PAH emission and star formation in the host of the z~2.56 Cloverleaf QSO
Accepted for publication in ApJL
PAH emission and star formation in the host of the z∼2.56
Cloverleaf QSO
D. Lutz1, E. Sturm1, L.J. Tacconi1, E. Valiante1, M. Schweitzer1 H. Netzer2, R. Maiolino3,
P. Andreani4, O. Shemmer5, S. Veilleux6
ABSTRACT
We report the first detection of the 6.2µm and 7.7µm infrared ‘PAH’ emis-
sion features in the spectrum of a high redshift QSO, from the Spitzer-IRS spec-
trum of the Cloverleaf lensed QSO (H1413+117, z∼2.56). The ratio of PAH
features and rest frame far-infrared emission is the same as in lower luminosity
star forming ultraluminous infrared galaxies and in local PG QSOs, supporting
1Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
lutz@mpe.mpg.de, sturm@mpe.mpg.de, linda@mpe.mpg.de, valiante@mpe.mpg.de,
schweitzer@mpe.mpg.de
2School of Physics and Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty
of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel
netzer@wise1.tau.ac.il
3INAF, Osservatorio Astronomico di Roma, via di Frascati 13, 00040 Monte Porzio Catone, Italy
maiolino@ao-roma.inaf.it
4ESO, Karl-Schwarzschildstraße 2, 85748 Garching, Germany
pandrean@eso.org
5Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University,
University Park, PA 16802, USA
ohad@astro.psu.edu
6Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
veilleux@astro.umd.edu
http://arxiv.org/abs/0704.0133v1
– 2 –
a predominantly starburst nature of the Cloverleaf’s huge far-infrared luminosity
(5.4 × 1012L⊙, corrected for lensing). The Cloverleaf’s period of dominant QSO
activity (LBol ∼ 7 × 10
13L⊙) is coincident with an intense (star formation rate
∼ 1000M⊙yr
−1) and short (gas exhaustion time ∼ 3× 107yr) star forming event.
Subject headings: galaxies: active, galaxies: starburst, infrared: galaxies
1. Introduction
Redshifts ∼2.5 witness both the ‘quasar epoch’ with peak number density of luminous
accreting black holes (e.g. Schmidt et al. 1995) and the peak in number of the most intense
star forming events as traced by the submillimeter galaxy population (Chapman et al. 2005),
suggestive of a relation of the two phenomena. Detailed evolutionary connections between
massive starbursts and QSOs have been discussed for many years (e.g. Sanders et al. 1988;
Norman & Scoville 1988) and form an integral part of some recent models of galaxy and
merger evolution (e.g. Granato et al. 2004; Springel et al. 2005; Hopkins et al. 2006). Phases
of intense star formation coincident with the active phase of the quasars are a natural
postulate of such models but have been exceedingly difficult to confirm and quantify due to
the effects of the powerful AGN outshining tracers of star formation at most wavelengths.
Perhaps the strongest constraint on the potential significance of star formation in QSOs
comes from the far-infrared part of their spectral energy distribution (SED). Indeed, this far-
infrared emission has been interpreted as due to star formation (e.g. Rowan-Robinson 1995),
but alternative models successfully ascribe it to AGN heated dust, by postulating a dust
distribution in which relatively cold dust at large distance from the AGN has a significant
covering factor, for example in a warped disk configuration (Sanders et al. 1989). Additional
diagnostics are needed to break this degeneracy.
CO surveys of local QSOs (e.g. Evans et al. 2001; Scoville et al. 2003; Evans et al. 2006)
have produced a significant number of detections of molecular gas reservoirs that might power
star formation. Depending on the adopted ‘star formation efficiency’ SFE=LFIR/LCO the
detected gas masses may be sufficient or not for ascribing the QSO far-infrared emission to
star formation. Optical studies have identified significant ‘post-starburst’ stellar populations
in QSOs (Canalizo & Stockton 2001; Kauffmann et al. 2003). On the other hand, Ho (2005)
suggested low star formation in QSOs, perhaps actively inhibited by the AGN, on the basis
of observations of the [OII] 3727Å line. We have used the much less extinction sensitive mid-
infrared PAH emission features to infer that in a sample of local (PG) QSOs, star formation
is sufficient to power the observed far-infrared emission (Schweitzer et al. 2006).
– 3 –
The observational situation remains complex for high redshift QSOs. Metallicity studies
of the broad-line region suggest significant enrichment by star formation (e.g. Hamann & Ferland
1999; Shemmer et al. 2004) but may not be representative for the host as a whole. Submm
and mm studies of luminous radio quiet QSOs have produced significant individual detec-
tions of dust emission of some QSOs, as well as statistical detection of the entire popula-
tion (e.g. Omont et al. 2003; Priddey et al. 2003; Barvainis & Ivison 2002). These suggest
potential starburst luminosities up to and exceeding 1013L⊙. CO studies have detected
large gas reservoirs in many high-z QSOs (see summaries in Solomon & Vanden Bout 2005;
Greve et al. 2005). Emission from high density molecular gas tracers has been detected in
some of the brightest systems (Barvainis et al. 1997; Solomon et al. 2003; Carilli et al. 2005;
Riechers et al. 2006; Garćıa-Burillo et al. 2006; Guélin et al. 2007) and may well originate in
dense high pressure star forming regions, but AGN effects on chemistry and molecular line
excitation could also play a role (e.g. Maloney et al. 1996). Finally, the [CII] 157µm rest
wavelength fine structure line was detected in the z=6.42 quasar SDSS J114816.64+525150.3
(Maiolino et al. 2005) at a ratio to the rest frame far-infrared emission similar to the ratio
in local ULIRGs, consistent with massive star formation.
We have initiated a program extending the use of mid-infrared PAH emission as star
formation tracer to high redshift QSOs. In this Letter, we use Spitzer mid-infrared spectra to
detect and quantify star formation in one of the brightest and best studied z∼2.5 QSOs, the
lensed Cloverleaf (H1413+117, Hazard et al. 1984; Magain et al. 1988). We adopt Ωm = 0.3,
ΩΛ = 0.7 and H0 = 70 kms
−1 Mpc−1.
2. Observations and Results
We obtained low resolution (R∼ 60 − 120) mid-infrared spectra of the Cloverleaf QSO
using the Spitzer infrared spectrograph IRS (Houck et al. 2004) in staring mode on July 24,
2006, at J2000 target position RA 14h15m46.27s, DEC +11d29m43.40s. The IRS aperture
includes all lensed images. 30 cycles of 120sec integration time per nod position were taken
in the LL1 (19.5 to 38.0 µm) and 15 cycles in the LL2 (14.0 to 21.3 µm) module, leading to
effective on-source integration times of 2 and 1 hours, respectively. We use the pipeline 14.4.0
processed basic calibrated data, own deglitching and coaddition procedures, and SMART
(Higdon et al. 2004) for extraction. When combining the two orders into the final spectrum,
we scaled the LL2 spectrum by a factor 1.02 for best match in the overlapping region.
Fig. 1 shows the IRS spectrum embedded into the infrared to radio SED of the Clover-
leaf, and Fig. 2 the IRS spectrum proper, together with the location of key features in the
corresponding rest wavelength range. The rest frame mid-infrared emission is dominated by
– 4 –
a strong continuum, approximately flat in νFν , due to dust heated by the powerful active
nucleus to temperatures well above those reached in star forming regions. Superposed on
this continuum are emission features, which we identify with the 6.2µm and 7.7µm aromatic
‘PAH’ emission features normally detected in star-forming galaxies over a very wide range
of properties. As expected for a Type 1 AGN, there are no indications for the ice (6µm)
or silicate (9.6µm) absorptions seen in heavily obscured galaxies. None of the well-known
emission lines in this wavelength range is bright enough to be significantly detected in this
low resolution spectrum, although we cannot exclude a contribution of [NeVI] 7.64µm to
the 7.7µm feature. Adopting standard mid-infrared low resolution diagnostics (Genzel et al.
1998; Laurent et al. 2000), the weak PAH features on top of a strong continuum agree with
the notion that the Cloverleaf is energetically dominated by its AGN. The detection of PAH
features with several mJy peak flux density in a z∼2.6 galaxy, however, implies intense star
formation, which we discuss in conjunction with other properties of the Cloverleaf.
By fitting a Lorentzian superposed on a local polynomial continuum, we measure a
flux of 1.52×10−21Wcm−2 for the 6.2µm feature, with a S/N of 6. The 7.7µm feature is
more difficult to quantify. Schweitzer et al. (2006) have discussed PAHs as star formation
indicators in local (PG) QSOs, PAHs are also detected in the average QSO spectrum of
Hao et al. (2007). The AGN continuum of those QSOs shows superposed silicate emission
features at & 9µm (see also Siebenmorgen et al. 2005; Hao et al. 2005). If PAH emission
is additionally present, the PAH features partly ‘fill in’ the minimum in the AGN emission
before the onset of the silicate feature (see Fig. 2 of Schweitzer et al. (2006)), causing a
seemingly flat overall spectrum. In reality, there is simultaneous presence of AGN continuum,
the 6.2-8.6µmPAH complex, silicate emission, and more PAH emission at longer wavelengths.
From inspection of Fig. 2, a similar co-presence of PAH and silicate emission is observed for
the Cloverleaf. We note that the presence of silicate emission in the luminous Cloverleaf
Type 1 QSO, other high z Type 1 QSOs (Maiolino et al., in prep) as well as in luminous
Type 2 QSOs (Sturm et al. 2006; Teplitz et al. 2006) has implications for the location of
this cool silicate component (∼200K, Hao et al. 2005) in unified AGN schemes. We leave
further discussion of the properties of silicates to a future paper. For the 7.7µm feature of
the Cloverleaf, we adopt a flux of 6.1×10−21Wcm−2. This flux was determined by scaling a
PAH template (ISO spectrum of M82, Sturm et al. 2000) to the measured Cloverleaf 6.2µm
feature flux, and then fitting three lorentzians to represent the 6.2, 7.7, and 8.6 features plus
a local polynomial continuum to this template. Similar Lorentzian fits were also used for
local comparison objects discussed below. Brandl et al. (2006) have quantified the scatter
of the 6.2 to 7.7µm flux ratio in starbursts, with 0.07 in the log this scatter indicates the
modest uncertainty induced by tying the longer wavelength features to the 6.2µm one. The
result of subtracting the scaled M82 template from the Cloverleaf spectrum is indicated
– 5 –
in Fig. 2, and shows a combination of continuum and silicate emission very similar to local
QSOs. Directly measuring the 7.7µm flux by fitting a single Lorentzian plus local continuum
to the Cloverleaf spectrum gives a ∼40% lower feature flux, which would be a systematic
underestimate because of the complexity of the underlying continuum/silicates discussed
above.
3. Intense star formation in the host of the Cloverleaf QSO
The Cloverleaf SED (Fig. 1) shows strong rest frame far-infrared emission in addition
to the AGN heated dust emitting in the rest frame mid-infrared. Weiß et al. (2003) decom-
posed the SED into two modified blackbodies of temperature 50 and 115K, the rest frame
far-infrared (40-120µm) luminosity of 5.4 × 1012L⊙ is dominated by the colder component
and could largely originate in star formation. Comparison of PAH and far-infrared emission
can shed new light on this question. The bolometric (rather than rest-frame far-infrared)
luminosity of the Cloverleaf will still be dominated by the AGN. We estimate LBol extrap-
olating from the observed rest frame 6µm continuum which for a mid-infrared spectrum
with weak PAH but strong continuum will be AGN dominated (Laurent et al. 2000). Using
LBol ∼ 10× νLν(6µm) based on an Elvis et al. (1994) radio-quiet QSO SED, the AGN lu-
minosity is ∼ 7 × 1013L⊙. A similar estimate ∼ 5 × 10
13L⊙ is obtained from the rest frame
optical (observed near-infrared; Barvainis et al. 1995) continuum, tracing the AGN ionizing
continuum, and the same global SED.
Schweitzer et al. (2006) have measured PAH emission in local QSOs and compared the
PAH to far-infrared emission ratio to that for starbursting ULIRGs, i.e. those among a larger
ULIRG sample not showing evidence for dominant AGN and not having absorption domi-
nated mid-infrared spectra. Fig. 3 places the Cloverleaf on their relation between 7.7µm PAH
luminosity and far-infrared luminosity. L(PAH)/L(FIR) is 0.014 for the Cloverleaf, very close
to the mean value for the 12 starburst-dominated ULIRGs of < L(PAH)/L(FIR) >= 0.0130.
The scatter of this relation is 0.2 in the log for these 12 comparison ULIRGs, indicating the
minimum uncertainty of extrapolating from the PAH to far-infrared emission. The Clover-
leaf thus extends the relation between PAH and far-infrared luminosity for the local QSOs
and ULIRGs to ∼5 times larger luminosities. Its PAH emission is consistent with an ex-
tremely luminous starburst of ULIRG-like physical conditions powering essentially all of the
rest frame far-infrared emission.
Teplitz et al. (2006) present the IRS spectrum of the lensed FIR-bright Type 2 AGN
IRAS F10214+4724 at similar redshift. They report a marginal feature at 6.2µm rest wave-
length which they do not interpret as PAH given the lack of a 7.7µm maximum. Given
– 6 –
the strength of silicate emission in this target, PAH emission may be present in the blue
wing of the silicate feature without producing a maximum, and such a component may be
suggested by comparing their Fig. 1 with the later onset of silicate emission in the spectra
of local QSOs. The tentative 6.2µm peak in IRAS F10214+4724 has similar peak height as
the Cloverleaf PAH feature, in line with our interpretation and the similar rest frame FIR
fluxes of the two objects.
With ∼5-10% of its total luminosity originating in the rest frame far-infrared and by star
formation, the Cloverleaf is within the range of local QSOs, and not a pronounced infrared
excess object. Specifically, its ratio of FIR to total luminosity and the ratio of rest frame far-
infrared (60µm) to mid-infrared (6µm) continuum are about twice those of the Elvis et al.
(1994) radio-quiet QSO SED. Adopting the conclusion of Schweitzer et al. (2006) that star
formation already dominates the FIR emission of local PG QSOs and considering the modest
FIR ‘excess’ of the Cloverleaf compared to the Elvis et al. (1994) SED then suggests only
a small AGN contribution to its FIR luminosity. Other z∼2 QSOs may have lower ratios
of FIR and total luminosity, and conversely larger AGN contributions to their more modest
FIR emission, though. After correcting for lensing, the Cloverleaf submm flux is a factor ∼2
above the typical bright z∼2 QSOs of Priddey et al. (2003) whose rest frame B magnitudes
in addition are typically brighter than the delensed Cloverleaf.
Submillimeter galaxies host starbursts of similar luminosity as the Cloverleaf, at similar
redshift. Lutz et al. (2005) and Valiante et al. (2007) have obtained IRS mid-infrared spec-
tra of 13 SMGs with median redshift 2.8, finding mostly starburst dominated systems. A
comparison can be made between PAH peak flux density and flux density at rest wavelength
222µm which is obtained with minimal extrapolation from observed SCUBA 850µm fluxes.
Combining the 7.7µm feature peak of 5.1mJy (Fig. 2) with a ν3.5 extrapolation of the SCUBA
flux of Barvainis & Ivison (2002) places the Cloverleaf at Log(SPAH7.7/S222µm) ∼ −1.2, near
the center of the distribution of this quantity for the SMGs of Valiante et al. (2007, their
Fig.4). Like the SMGs, the Cloverleaf appears to host a scaled up ULIRG-like starburst,
but with superposition of a much more powerful AGN, also in comparison to the gas mass.
Tracers of high density gas, in particular HCN but also HCO+ have been detected in a
few high redshift QSOs including the Cloverleaf (Barvainis et al. 1997; Solomon et al. 2003;
Riechers et al. 2006). Their ratio to far-infrared emission is similar to the one for Galactic
dense star forming regions, and has been used to argue for dense, high pressure star forming
regions dominating the far-infrared luminosity of these QSOs as well as of local ULIRGs (e.g.
Solomon et al. 2003). Intense HCN emission is observed also from X-ray dominated regions
close to AGN (e.g. Tacconi et al. 1994), and there is ongoing debate as to the possible
contributions of chemistry and excitation in X-ray dominated regions, and other effects like
– 7 –
radiative pumping, to the emission of dense molecular gas tracers in ULIRGs and QSOs
(Kohno 2005; Imanishi et al. 2006; Graćıa-Carpio et al. 2006). Unlike HCN, PAH emission
is severely reduced in X-ray dominated regions close to AGN (Voit 1992) and provides an
independent check of the effects of the AGN on the molecular gas versus the role of the host
and its star formation. In a scenario where XDRs dominate the strong HCN emission and
the hosts PAH emission, reproducing the consistent ratios of these quantities to rest-frame
far-infrared over a wide range of far-infrared luminosities would thus require a considerable
amount of finetuning. In contrast, these consistent ratios are a natural implication if all
these components are dominated by ULIRG-like dense star formation.
Our detection of PAH emission is strong support to a scenario in which the Cloverleaf
QSO coexists with intense star formation. Applying the Kennicutt (1998) conversion from
infrared luminosity to star formation rate to LFIR = 5.4× 10
12L⊙ suggests a star formation
rate close to 1000 M⊙yr
−1, which can be maintained for a gas exhaustion timescale of only
3×107yr, for the molecular gas mass inferred by Weiß et al. (2003). At this time resolution,
the period of QSO activity coincides with what likely is the most significant star forming
event in the history of the Cloverleaf host.
This work is based on observations made with the Spitzer Space Telescope, which is
operated by the Jet Propulsion Laboratory, California Institute of Technology, under a con-
tract with NASA. Support for this work was provided by NASA under contracts 1287653
and 1287740 (S.V.,O.S.). We thank the referee for helpful comments.
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– 11 –
Table 1. Cloverleaf properties
Quantity Value Reference
Redshift z 2.55784 Weiß et al. (2003)
Amplification µL 11 Venturini & Solomon (2003)
F(PAH 6.2µm) 1.5× 10−21Wcm−2 this work
F(PAH 7.7µm) 6.1× 10−21Wcm−2 this work
L(PAH 7.7µm)a 7.6× 1010L⊙ this work
L(40-120µm)a 5.4× 1012L⊙ Weiß et al. (2003)
M(H2)
a 3.0× 1010M⊙ Weiß et al. (2003)
LBol(QSO)
∼ 7× 1013L⊙ this work, 10× νLν(6µm)
aCorrected for lensing amplification 11 and to our adopted cosmology Ωm = 0.3, ΩΛ = 0.7
and H0 = 70 kms
−1 Mpc−1 (DL=20.96 Gpc).
– 12 –
Fig. 1.— Infrared to radio spectral energy distribution for the Cloverleaf QSO. The
IRS spectrum (continuous line) is supplemented by photometric data from the literature
(Barvainis et al. 1995; Alloin et al. 1997; Barvainis & Lonsdale 1997; Hughes et al. 1997;
Benford 1999; Rowan-Robinson 2000; Solomon et al. 2003; Weiß et al. 2003). The ISOCAM-
CVF spectrum of Aussel et al. (1998) is indicated by the short dotted line. The ISO 12µm
flux appears too high while the other mid-infrared data are consistent within plausible cali-
bration uncertainties.
– 13 –
Fig. 2.— IRS spectrum of the Cloverleaf QSO. The PAH emission features as well as the
expected locations of strong spectral lines in this wavelength range are marked. The dotted
line shows the spectrum after the subtraction of a PAH template (spectrum of M82, see also
bottom of figure), redshifted and scaled to the measured strength of the Cloverleaf 6.2µm
PAH feature. Note that the noise in IRS low resolution spectra increases strongly from
∼33µm towards the long wavelength end.
– 14 –
Fig. 3.— Relation of 7.7µm PAH luminosity and rest frame FIR luminosity for the Cloverleaf
and for local PG QSOs and starbursting ULIRGs from Schweitzer et al. (2006).
	Introduction
	Observations and Results
	Intense star formation in the host of the Cloverleaf QSO
