Issue |
A&A
Volume 508, Number 2, December III 2009
|
|
---|---|---|
Page(s) | L21 - L25 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200913407 | |
Published online | 19 November 2009 |
A&A 508, L21-L25 (2009)
LETTER TO THE EDITOR
Ly
emitters: blue
dwarfs or supermassive ULIRGs? Evidence for a transition with redshift![[*]](/icons/foot_motif.png)
K. K. Nilsson1 - P. Møller2
1 - ST-ECF, Karl-Schwarzschild-Straße 2, 85748 Garching bei München,
Germany
2 - European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748
Garching bei München, Germany
Received 5 October 2009 / Accepted 16 November 2009
Abstract
The traditional view that Ly emission and dust
should be mutually exclusive
has been questioned more and more often; most notably, the observations
of
Ly
emission
from ULIRGs seem to counter this view. In this
paper we seek to address the reverse question. How large a fraction of
Ly
selected
galaxies are ULIRGs? Using two samples of 24/25 Ly
emitting galaxies at
z = 0.3/2.3, we perform this test, including results
at z = 3.1, and find that, whereas the
ULIRG fraction at z = 3.1 is very small, it
systematically increases
towards lower redshifts. There is a hint that this evolution may be
quite sudden and that it happens around a redshift of
.
After measuring the infrared luminosities of the Ly
emitters,
we find that they are in the normal to ULIRG range in the lower
redshift sample, while the higher redshift galaxies all have
luminosities in the ULIRG category. The Ly
escape fractions for
these infrared bright galaxies are in the range 1-100% in the z
= 0.3 galaxies, but are very low in the z = 2.3
galaxies, 0.4% on average. The unobscured star formation rates are very
high, ranging from 500 to more than 5000
yr-1,
and the dust attenuation derived are in the range
0.0 < AV
< 3.5.
Key words: cosmology: observations - galaxies: high- redshift
1 Introduction
Galaxies at high redshift come in all kinds of flavours. Depending on the search criteria, the sources found may be dusty or dust-free, more or less massive, star forming or quiescent. Some of the most impressive beasts of the high redshift zoo are the sub-mm and ultra-luminous infrared galaxies: SMGs (Ivison et al. 1998, 2000; Blain et al. 2002; Chapman et al. 2005) and ULIRGs (Sanders & Mirabel 1996). These galaxies show star formation rates in the hundreds or thousands of solar masses per year, with most of their light re-processed into the infrared or sub-mm wavelengths due to large amounts of dust. On the other end of the scale, Ly
Only a few publications have reported finding red and/or dusty
Ly emitters
(Stiavelli et al. 2001; Colbert
et al. 2006;
Lai et al. 2007;
Finkelstein et al. 2009;
Nilsson et al. 2009).
Considering the traditional view that even small amounts of dust would
quench any Ly
emission
(for a discussion, see Pritchet 1994), the
detection of the Ly
line
in dusty galaxies would seem surprising, but even more so the
detection of Ly
in
a sample of sub-mm galaxies (Chapman et al. 2003, 2005). Likely
explanations for the existence of dusty galaxies with Ly
emission
are either special geometrical alignments or strong local variations in
the dust-to-gas ratios in a clumpy medium (Neufeld 1991; Hansen
& Oh 2006).
Observational evidence that Ly
emission is commonly seen
among sub-mm galaxies does not, however, constitute evidence for the
opposite,
since sub-mm galaxies are rare and because there are few ways other
than Ly
to
determine the redshift of very red galaxies. It follows
that the overlap reported so far could well be an observational
selection bias.
The fundamental questions that remain are therefore whether i) sub-mm
galaxies and ULIRGs are common among Ly
emitters and
ii) whether the ULIRG fraction evolves with
redshift. This Letter aims to address
those two questions. We here present the infrared properties of two
samples of galaxies found through their Ly
emission at z
= 0.3 and 2.3. We show that they have infrared fluxes
well into the ULIRG regime. This sample is unique, bridging the gap
between low-mass star forming and large star-bursting galaxies.
Throughout this paper, we assume a cosmology with H0=72 km s-1 Mpc-1,
and
.
2 Data
![]() |
Figure 1: Histogram of infrared luminosities of all IR detected sources as derived with Eq. (1). At both redshifts, the hatched area marks genuinely certain LAE galaxies, while the open areas are AGN or unclassified objects. Solid vertical lines mark the LIRG/ULIRG definitions. |
Open with DEXTER |
The Ly candidate
samples studied here come from Deharveng et al. (2008, z=0.3,
hereafter z03) and Nilsson et al. (2009, z=2.3,
hereafter z23). The z03 sample consists of a total of 31 Ly
emitting
galaxies at 0.1 < z
< 0.4 in the ECDF-S and ELAIS-S fields, found in a survey volume
of
Mpc3.
Of the 31, one is observed to be an AGN by Cowie
et al. (2009)
and 11 are not included in their classification. At z
= 2.3, the sample includes 187 Ly
emitting candidates
at redshifts 2.21< z
<2.31, and are spread over a
0.2 deg2
area in the central COSMOS field (the survey volume is
Mpc3).
Of the 187 candidates, 27 are considered AGN, based
on detections in public Chandra and/or XMM X-ray
images. The ECDF-S, ELAIS-S and COSMOS fields are covered by the SWIRE
(Lonsdale et al. 2003) and
S-COSMOS (Sanders et al. 2007) Spitzer
surveys. The photometry of all the z23 candidates, in all Spitzer
bands from
m, will be
presented in a forthcoming publication (Nilsson et al., in
prep.). Here we focus on the results found for the photometry in the
m and
m (MIPS)
bands for the z03 and z23 samples, respectively, corresponding
to
m in the
restframe of both samples.
For the z03 emitters, aperture photometry centred on the
Ly coordinates
was performed. The aperture radius was 4.5'',
selected to be consistent with the z23 results. The limiting
sensitivity is a few times ten
Jy. Of the 31 candidates, 24 are
detected at >
,
of which
Cowie et al. (2009)
classified one as an AGN and 17 as galaxies.
In COSMOS, both a deep and a wide survey has been published in
the MIPS band. The deep survey covers 33% of the field surveyed for
Ly
,
and the sensitivities reached in the two surveys were 71 and 150
Jy,
respectively (5
).
To find counterparts to the Ly
emitters, the public
catalogues were searched within 4 pixels (4.8'', i.e.
FWHM
of the MIPS PSF) radii of each source and 25 counterparts were found.
Of these, 16 are also X-ray detected, and are considered to be AGN
based on their R band-to-X-ray flux
ratios. The
fluxes of the galaxies in the 8/24
m bands are found in
Table 1.
3 Infrared
properties of Ly
emitters
![]() |
Figure 2:
IR to Ly |
Open with DEXTER |
At z = 2.25, the MIPS m band corresponds to
restframe 5.9-8.8
m.
Correspondingly, the
m
IRAC band covers 4-8.8
m for the z03 emitters. To convert this
mid-infrared luminosity to the total infrared luminosity, we use the
conversion of Chary & Elbaz (2001):
The values derived can be found in Table 1, and a histogram of the total infrared luminosities is shown in Fig. 1. At each redshift, the infrared luminosities for the full samples (25 and 24, respectively), as well as the subsamples with certain identifications, are shown. It is seen that the flux limits at the two redshifts cause the overlap between the high and z03 sample to be very small. All of the z23 sources are consistent with ULIRG luminosities (





Table 1: IR fluxes, star formation rates and derived dust attenuation.
In Fig. 2
the Ly luminosities
are shown as a function of the infrared luminosities of the galaxies.
Here and in the following analysis, we have chosen to be
conservative and have excluded all AGN from the samples.
In Sect. 4.1
we return to the question of AGN and
test how robust the results are against AGN inclusion.
In Fig. 2
the z23 LAE candidates with MIPS detections seem to follow a
given
trend between the two flux measurements. The best-fit ratio between
Ly
and
infrared luminosity is
0.02%.
In the z03 sample, the Ly
luminosity
interestingly stays constant as a function of infrared luminosity.
This indicates that the physical processes governing the Ly
and
the IR luminosities are not related at low Ly
and/or IR
luminosities, although the relation seen at bright luminosities is
based on small number statistics. As the blue points in this
case (sources with no previous identification as either galaxy or AGN)
are mixed in the population of galaxy LAEs based on Ly
luminosity,
these are hereafter considered as normal LAE candidates.
![]() |
Figure 3:
Star formation rate ratios between Ly |
Open with DEXTER |
The bolometric luminosity can be calculated from the infrared and
ultraviolet luminosity according to
where
![]() |
(3) |
This can further be converted to a dust unobscured star formation rate, assuming that the bolometric luminosity includes all the re-processed light from star forming regions (Kennicutt 1998):
The star formation rates found from the bolometric luminosity are in the range










In Meurer et al. (1999), a
relation between the ratio of infrared to ultraviolet flux and the dust
attenuation A1600 was
derived:
The spread in A1600 for the galaxies here is large. The attenuation in both samples reaches as high as eight magnitudes in A1600. In the z03 sample the attenuation spreads down to one magnitude, whereas the z23 galaxies all have A1600 > 3. The distribution is flat in both samples. A plot of the SFR ratios against the dust attenuation derived according to this equation is found in Fig. 3. The SFR ratios, which are proxies of the Ly


4 ULIRG fraction and redshift evolution
The finding that Ly emitting
galaxies can also be very IR-bright, hence dusty, is not a novel
result. Chapman et al. (2003, 2005) showed
that a set of sub-mm galaxies had Ly
in emission. Very
red LAEs have also been found in other surveys. At
,
Nilsson et al. (2007; N07,
erg s-1)
found one red LAE in a sample of 24 and Lai et al. (2008; L08,
erg s-1)
presented four red LAEs among 162, also at
.
To determine the fraction of ULIRGs in the
surveys, we studied the ULIRG template SEDs of Vega
et al. (2008)
and find that z=3.1 ULIRGs must be detected in the Spitzer
IRAC bands to the flux limits of ECDF-S. Furthermore, the colours in
the IRAC bands have to be red;
.
In the N07 sample, only one object is detected in the IRAC bands, with R-Ch4
= 6.5, resulting in one ULIRG in this sample (1/24). In the L08 sample,
18 galaxies are detected in the IRAC bands. However, the colours of
these objects, including the red objects, are all Ch1-Ch4
< 2 (their Fig. 2). As a result, no ULIRGs are found in their
survey (0/162). Combining these two samples, the number of ULIRGs at
are 1 out of 186, resulting in a ULIRG fraction of
0.5+1.3-0.5%, where the
error bars are based on Poisson statistics. Finally, Colbert
et al. (2006)
found that three out of 22
LAEs (
14+13-7%,
excluding their detections of ULIRGs in Ly
blobs
, see also Francis
et al. 2001)
were bright in the observed infrared, and that they were ULIRGs. The
percentage of ULIRGs in the z = 2.3 sample
presented here is 14+8-5%,
if only the objects in the deep survey are considered (7/50). This
number is a lower estimate, as the sensitivity of the deep COSMOS MIPS
data only reaches
at z = 2.3. In the z = 0.3
sample, the results are complete in infrared luminosity, and the
percentage of ULIRGs in the sample is
20+12-8% (6/30), where we
again have included 6 unclassified objects but not the AGN. We
return to the robustness also of this result against AGN
inclusion in Sect. 4.1
below.
![]() |
Figure 4:
ULIRG fraction in Ly |
Open with DEXTER |
In Fig. 4
the ULIRG fractions are plotted as a function of redshift.
The data points are few, but there is a clear trend for the ULIRG
fraction to grow from high redshifts up to the present day. There
even seems to be a hint that the transition from almost zero ULIRG
fraction to the current value is rather sudden. For the sole
purpose of illustrating the time and steepness of the transition, we
overplot in Fig. 4
a hyperbolic tangents function of
the form:
In this equation, UF is the ULIRG fraction,






4.1 Robustness of the result
For the definition of the Ly-selected ULIRG subsamples we
used the common definition based on
.
IR-selected ULIRG
samples are known to have a significant (15-50%) fraction of
galaxies with AGN components, but it is also known that
their total luminosity is often dominated by star formation rather
than by the AGNs (Veilleux et al. 2009).
Nevertheless, to make sure that our
Ly
selection
did not bias the results, we chose the most
conservative approach and therefore removed all X-ray detected objects
from
the samples. We now ask the question of whether a less conservative
approach is possible, and if that would change the conclusions.
We therefore searched for an additional test to certify that
a candidate is a ULIRG. Using the ULIRG template SEDs of Vega
et al. (2008),
we find that
a useful index at z=2.3 is optical R
band minus Ch4 of IRAC. All
Vega templates have R-Ch4>2.7,
see Fig. 5.
The points in the plot show all the LAE candidates at z=2.3
from Nilsson et al. (2009) with
>
detections in R and Ch4. The
black points are those detected with MIPS, whereas the green points are
not MIPS-detected. X-ray detected objects are marked with a pink ring.
Four ULIRGs are not detected in Ch4 and are shown
with upper limits. It can be seen from this plot that the flux
selection with MIPS very cleanly samples the region of colour space
where ULIRGs are expected to be, except the very brightest AGN that are
too blue for this selection. Selecting ULIRGs based on the infrared
flux and the R-Ch4 colour, and
including all objects that have the correct colours within
,
changes the z=2.3 sample by increasing the number
of ULIRGs from seven of a total 50 LAE candidates in the first
selection to eight of a total 60 candidates in the second selection.
The new fraction is 13+7-5%,
in perfect agreement with the initial result.
![]() |
Figure 5:
R-Ch4 colours for the LAEs
at z=2.3 with at least |
Open with DEXTER |
The ULIRG fraction at z=0.3 was obtained using the
Deharveng et al. (2008) sample
but excluding the AGN reported by
Cowie et al. (2009).
If the AGN are included, the ULIRG fraction drops to
19+12-8% rather than the
previous 20+12-8%,
while
ignoring the Deharveng et al. (2008) sample.
Using only the sample
of Cowie et al. (2009) gives a
ULIRG fraction of 12+16-4%.
All of those results are
identical to within
errors, so the z=0.3 result
is also robust against details of the sample selection.
5 Conclusion
We have here presented the infrared properties of two samples of Ly

- i)
- At all redshifts below z = 3, a
non-zero fraction of LAEs are found to be ULIRGs. This
result directly contradicts the classic view that dust and Ly
emission are mutually exclusive, and it holds together with the finding that Ly
emission at high redshifts correlate with metallicity (Møller et al. 2004) makes the case that we must re-think the importance of dust for the Ly
escape fraction.
- ii)
- There is evidence for a strong evolution in ULIRG fraction
from redshift 3.1 to the present
universe. The fractions derived have been shown to be very robust
against different selection criteria. This explains why there was
severe disagreement about the colour of Ly
galaxies originally. At redshifts around three they were reported to be young, blue, low-dust starbursts (Warren & Møller 1996; Fynbo et al. 2000), while at redshifts closer to two they were reported to be red/dusty (Stiavelli et al. 2001).
- iii)
- There may be evidence that the evolution of the ULIRG fraction is not linear in time, but rather that there is a jump from redshift three to two, and then a more gradual evolution to the present time. Even though the evidence is merely suggestive at present, this jump does coincide with the number density evolution of quasars and suggests a connection between those classes of objects. This question bears strongly on the connection between starbursts and the formation of quasars, so it should be investigated more thoroughly via dedicated surveys of LAEs at several different redshifts in the range z = 2 to z = 3.5. We provide a simple formalism in the form of a three parameter function which is well suited to future tests of how sharp the transition is.







The authors wish to thank Daniel Schaerer and Anne Verhamme for useful discussions. We would also like to thank the anonymous referee for useful comments that have significantly improved the presentation of our results.
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Online Material
Table 1: IR fluxes, star formation rates and derived dust attenuation.
Footnotes
- ... redshift
- Full Table 1 is only available in electronic form at http://www.aanda.org
- ...
blobs
- It is unclear what mechanisms power Ly
blobs, and if they are a distinct population of objects or simply the tail of the size distribution of LAEs. Due to these uncertainties, we chose a conservative selection and disregard these sources here.
All Tables
Table 1:
IR fluxes, star formation rates and derived dust attenuation.
Full Version
Table 1: IR fluxes, star formation rates and derived dust attenuation.
All Figures
![]() |
Figure 1: Histogram of infrared luminosities of all IR detected sources as derived with Eq. (1). At both redshifts, the hatched area marks genuinely certain LAE galaxies, while the open areas are AGN or unclassified objects. Solid vertical lines mark the LIRG/ULIRG definitions. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
IR to Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Star formation rate ratios between Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
ULIRG fraction in Ly |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
R-Ch4 colours for the LAEs
at z=2.3 with at least |
Open with DEXTER | |
In the text |
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