig. 11 is the
galaxy counterpart of the
DLA system, we gathered
spectra of Q 2138-4427 using both FORS 1 Multi-Object Spectroscopy
(MOS) and long-slit spectroscopy. The details of the MOS observations
were given in Paper I. The positions of the two MOS slitlets that
covered the QSO are marked with dotted lines in the left panel of
Fig. 11. One of the
-wide MOS slitlets covered
both the QSO and the candidate DLA galaxy counterpart while the second
MOS slitlet was oriented at a position angle about
smaller. None of these spectra showed Ly
emission from the
DLA absorber. However, the atmospheric conditions during the MOS
observations were quite poor (see Paper I) and we consequently
obtained a deeper (
5 h total integration time) FORS 1
long-slit spectrum with a
-wide slit and the G600 B grism
covering the candidate under very good seeing conditions (
in the combined spectrum). The position of the long
slit is shown with dashed lines in Fig. 11. The 2D
long-slit spectrum is displayed in the upper two panels of
Fig. 12. We used the spectral PSF II optimal extraction
algorithm (Møller et al. 2000) to subtract the emission from the QSO in
the wings of the DLA trough. No Ly
emission from the
candidate DLA galaxy counterpart is detected.
We conclude that the source seen in Fig. 11 is not an LAE
at z=2.85. Possible Ly
emission from this source should be
fainter than the
very low detection limit of
erg s-1 cm-2. Within
from the QSO
line-of-sight, our detection limit for Ly
emission is even
lower (by less than a factor of two though) due to the combined limits
from both the MOS and long-slit spectra. The nearest confirmed LAE is
LEGO2138_36 (Paper I) and it is situated at a distance of
from the QSO line-of-sight. This means that if the
DLA galaxy counterpart is an LAE it should actually be fainter than
all of the LAEs detected in our survey.
![]() |
Figure 12:
a) portion of the 2D long-slit spectrum of Q 2138-4427
centered on the Ly |
Open with DEXTER |
What could be the origin of the source seen in Fig. 11?
One possibility is that this source is associated to the galaxy
counterpart of the lower redshift,
DLA system
toward Q 2138-4427. In this case the emission detected in the
narrow-band image would be continuum emission. Making this
assumption, we estimate a broad-band magnitude of
B(AB
AB)=25.7 for this object. In the lower two panels
of Fig. 12, we show the 2D spectrum of the QSO around
the corresponding DLA trough. No Ly
emission is detected
at more than the
confidence level. There are two
peaks in the spectrum however but we consider these too uncertain to
allow further discussion. Another interesting possibility is that the
source is part of the host galaxy of Q 2138-4427.
In none of the other two fields do we see any evidence of a narrow band source close to the QSO position after PSF subtraction. Note, however, that for these sight lines the intervening absorbers are only Lyman-limit systems so the fluxes from the QSOs are much less reduced by the absorption systems than for Q 2138-4427.
5.1 Serendipitously detected LAEs
The combined FORS 1 2D long-slit spectrum covers a solid angle of
arcsec2 and the wavelength range from 3600 Å to 6000 Å. This spectrum is therefore sensitive to LAEs with redshifts
in the range 2.0<z<3.9. Assuming a constant source density of
approximately ten LAEs per arcmin2 per unit redshift, as is found
in our survey for
LAEs down to a flux detection limit of
(corresponding to a Ly
flux of about
erg s-1 cm-2), we expect to
serendipitously observe about six LAEs in the long slit. This number
is of course only a rough estimate as the sensitivity of the spectrum
is a function of wavelength and the volume density of LAEs down to the
given flux detection limit is likely to be a strong function of
redshift. In the 2D spectrum, we actually detect six emission-line
objects with no or very faint continua. These are likely to be LAEs at
redshifts 2.30, 2.45, 2.69, 2.86 (one of which is LEGO2138_12,
Paper I), 3.03 and 3.61 (see Fig. 13). In
Table 6 we give the redshift and broad-band
magnitudes of the serendipitously detected LAEs. Two of the systems
were not detected in our broad-band imaging. For the others, it can be
seen that their broad-band magnitudes match the typical magnitudes of
the photometrically selected LAEs from our survey. With respect to the
colour indices we find values around zero which is towards the blue
end of the typical candidates.
![]() |
Figure 13: 2D spectra of probable LAEs serendipitously observed in the FORS 1 long slit spectrum. The redshifts range from z=2.30 to 3.61. The z=2.86 object is LAE2138_12 previously observed also in one of the MOS slitlets (see Paper I). |
Open with DEXTER |
Table 6: Properties of the serendipitously detected LAEs.
6 Discussion and conclusions
The aim of our survey was to try to bridge the gap between emission
selected and absorption selected galaxies at .
To reach
this goal we have performed the currently deepest narrow-band survey
for Ly
emitting galaxies at
using narrow-band
imaging at the VLT. Our survey was successful in establishing the
existence of a large number of galaxies below the flux limit of the
Lyman-break surveys (R=25.5). We reach a surface density of LAEs of
the order of 10 per arcmin-2 per unit redshift. This is about an
order of magnitude greater than for R<25.5 LBGs. It is also about a
factor of two higher than that found by other deep surveys for LAEs at
(e.g. Gronwall et al. 2007). This difference mainly
reflects the varitation in depth between the surveys, since if we
impose a luminosity limit consistent with that of Gronwall et al. (2007)
then we include only 42 of our 83 LAEs consistent which results in
only about 20% more galaxies in our sample with respect to the
Gronwall et al. (2007) results. This marginal overdensity we attribute to
our survey targeting the environments of DLAs and Lyman-limit systems
and not a blank field.
Our survey targeted three fields in which we photometrically selected
89 emission line candidates. Of these, 63 were confirmed to be
emission line galaxies, however, four of the emission galaxies were
foreground systems. In total we thus identified and spectroscopically
confirmed 59 LAEs in the three fields. Three candidates were not
observed as we could not fit them into the available masks. This
corresponds to a spectroscopic confirmation rate of about 73% or 69%
excluding the interlopers. Comparison of the properties of the
spectroscopically confirmed and not confirmed candidates showed that
the non-confirmed cases were mostly in the faint end of the narrow
band magnitude range and any other related quantity (EW, Ly flux and luminosity) while the broad band properties were similar for
the two groups. Some of the confirmed candidates are only detected at
low signal-to-noise in the spectroscopy, most noteably
LAE1202_09. This LAE seems to be extended both spatially and in
velocity causing the signal-to-noise in the spectroscopy to be lower
than for more compact candidates with only marginally resolved lines.
Hence, some of the unconfirmed candidates maybe more broadlined
systems. We conclude that the non-confirmed systems must be a mix of
spurious candidates and objects with too faint emission lines to be
detected by our spectroscopic follow-up. The properties of the
confirmed sample of LAEs are comparable to that found by other authors
as shown in Figs. 8 and 10, except being
deeper. The depth of the R-band data does not allow for a detailed
discussion, but about 90% of the selected emission line galaxies are
fainter than the R=25.5 limit for Lyman-break surveys.
Since our survey was started in 2000 we have learned a lot more about
the faint end of the luminosity function at .
The study of
LAEs has progressed substantially both in sample sizes and in the
range of redshifts that have been probed from z=2(Nilsson et al. 2009; Fynbo et al. 2002) to
(Ota et al. 2008; Iye et al. 2006). Also, using gamma-ray bursts (GRBs) it has
been found that a significant fraction of massive stars at these
redshifts die in extremely faint galaxies, e.g.
for the
host galaxy of GRB030323 at z=3.37 (Vreeswijk et al. 2004) and
R>29.5 for the host galaxy of GRB020124 at z=3.20(Berger et al. 2002; Hjorth et al. 2003). A statistical analysis of the
luminosities of GRB host galaxies again points to a large fraction of
the star formation being located at the faint end of the luminosity
function (Jakobsson et al. 2005; Fynbo et al. 2008). Also for continuum selected
galaxies there has been significant progress. Sawicki & Thompson (2006) and
Reddy & Steidel (2008) used the Lyman-break technique to push to
significantly fainter limits confirming an extremely steep faint end
slope.
As for the other (faint) end of the bridge, the study of the galaxy
counterparts of DLAs, there has been disappointingly little
progress. We still only have a few spectroscopically confirmed
counterparts of high-z DLAs (Møller et al. 2002; Christensen et al. 2007; Møller et al. 2004).
An
interesting development on that issue is the tentative evidence of a
luminosity-metallicity relation at place at (Ledoux et al. 2006; Møller et al. 2004) (see
also Pontzen et al. 2008; Fynbo et al. 2008). This implies that targeted searches
for the galaxy counterparts of metal rich DLAs could have
substantially higher success rates than for randomly selected DLAs,
but this remains to be confirmed. It
is also consistent with the nondetection of the galaxy counterpart of
the DLA towards Q2138-4427 as this DLA has a relatively low
metallicity of about [Zn/H]=-1.74 (Ledoux et al. 2006). Another very
interesting recent discovery is that of extemely faint, extended LAEs
detected spectroscopically by Rauch et al. (2008) and argued by the same
authors to be a population of galaxies responsible for the bulk of the
DLAs (see also Barnes & Haehnelt 2008). The argument appears very
convincing, and if confirmed by more actual detections of DLA galaxy
counterparts this means that we now have bridged the gap between
absorption and emission selected galaxies at
.
Indepedently of the issue of bridging the gap between absorption and emission
selected galaxies it is clear that there is a very numerous population of
high-z galaxies occupying the faint end of the luminosity function.
There is growing evidence that this population of faint galaxies plays an
important role for many important processes in the early Universe. These
galaxies by far dominate the emission of ultraviolet light
(e.g., Jakobsson et al. 2005; Fynbo et al. 2008) and most likely also the
emission of ionising radiation (Faucher-Giguère et al. 2008; Loeb 2008; Bianchi et al. 2001).
They also most likely contain a large fraction of the total metal budget
in galaxies and are responsible for a large fraction of the
enrichment of the intergalactic medium at
(e.g., Sommer-Larsen & Fynbo 2008). Currently it is extremely difficult
to infer more detailed astrophysical properties (e.g., metallicities,
dust content, stellar populations, masses, etc.) for this class of objects.
In a few cases like DLAs and GRB host galaxies we can infer several of these
properties, but in general we cannot. With the advent of 30 m class telescopes
the future looks more promising.
Acknowledgements
We thank the anonymous referee for useful comments. We are grateful to Dr. M. Ouchi and to M. Rauch for providing us with comparison data. The Dark Cosmology Centre is funded by the Danish National Research Foundation. LFG acknowledges financial support from the Danish Natural Sciences Research Council. M.L. acknowledges the Agence Nationale de la Recherche for its support, project number 06-BLAN-0067
Appendix A: Data for individual LAE candidates
A.1 Spectroscopically confirmed candidates
Table A.1: Properties of the 18 confirmed LAEs in the field of BRI 1202-0725.
Table A.2: Properties of the 18 confirmed LAEs in the field of BRI 1346-0322.
Table A.3: Properties of the 23 confirmed LAEs in the field of Q 2138-4427.
Table A.4:
Properties of the non-confirmed LAE candidates.
This section gives the individual properties for the spectroscopically
confirmed candidates in each of the three survey fields. The
magnitudes in the tables are total magnitudes taken to be the
SExtractor MAG_AUTO (Bertin & Arnouts 1996). From the total narrow-band
magnitude we compute Ly
This section gives the properties of the emission line candidates for
which the nature could not be assessed through the available
spectroscopic data. The properties are measured as detailed in the
previous section. The restframe EWs are given assuming the redshift
corresponding to the central wavelength of the filter.
flux, luminosity and star formation
rates. The EWs are computed based on colour indices computed from the
isophotal magnitudes. The properties are derived as described in
detail in Fynbo et al. (2002). For objects where the measured flux was
below the
level are indicated as lower/upper limits.
A.2 Candidates not confirmed by the available spectroscopy
References
Footnotes
- ... survey
- Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs 67.A-0033, 267.A-5704, 69.A-0380, 70.A-0048, and 072.A-0073.
- ... Paper I
- Note that due to an error in the photometric zeropoints used in that paper the listed broad band detection limits are slightly different from the original ones (by about 0.15 mag). This is not affecting the conclusions of that paper.
All Tables
Table 1:
Log of imaging observations with FORS1 and 2 of the three
surveyed fields. The
detection limits are computed for a
3
diameter aperture. The data for BRI 1346-0322 and
Q 2138-4427 are reproduced from Paper I.
Table 2: Log of spectroscopic observations with FORS2.
Table 3: Overview of LAE samples from the three fields included in the present survey.
Table 4: Summary of the main properties of our and the comparison samples.
Table 5: Results of fitting a Schechter function to the LF.
Table 6: Properties of the serendipitously detected LAEs.
Table A.1: Properties of the 18 confirmed LAEs in the field of BRI 1202-0725.
Table A.2: Properties of the 18 confirmed LAEs in the field of BRI 1346-0322.
Table A.3: Properties of the 23 confirmed LAEs in the field of Q 2138-4427.
Table A.4: Properties of the non-confirmed LAE candidates.
All Figures
![]() |
Figure 1: Transmission curves of the narrow- and broad-band filters used for the observations of BRI 1202-0725. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Left panel: colour-colour diagram for simulated
galaxies based on Bruzual & Charlot (1995) galaxy SEDs
(Bruzual & Charlot 2003). The open squares are 0< z <1.5 galaxies with
ages ranging from a few to 15 Gyr and the open triangles are
1.5<z<3.0 galaxies with ages ranging from a few Myr to 1 Gyr. The
dotted box encloses the simulated galaxy colours. The dashed line
indicates colours of objects with a particular broad-band colour and
with an SED in the narrow-band filter ranging from an absorption line
in the upper right part to a strong emission line in the lower left
part of the diagram. Middle panel: colour-colour diagrams for
all objects detected at S/N>5 in either the narrow band or the Vband in the BRI 1202-0725. In the lower left part of the diagram
there is a large number of objects with excess emission in the narrow
filter. Candidate LAEs are those objects with a 1 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Our equivalent width selection criterion is illustrated by
plotting the line equivalent width against the Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
|
Open with DEXTER | |
In the text |
![]() |
Figure 6: Redshift distribution of LAEs in the field of BRI 1202-0725 relative to the filter transmission curve. The redshifts of LAEs fill out the volume probed by the filter. |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Total magnitude (MAG_AUTO) distribution of the LAEs in
Narrow- ( top) and R-band ( bottom). The curves are averages between
fields and error bars are the corresponding standard deviations. Open
circles and dotted lines mark the distribution for all photometrically
selected candidates, while filled symbols and solid lines mark the
confirmed Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Rest frame equivalent width distribution for the confirmed
(solid symbols and line) and entire (open symbols and dashed line)
samples ( upper panel). In the lower panel the distribution for the
confirmed sample is compared with that of (Gronwall et al. 2007, dot-dashed
line) and (Ouchi et al. 2008, dashed line). The triangles
indicate cases for which only lower limits (at the |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Completeness as function of Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
The derived differential Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 11:
FORS 1 narrow-band image centered on Q 2138-4427 ( left
panel). North is up and East is to the left. The wavelength at peak
transmission of the (He II) narrow-band filter used corresponds
to Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 12:
a) portion of the 2D long-slit spectrum of Q 2138-4427
centered on the Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 13: 2D spectra of probable LAEs serendipitously observed in the FORS 1 long slit spectrum. The redshifts range from z=2.30 to 3.61. The z=2.86 object is LAE2138_12 previously observed also in one of the MOS slitlets (see Paper I). |
Open with DEXTER | |
In the text |
Copyright ESO 2009
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.