Issue |
A&A
Volume 519, September 2010
|
|
---|---|---|
Article Number | L1 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201015424 | |
Published online | 06 September 2010 |
LETTER TO THE EDITOR
[CII] line emission in BRI 1335-0417 at z = 4.4![[*]](/icons/foot_motif.png)
J. Wagg1 - C. L. Carilli2 - D. J. Wilner3 - P. Cox4 - C. De Breuck5 - K. Menten6 - D. A. Riechers7, - F. Walter8
1 - European Southern Observatory, Casilla 19001, Santiago, Chile
2 - National Radio Astronomy Observatory, PO Box 0, Socorro, NM, 87801, USA
3 - Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4 - Institute de Radioastronomie Millimétrique, Saint-Martin-d'Hères, 38406, France
5 - European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching bei München, Germany
6 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 71, 53121 Bonn, Germany
7 - California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
8 - Max-Planck-Institute for Astronomy, Königsstuhl 17, 69117 Heidelberg, Germany
Received 19 July 2010 / Accepted 6 August 2010
Abstract
Using the 12-m APEX telescope, we have detected redshifted emission from the 157.74 m [CII] line in the z=4.4074 quasar BRI 1335-0417. The linewidth and redshift are in good agreement with previous observations of high-J CO line emission. We measure a [CII] line luminosity,
,
making BRI 1335-0417 the most luminous, unlensed [CII] line
emitter known at high-redshift. The [CII]-to-FIR luminosity ratio of
10-4 is
3
higher than expected for an average object with a FIR luminosity
,
if this ratio were to follow the trend observed in other
FIR-bright galaxies that have been detected in [CII] line
emission. These new data suggest that the scatter in the [CII]-to-FIR
luminosity ratio could be larger than previously expected for high
luminosity objects. BR1335-0417 has a similar FIR luminosity and
[CII]/CO luminosity compared to local ULIRGS and appears to be a
gas-rich merger forming stars at a rate of a few thousand solar
masses per year.
Key words: galaxies: high-redshift - galaxies: ISM
1 Introduction
The dominant interstellar medium (ISM) gas cooling line in star-forming galaxies is the 157.74 m fine structure line of [CII]
,
which typically exhibits a luminosity
0.1-1% that of the far-infrared (FIR) luminosity (rest-frame 42.5 to 122.5
m) in quiescent galaxies, and less than 0.1% in ultra-luminous infrared galaxies (ULIRGs; Crawford et al. 1985; Stacey et al. 1991; Wright et al. 1991; Malhotra et al. 1997). The bulk of [CII] line emission arises in warm (
200 K) and dense (
= 3
103 cm-3)
gas associated with photo dissociation regions (PDRs) found at the
surface between neutral and ionized gas clouds. [CII] line
emission therefore provides a powerful means of studying star-formation
and gas kinematics in galaxies.
Although it is not possible to detect the [CII] emission line in nearby
galaxies using ground-based facilities, successful observations of this
line at
have been achieved with submm/mm-wavelength telescopes. The first detection of [CII] line emission in the z=6.4 quasar host galaxy, J1148+5251, was made using the IRAM 30 m telescope (Maiolino et al. 2005),
confirming that the [CII]-to-FIR luminosity ratio is decreasing in such
hyper-luminous infrared galaxies (e.g. Malhotra et al. 2001; Maiolino et al. 2009). Subsequent interferometric imaging with the IRAM Plateau de Bure Interferometer (PdBI) by Walter et al. (2009) showed that [CII] emission traces a large burst of star-formation over physical scales of
1.5 kpc,
a size nearly an order of magnitude larger than that of the
nuclear starbursts observed in nearby galaxies like Arp220. This
emission line has also been detected in other high-redshift quasar host
galaxies, including BR1202-0725 at z = 4.7 with the SMA (Iono et al. 2006), and in the lensed object BRI 0952-0115 at z=4.4 with the Atacama Pathfinder Experiment (APEX) telescope (Maiolino et al. 2009).
Similarly,
strongly lensed luminous infrared starburst galaxies have been detected
in [CII] line emission, including a CSO detection of [CII] in
MIPS J1428+3526 at z=1.3 (Hailey-Dunsheath et al. 2010) and a tentative detection in SMM J2135-0102 at z = 2.3 with Herschel (Ivison et al. 2010).
BRI 1335-0417 is a quasar host galaxy at z = 4.4 whose AGN emits weak Ly,
N V and C IV line emission (Storrie-Lombardi et al. 1996), and has a likely black hole mass of 6
109
(Shields et al. 2006). Thermal dust continuum emission detected at
1.25 mm with the IRAM 30 m (Omont et al. 1996) suggests the presence of a significant quantity of dust (
10
), which shorter wavelength 350
m observations constrain to have a temperature of
43 K (Benford et al. 1999). The implied far-infrared luminosity of 3.1
10
is consistent with a massive burst of star-formation in a heavily obscured system, one which is being fueled by
9
10
of warm molecular gas, as indicated by the presence of CO J = 2-1 and CO J = 5-4 line emission (Guilloteau et al. 1997; Carilli et al. 1999, 2002; Riechers et al. 2008).
In spite of its tremendous apparent luminosity, BRI 1335-0417
does not appear to be gravitationally lensed (Storrie-Lombardi
et al. 1996), but is rather
a massive galaxy in the process of formation via a gas-rich,
or ``wet'' merger (as revealed by spatially resolved
CO imaging by Riechers et al. 2008).
Here, we present APEX observations of [CII] line emission in BRI 1335-0417. We adopt the following
-dominated cosmological parameters: H0 = 71 km s-1 Mpc-1,
,
(Spergel et al. 2007).
2 Observations and data analysis
Observations of [CII] line emission in BRI 1335-0417 were made
with the 12 m APEX telescope in November and December
of 2009. The data were obtained over the course of five nights,
as listed in Table 1,
for a total of 20.3 h of observing time. The weather conditions
were generally dry, with a median precipitable water vapour, PWV
0.5 mm. At a redshift of
z = 4.4074, determined from the CO J = 5-4 emission line observed in BRI 1335-0417 by Guilloteau et al. (1997),
the 1900.539 GHz [CII] line is redshifted to 351.47 GHz,
accessible with the APEX-2 receiver. For this frequency, we assume
a gain of 41 Jy/K in converting Ta* to flux density (Güsten et al. 2006)
and the absolute calibration uncertainties are expected to be 12%.
The fast Fourier-transform spectrometer was set up to sample
1000 km s-1 of bandwidth (two overlapping 1 GHz units), sufficient to cover the [CII] velocity width expected from the CO J = 5-4 line (
420 km s-1; Guilloteau et al. 1997),
assuming that the two species arise within a common region. The
beamsize of these APEX-2 observations is 17.8'', much larger than
the physical extent of the CO line emission (Riechers et al. 2008).
Table 1: [CII] observations of BRI 1335-0417.
Data were analyzed using the GILDAS CLASS software package. Before creating the final, average spectrum, each scan was inspected for large frequency scale spectral baseline irregularities. After removing scans that clearly contained evidence for large scale instabilities, the total on-source integration time of scans contributing to the final spectrum is 214.1 min. Much of the large overhead cost associated with these observations is due to the use of the wobbling subreflector, essential for obtaining stable spectral baselines. Before adding the spectra, a constant, zeroth order baseline was subtracted from the spectrum of each scan, determined by fitting across the off line channels. Such a conservative baseline fitting approach was adopted so as to avoid introducing spurious line emission into the final spectrum. Although there may be some uncertainty in the baseline subtraction, there is sufficient bandwidth for us to remove the continuum emission.
3 Results
![]() |
Figure 1: APEX spectrum of [CII] line emission redshifted to 351.47 GHz where the velocity scale is plotted relative to the CO J = 5-4 redshift z = 4.4074. The spectrum is plotted at a resolution of 58 km s-1 and the long dashed line shows the fitted Gaussian with parameters listed in Table 2. |
Open with DEXTER |
Figure 1 shows the APEX spectrum of [CII] line emission in BRI 1335-0417 where the continuum emission (expected to be 14 mJy; McMahon et al. 1999) is absent due to the baseline subtraction. The rms in the final spectrum is
0.4 mK (16.4 mJy) per 75 km s-1
channel, calculated from the off-line channels away from the edge of
the band. This is consistent with the predicted rms of 0.36 mK,
estimated using the total on-source observing time and a PWV of
0.5 mm.
The [CII] line emission is observed in the central channels of the
spectrum, with a total integrated intensity of 26.6
4.3 Jy km s-1. Following the definition for line luminosity given by Eq. (1) in Solomon et al. (1992), we calculate a [CII] line luminosity of
for BRI 1335-0417. Assuming that a single Gaussian profile is a
good approximation to the [CII] line, we determine the best-fit
parameters presented in Table 2. The line width and central frequency are consistent with that of the previously detected CO J = 5-4 line, measured at z=4.4074 with a linewidth of 420
60 km s-1.
Table 2: [CII] line parameters for BRI 1335-0417.
It has been proposed that the [CII] line luminosity can be used to
estimate the obscured star-formation rate (SFR) in ultra- and
hyper-luminous infrared galaxies such as BRI 1335-0417. Maiolino
et al. (2005) use the calibration between FIR luminosity and SFR given by Kennicutt (1998) to derive a relationship between
and SFR for systems with
.
We note that Boselli et al. (2002) find a lower conversion factor to infer SFR from
.
This is because the calibration of the relationship is based on the
FIR luminosity, and it is observed that higher luminosity systems
exhibit a decreasing [CII]-to-FIR luminosity ratio (Malhotra
et al. 1997; Maiolino et al. 2009). Given the large apparent FIR luminosity in BRI 1335-0417, we adopt Eq. (1) from Maiolino et al. (2005) to estimate a SFR
yr-1 (but see the discussion in Sect. 4.1).
4 Discussion
4.1 [CII] line emission, FIR luminosity, and star-formation activity
Although the star-formation rate inferred from the [CII] line
luminosity is extreme, the applicability of the relationship used
here depends on whether BRI 1335-0417 has a similar
[CII]-to-FIR luminosity ratio as the other ultra- and
hyper-luminous infrared galaxies from which this relationship is
calibrated. The FIR luminosity determined by Benford et al. (1999) is
and implies an obscured star-formation rate of 4650
following Kennicutt (1998),
if all of the FIR luminosity is powered by star-formation
rather than AGN activity. Indeed, the AGN must be in a low
state of activity as the brightness temperature measured from
1.4 GHz VLBI imaging is only
3.5
104 K, and extended over scales of 1-2 kpc (Momjian et al. 2007),
reminiscent of synchrotron emission arising from star-forming galaxies
and not a central AGN. From the [CII] line luminosity
measured in BRI 1335-0417, we calculate,
10-4, which is
3
higher than expected if one adopts the luminosity relationship
determined for the other similarly luminous high-redshift objects
detected in [CII] line emission (Maiolino et al. 2009).
This would suggest that the star-formation rate inferred from the
[CII] line luminosity following the Maiolino et al. (2005) relation is likely an overestimate of the true rate by a similar factor (
3) in the case of BRI 1335-0417.
Given the large scatter in the [CII]-to-FIR luminosity relationship observed in luminous and ultra-luminous infrared galaxies, we do not consider the observed [CII] luminosity in BRI 1335-0417 to be unusual. This ratio has been shown to depend on dust temperature rather than FIR luminosity (Malhotra et al. 2001), which we have not considered for the high-redshift sample. It is possible that AGN heating of the dust plays a significant role here, however both J1148+5251 and BRI 0952-0115 are believed to contain AGN, and they contribute to the calibration of the high luminosity end of the [CII]-to-FIR luminosity relationship. The radio-to-FIR spectral energy distribution of BRI 1335-0417 is very similar to that of the nuclear starburst galaxy M 82 (Carilli et al. 1999), further suggestive of a starburst origin for the FIR luminosity in this object. Sub-arcsecond resolution imaging of both the dust continuum and [CII] line emission would enable us to measure the spatial distribution of these two components of the interstellar medium, and may help determine if the AGN plays a significant role in heating the dust.
4.2 Physical conditions of the gas
Comparing the line luminosities in different molecular and atomic
species can provide useful constraints on the cooling budget and
conditions of the star-forming gas within BRI 1335-0417.
Walter et al. (in prep.) obtain sensitive limits to the
luminosity in the CI(
)
and CI(
) lines emitting from BRI 1335-0417. The upper-limit to the luminosity in the CI(
) transition
indicates that this line has less than 2% of the luminosity in the
[CII] line detected here. This is consistent with observations of
the z=6.4 quasar J1148+5251, for which the CI(
3P2-3P1) line contributes
2% of the ISM gas cooling compared to the [CII] emission line (Maiolino et al. 2005; Walter et al. 2009; Riechers et al. 2009).
We can also compare the [CII] line luminosity to that of the CO J = 2-1 line emission used to infer the total mass of the cold molecular gas (Carilli et al. 1999, 2002; Riechers et al. 2008).
The intensity ratio between the high and low order CO lines
suggests that the CO excitation is similar to that of
BRI 1202-0725, whose gas has a kinetic temperature,
60 K and
a density, 104.1 cm-3 (Riechers et al. 2006, 2008). High resolution imaging of the CO J = 2-1
line emission reveals at least three components, indicative of an
ongoing merger between multiple gas-rich systems (Riechers et al. 2008). If we assume a constant brightness temperature ratio between the J = 2-1 and J = 1-0
CO line transitions, consistent with that observed in other
high-redshift quasar host galaxies (e.g. Riechers et al. 2006), then we can calculate the line lumiosity ratio of the CO J = 1-0 and the [CII] lines for comparison with other galaxies. We calculate a line luminosity ratio,
480.
The [CII]-to-CO line luminosity ratio in BRI 1335-0417 is similar to that of the z=4.7 quasar
host galaxy, BR1202-0725, however the [CII]-to-FIR luminosity
ratio is larger in BRI 1335-0417, as discussed previously.
From these luminosity ratios, models for gas in PDR regions
(e.g. Kaufman et al. 1999) can be used to constrain the gas density (n) and FUV flux (G0, expressed in units of 1.6
10-3 erg cm-2 s-1). The observations of BRI 1335-0417 are consistent with dense gas (n
105 cm-3) illuminated by a FUV flux,
.
These luminosity ratios and inferred physical conditions are similar to
those found in BR1202-0725 as well as local ULIRGs
(e.g. Hailey-Dunsheath et al. 2010). It would therefore
seem that BRI 1335-0417 is indeed an extreme version of a local
starburst ULIRG.
5 Summary
We have used the APEX telescope to observe the 157.74 m atomic cooling line of [CII] in the
z = 4.4074 quasar host galaxy BRI 1335-0417. The line is detected with an apparent luminosity of
,
making it the most intrinsically luminous [CII] line emitter so
far at high-redshift. The high luminosity results in a superior
[CII]-to-FIR luminosity ratio,
10-4,
when compared to other high-redshift galaxies of comparable
luminosities. This is consistent with the picture that
BRI 1335-0417 is an extremely luminous starburst galaxy in the
process of undergoing a ``wet'' merger.
With the future capabilities of ALMA, observations of redshifted
FIR emission lines will be possible for less luminous,
high-redshift star-forming galaxies. For example, detecting the
[CII] line emission from a z=4.5 object with a line strength 100
weaker than that of BRI 1335-0417
would require only
4 h
of observing with the array of 12 m ALMA antennas. Surveys of
[CII] line emission with ALMA would therefore allow us to probe
the dynamics and obscured star-formation in objects whose
star-formation rates were comparable to that of the Milky Way when
the Universe was only 1.4 Gyr old.
We thank the staff of the APEX telescope for help with the observations and data analysis. In particular we thank Thomas Stanke, Andreas Lundgren, Rodrigo Parra, Francisco Montenegro, Giorgio Siringo, and Claudio Agurto. D.R. acknowledges support from from NASA through Hubble Fellowship grant HST-HF-51235.01 awarded by STScI, operated by AURA for NASA, under contract NAS 5-26555. We thank the referee for useful comments and suggestions on the manuscript.
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Footnotes
- ... 4.4
- Based on data collected with the ESO APEX telescope under program ID 084A-0030.
- ...
- Hubble Fellow.
- ... APEX
- APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.
- ... BRI 1335-0417
- A 0.7 mJy peak [CII] line would correspond to, SFR
yr-1, following the Bosselli et al. (2002) calibration.
All Tables
Table 1: [CII] observations of BRI 1335-0417.
Table 2: [CII] line parameters for BRI 1335-0417.
All Figures
![]() |
Figure 1: APEX spectrum of [CII] line emission redshifted to 351.47 GHz where the velocity scale is plotted relative to the CO J = 5-4 redshift z = 4.4074. The spectrum is plotted at a resolution of 58 km s-1 and the long dashed line shows the fitted Gaussian with parameters listed in Table 2. |
Open with DEXTER | |
In the text |
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