A&A 467, 955-969 (2007)
A. Weiß1 - D. Downes2 - R. Neri2 - F. Walter3 - C. Henkel1 - D. J. Wilner4 - J. Wagg4,5 - T. Wiklind6
1 - MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
2 - IRAM, Domaine Universitaire, 38406 St-Martin-d'Hères, France
3 - MPIA, Königstuhl 17, 69117 Heidelberg, Germany
4 - Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138, USA
5 - Instituto Nacional de Astrofisica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, Puebla, Mexico
6 - ESA-Space Telescope Division, STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
Received 26 July 2006 / Accepted 19 February 2007
We report the detection of the CO 4-3, 6-5, 9-8, 10-9, and 11-10 lines in the Broad Absorption Line quasar APM 08279+5255 at z=3.9 using the IRAM 30 m telescope. We also present IRAM PdBI high spatial resolution observations of the CO 4-3 and 9-8 lines, and of the 1.4 mm dust radiation as well as an improved spectrum of the HCN(5-4) line. Unlike CO in other QSO host galaxies, the CO line SED of APM 08279+5255 rises up to the CO(10-9) transition. The line fluxes in the CO ladder and the dust continuum fluxes are best fit by a two component model, a "cold'' component at 65 K with a high density of n(H2) = cm-3, and a "warm'', 220 K component with a density of cm-3. We show that IR pumping via the 14 m bending mode of HCN is the most likely channel for the HCN excitation. From our models we find, that the CO(1-0) emission is dominated by the dense gas component which implies that the CO conversion factor is higher than usually assumed for high-z galaxies with . Using brightness temperature arguments, the results from our high-resolution mapping, and lens models from the literature, we argue that the molecular lines and the dust continuum emission arise from a very compact ( pc), highly gravitationally magnified ( m= 60-110) region surrounding the central AGN. Part of the difference relative to other high-z QSOs may therefore be due to the configuration of the gravitational lens, which gives us a high-magnification zoom right into the central 200-pc radius of APM 08279+5255 where IR pumping plays a significant role for the excitation of the molecular lines.
Key words: galaxies: formation - galaxies: high-redshift - ISM: molecules - galaxies: individual: APM 08279+5255 - cosmology: observations - galaxies: quasars: emission lines
The existence of massive reservoirs of molecular gas at high redshifts has now been established in quasars, submillimeter galaxies and radio galaxies out to the highest redshifts (see review by Solomon & Vanden Bout 2005, and references therein). It is now of interest to study in detail the excitation conditions of the molecular gas in these high-redshift systems to search for differences of the gas properties among high-z sources and to relate them to the properties of their host galaxies. One way to do this is by studying multiple CO transitions from individual key sources (hereafter referred to as "CO line spectral energy distributions (SEDs)''). We have recently reported observations for the gravitationally lensed galaxy SMM J16359+6612 (Weiss etal. 2005a). Here we focus on new observations of one of the brightest quasars at high redshifts, the broad absorption line (BAL) quasar APM 08279+5255 at z=3.9 (Irwin etal. 1998).
|Figure 1: IRAM 30 m telescope spectra of CO 4-3, 6-5, 9-8, 10-9, 11-10 and the average CO spectrum from APM 08279+5255, with Gaussian fit profiles superimposed. Velocity scales are relative to a redshift of z=3.911. The velocity resolution for individual spectra is 50 kms-1. For the average spectrum, the velocity resolution is 22 kms-1. The individual spectra are plotted on the same scale in flux density ( left axis).|
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APM 08279+5255 has a stunning apparent luminosity of , one of the highest in the universe (Irwin etal. 1998). High-resolution imaging and spectroscopy by Ledoux etal. (1998) initially showed the optical source to have two images, A and B, separated by , and showing the same spectrum, indicating that APM 08279 is gravitationally lensed. Subsequently Ibata etal. (1999) and Egami etal. (2000) found a third image, C, north of A, implying that this is a rare cusped lens with an odd number of images. The multiple image structure has also been seen in X-rays (Chartas etal. 2002). The lens modeling by Egami etal. (2000) indicates a high magnification factor of 100 at optical wavelengths. The object's true bolometric luminosity would therefore be , still making it one of the most luminous quasars. APM 08279 was also detected in the mm and submm dust continuum, and its SED peaks near 30 m (restframe), consistent with a single component dust fit with K (Lewis etal. 1998) - significantly warmer than the dust emission in other high-z QSOs with typical temperatures of 50 - 80 K (see e.g. Beelen etal. 2006).
Molecular gas in APM 08279 object has been first detected through observations of the CO(4-3) and CO(9-8) lines (Downes etal. 1999; hereafter D99) using the IRAM interferometer. These high-J CO detections were followed up by observations of the CO(1-0) and CO(2-1) lines at the VLA (Papadopoulos etal. 2001; Lewis etal. 2002a). Based on the earlier observations, initial gravitational lens models suggested the CO lines arise in a compact ( pc) disk surrounding the quasar and that the CO was magnified by a factor of 5 to 20 (D99; Lewis etal. 2002a,b). Based on the morphology of the low-J CO transitions Papadopoulos etal. (2001) suggested the presence of extended molecular gas emission in this source. Subsequent GBT observations by Riechers etal. 2006 and sensitive new high-resolution VLA imaging (Riechers etal. in prep.), however, do not show evidence for such a resolved molecular gas component: the CO emission appears to be co-spatial to the emission seen in the NIR.
While the size and mass (109 to 1010 ) of the molecular gas toroid in APM 08279 is similar to those derived for other high-z QSOs (Solomon & Vanden Bout 2004) the mere detection of the CO(9-8) line in APM 08279 already indicates that the physical properties of its molecular gas phase are more extreme than what is found in other high-z QSOs. Another sign of these more extreme conditions is the detection of HCN(5-4) in this source, which also indicates high gas densities (Wagg etal. 2005). To better constrain the excitation of the molecular gas, we here present a detailed study of the CO line SED in this source. The observations were obtained at the IRAM 30 m telescope (CO 4-3, 6-5, 9-8, 10-9, and 11-10 lines). We also present Plateau de Bure interferometer long-baseline observations of the CO 4-3, 9-8, and 1.4 mm dust radiation that have higher resolution than the earlier Bure maps of this source (D99).
The most remarkable result of our CO line SED study of APM 08279 at the 30 m telescope is that we detect the CO(10-9) and the CO(11-10) lines (Fig. 1). To search for differences in the line profile between the mid-J and high-J lines of CO we averaged the CO(9-8) CO(10-9) and CO(11-10) spectra with equal weight for each line. The linewidth from this high-J CO spectrum is kms-1 , which agrees with the value derived from CO(4-3) alone ( kms-1; D99). From the average profile of all CO lines (Fig. 1), we derive a CO linewidth of kms-1 and a CO redshift of . Our new 30 m CO(4-3) line intensity agrees with both the earlier (D99) and our new PdBI measurement. The 30 m CO(9-8) line intensity also agrees well with our new PdBI observations. Both new measurements yield a flux density slightly higher, but consistent, within the calibration error, with the previous measurements (D99).
We also observed the CO(4-3) and CO(9-8) lines with the IRAM Plateau de Bure interferometer in the long-baseline A and B configurations. The resulting data have an equivalent 6-antenna on-source integration time of 15 h, with baselines from 24 to 410 m, giving naturally-weighted synthesized beams of at 3.2 mm and at 1.4 mm. Receiver temperatures were 45 to 65 K at both wavelengths. The spectral correlators covered 910 kms-1 at 3.2 mm and 760 kms-1 at 1.4 mm, with resolutions of 8.0 and 3.6 kms-1 respectively. Amplitudes were calibrated with 3C84, 3C454.3, 3C273, and MWC349, and phases were calibrated with IAP 0749+540 and IAP 0804+499.
|Figure 2: Integrated CO(4-3) and CO(9-8) spectra from APM 08279+5255 obtained with the IRAM Interferometer. For both spectra the velocity offsets are given relative to a redshift of z=3.911. Upper panel: CO(4-3) profile above the dust continuum of 1.3 mJy. The channel width is 20 kms-1with an rms noise of 0.7 mJy. Lower panel: CO(9-8) profile above the dust continuum. The channel width is 20 kms-1 with an rms noise of 1.7 mJy. The dust continuum is 16.9 mJy.|
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To obtain high signal-to-noise spectra, we add the older short-baseline configuration D data from D99 to the long-baseline data. These combined data have equivalent 6-antenna integration times of 26 h at 3.2 mm and 23 h at 1.4 mm and naturally-weighted synthesized beams of at 3.2 mm and at 1.4 mm. The integrated CO spectra from this data cube are shown in Fig. 2. Such spectra of the total flux can be obtained by either spatially integrating over the source map, or by doing u,v-plane fits to the data in each channel, with the source centroid and source size fixed to (see below). The fit then gives the zero-spacing flux (total flux) in each channel. Both methods give the same result. For comparison with the 30 m spectra, we also fit these interferometer line profiles with single Gaussians. Table 1 summarizes our results. The CO linewidth and redshift derived from the average of the two PdBI CO spectra give kms-1 and . This redshift is higher than the corresponding value derived from the average 30 m spectrum but in good agreement with the HCN redshift measured with the PdBI. We attribute these differences to the lower S/N ratio and remaining baseline instabilities in the 30 m data. The continuum fluxes at 3.2 mm and 1.4 mm are 1.3 mJy and 16.9 mJy respectively - in agreement with the earlier data (D99).
To determine the astrometry and apparent size of the dust and CO emitting region we use the uniform weighted 1 mm data which has a spatial resolution of . This CO(9-8) and 1.4 mm dust maps (Fig. 3) yield a source position (Table 2) that agrees within the errors with the earlier result (D99) and coincides within with the optical quasar (revised optical position from Irwin etal. 1998), and with the non-thermal radio source detected at 1.4 GHz (Ivison 2006).
In its long-baseline A and B configurations, the IRAM Interferometer starts to resolve the source as the peak flux is no longer equal to the total flux. But the beam size is still too large to show whether the mm-source is in three images, as in the visible and near-IR (Ibata etal. 1999; Egami etal. 2000; or in a section of an Einstein ring Lewis etal. 2002a). We estimate an overall apparent size of the CO emission region from u,v-plane fits to the CO(9-8) data. A one-component elliptical Gaussian fit yields an equivalent single-component CO size of (Fig. 4).
Table 1: Line measurements in APM 08279+5255.
Table 2: Positions, sizes, and continuum flux densities.
|Figure 3: Map of an 8'' field around APM 08279 made with the IRAM Interferometer at 1.4 mm. The signal is the CO(9-8) line integrated over 760 kms-1, plus the 1.4 mm dust emission at the highest spatial resolution (uniform weighting, beam: ( lower left inset)). Contours start at 3 mJy beam-1 and increase in steps of 3. The peak is 19.4 mJy beam-1 (43) and the spatially-integrated flux is 33.9 mJy.|
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|Figure 4: Size measurement with the IRAM Interferometer: visibility amplitudes of the signal in the receivers' lower sideband at 1.4 mm. The signal is the CO(9-8) line integrated over 760 kms-1, plus the 1.4 mm dust emission. The plot shows the real part of the visibility amplitude vs. the projected antenna spacing, for u,v-plane data averaged in circular bins 40 m wide, with error bars of . The solid curve is a circular Gaussian fit with half-power width .|
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|Figure 5: HCN(5-4) spectrum from APM 08279+5255 obtained with the IRAM Interferometer. The line profile appears above the dust continuum of 1.3 mJy. The velocity scale is relative to a redshift of z=3.911, the beam is at PA 81, and the channel width is 15 MHz (49.84 kms-1). The rms noise in the spectrum is 0.47 mJy.|
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The continuum data is equally well fit with a single or 2-component
dust model. The single component fit is shown in
Fig. 6 (left). From this fit we find
The 2-component fit is shown in Fig. 6 (right). For convenience
we call these the "cold'' and "warm''
components. They correspond to the "starburst'' and "AGN'' components
respectively, in similar fits to the dust
spectrum by Rowan-Robinson (2000) and Beelen etal. (2006; see also
the multi-component fits to APM 08279 by Blain etal. 2003).
To reduce the number of free parameters, we assume that the overall size
of the dust emission region is similar to that of the CO lines, and
fix the equivalent (magnified) radius r0 to 1150 pc (from the 2-component CO
model). We then determine dust temperatures and
masses (times magnification m) of both components, and the relative
area filling factor of the warm and cold components. We fit these five
free parameters to the data points above 43 GHz (upper limit only)
and below the IRAS 25 m point.
|Figure 6: Single component ( left) and two component ( right) dust models for APM 08279+5255. The continuum fluxes are from Irwin etal. (1998), Lewis etal. (1998, 2002a), Downes etal. (1999), Egami etal. (2000), Papadopoulos etal. (2001), Barvainis & Ivison (2002), Wagg etal. (2005), Beelen etal. (2006) and this work.|
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For the "cold'' dust component, we find K and M(cold dust) (%). The "warm'' component has K, M(warm dust) (65%). The area filling factors we derive for the "cold'' and "warm'' dust components are % and % respectively. For the "cold'' component, the dust continuum becomes opaque for rest wavelengths shorter than 300 m ( mm). The implied FIR luminosity is (with integrated from 40-120 m restwavelength, Helou etal. 1985). The contributions from the cold and warm components are and respectively.
The peak and velocity-integrated fluxes of the CO lines (Table 1) increase with rotational quantum number up to the
9-8 line. Beyond this transition the CO line SED flattens or even
starts to decrease as for the 11-10 line. The peak of the CO line
SED occurs at 10-9 line.
The CO line luminosity,
in K kms-1 pc2, is defined by
The total H2 mass from the LVG models can be obtained using the CO
emitting area (expressed as r0) and the H2 column density calculated via
We first use a CO abundance per velocity gradient of [CO]/ . For this value, models with enough excitation to match the observed high-J fluxes, however, predict high CO opacities, even for CO(1-0), so the line luminosities are predicted to stay constant from CO(1-0) to (4-3), while the observations show a deficit in the lowest lines (Papadopoulos etal. 2001). For lower CO abundance per velocity gradient, the CO opacities are lower, because the implied CO column densities per velocity interval are reduced. For [CO]/ , most of the LVG models that fit the high-J lines also have low enough opacities to reproduce the observed fluxes of the low-J CO lines. Thus all the lines from CO(1-0) to CO(11-10) could be fit with a single-component LVG model. Similar low values for [CO]/ (i.e., a large velocity spread) in nearby starburst galaxies likely also explain the low 13CO/12CO ratios observed in their nuclei (e.g. Aalto etal. 1991; Paglione etal. 2001). In the following, we therefore fix [CO]/ to .
|Figure 7: Observed CO fluxes vs. rotational quantum number (CO line SED) for APM 08279+5255, obtained with the IRAM 30 m telescope (filled squares) and the IRAM Interferometer (open triangles) (D99; this paper). Errors include the calibration uncertainties. The fluxes for the 1-0 and 2-1 lines are from Papadopoulos etal. (2001, filled triangles), and Riechers etal. (2006, circle at J=1). The single-component LVG model fluxes are shown for ( , ) combinations as follows: solid line: (104.4 cm-3, 125 K); dashed line: (104.2 cm-3, 220 K); dotted line: (105.4 cm-3, 40 K); dashed-dotted line: (104.0 cm-3, 350 K). The inset shows a zoom, for the observed and model-predicted fluxes of the 1-0 and 2-1 lines. The two CO(1-0) data points are shown with a small offset in the rotational quantum number for legibility reasons.|
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|Figure 8: distribution for a single component LVG model fit to the observed line luminosity ratios (greyscale and grey contours, contours: , 4, 8, 10 and 20). The CO abundance per velocity gradient for the LVG models is [CO]/ . Black lines show gas to dust mass ratios of 60, 150 and 500 calculated from the LVG H2 mass for a dust mass of (single component dust fit, see Sect. 3.1). For the calculation of the LVG H2 mass we have used a turbulence line width of and a velocity gradient of (see also Sect. 4.2).|
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Figure 7 shows the results of fitting the observed CO fluxes with selected single-component LVG models. A good fit to the observations is provided by an H2 density of 104.4cm-3, a gas temperature of K and an equivalent (magnified) radius of r0=910 pc. For these parameters, the CO(1-0) optical depth is only 1.3. The CO(2-1)/CO(1-0) line luminosity ratio is then 1.25, in agreement with the observations (Papadopoulos etal. 2001). The best fitting solution is provided by n(H2) = 104.2 cm-3, K and r0=790 pc. Other temperature-density combinations with similar H2 pressure also match the data (Fig. 7, Fig. 8). For H2 densities below 104.0 cm-3, however, the model predictions fail to reproduce the observed fluxes in the high-J lines. The kinetic temperature is poorly constrained by our models. Lower-temperature models with H2 densities greater than 105 cm-3and K will also make the CO line SED turn over at the 10-9 line. For these cold, dense solutions, the turnover of the CO SED is much steeper than for the warm solutions, so in case better data were to become available, this predicted steep slope might allow us to distinguish between cold and warm gas. For the densest solutions, however, the models overestimate the 1-0 flux because the low-J opacities become too high. In Fig. 8 we show the results of a test for the single component LVG model. In this plot we also show the resulting gas to dust mass ratio. The plot demonstrates that solutions with an H2 density in excess of lead to very high LVG gas masses which imply unrealistically high gas to dust mass ratios (>1000). The warmest and coolest possible LVG solutions limit the equivalent (magnified) size of the emission region to the range pc. Using a gas to dust mass ratio of 500 as a plausible upper limit restricts the emission region further to pc.
|Figure 9: 2-component model for the CO lines. The dotted line represents the "cold'', dense gas, the dashed line the "warm'' gas and the solid line the sum of both components. Model parameters are listed in Table 3.|
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This 2-component model does not necessarily imply that the warmer gas is closer to the AGN. The two components could just as well be randomly mixed together in a single circumnuclear disk with an equivalent (magnified) radius of pc, with relative area filling factors of 75% and 25% for the cooler and warmer gas respectively - similar to the relative filling factors derived from the dust continuum. In contrast to the single component model the dense gas phase in this 2-component model only results in a gas to dust mass ratio of 150 as the dust mass in the cold dense component is much higher than the dust mass derived from a single component fit. Interestingly, the contribution of the cold, dense gas to the CO(1-0) line luminosity exceeds that of the warm gas phase. The (gravitationally-amplified) CO(1-0) line luminosities associated with both components are (cold) = K kms-1 pc2 and (warm) = K kms-1 pc2.
|Figure 10: Left: HCN line SED as a function of the IR radiation temperature for a H2 density of 105 cm-3, a kinetic temperature of 65 K and an delution factor for the IR radiation field of 10% for each model. All SEDs are normalized to the HCN(1-0) flux density for the pure collisional excitation model ( K). The plot exemplifies the large effect of IR pumping on the HCN excitation for dust temperatures above 100 K. Middle: HCN excitation in APM 08279+5255 for gas parameters as derived from the single component dust/CO model (n(H2) = 104.2 cm-3, K and r0=790 pc) as a function of the IR filling factor. For IR % or above the IR pumping boosts the HCN excitation such that the HCN(5-4) flux density is in agreement with the observations. Right: HCN excitation model for the two-component dust/CO model for APM 08279+5255. The dashed line represents the "warm'' gas with and IR illumination factor of unity, the dotted lines the "cold'', dense gas with pure collisional excitation (IR ) and with an IR illumination of 0.25%. The solid line the sum of both IR illuminated components. The IR radiation temperature for both components is 220 K.|
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Wagg etal. (2005) have argued that an increased HCN abundance may be responsible for the bright HCN emission. Their calculations suggest that a relative HCN abundance of [HCN/CO] 10-2, thus 2 orders of magnitude above the standard abundance ratio (Wang etal. 2004), is required in order explain the observed HCN and CO luminosities from a single gas component. Enhanced HCN abundances can be caused by an increased ionization rate in the vicinity of an AGN (see Wagg etal. 2005 and reference therein). Models however suggest that this increase is only modest ([HCN/CO] = , Lepp & Dalgarno 1996). For NGC 1068 Usero etal. (2004) suggested that X-rays drive the abundances ratio to [HCN/CO] 10-3. But even if we use this increased HCN abundance, single component models with kinetic temperatures similar to the dust temperature fail to reproduce the observed HCN(5-4) flux density by a factor of 2 or more.
In the context of the 2-component model the predicted HCN(5-4) luminosity increases because the density of the cold dense gas is much higher than that derived from the one-component model. But even for this gas phase, which has a density comparable to that derived for the HCN emitting volume in local IR luminous sources (Greve etal. 2006), the predicted HCN(5-4) luminosity is much smaller than the observed value. We can, however, increase the H2 density of the cold gas component if kinetic temperatures below the dust temperature are considered. An equally good fit to all the observed CO lines that also matches the HCN(5-4) luminosity can be obtained using a high-density component with n(H2) 105.7 cm-3, K, and a source equivalent (magnified) radius of pc as well as a warmer, lower-density component with n(H2) 104.0 cm-3, K, and pc. The high density for the cold gas component, however, leads again to a very high gas to dust mass ratio of 1000. Given the high metalicity of APM 08279 (2 to 5 times solar as suggested by X-ray observations of the iron K-shell absorption edge, Hasinger etal. 2002), this makes it very unlikely that the HCN(5-4) luminosity can be explained within a pure collisional excitation scheme.
Another potential mechanism to increase the HCN luminosity is to boost the HCN by infrared excitation, notably through the stretching and bending modes at 3, 5, and 14 m. Various studies have addressed this issue for local IR bright galaxies and concluded that IR pumping via the 14 m modes is not important compared to collisional excitation (e.g. Stutzki etal. 1988; Paglione etal. 1997; Gao & Solomon 2004). The dust SED of APM 08279, however, has a much higher contribution of warm dust compared to other sources studied in detail so far. Furthermore its dust temperature of 200 K is well beyond the minimum dust temperature of 110 K required for IR pumping via the 14 m bending mode to become efficient (Carroll & Goldsmith 1981).
To investigate the effect of the increased IR field in APM 08279+5255 on the HCN excitation we include the first vibrational bending mode (, see e.g. Thorwirth etal. 2003) in our LVG code. We neglected the vibrational mode as its excitation requires 4 times higher IR temperatures (Carroll & Goldsmith 1981). For the computation we take 20 rotational levels into account which leads in total to 58 energy levels due to the of the mode. The IR field is described by a greybody spectrum with a frequency dependence in analogy to Eq. (3). The illumination of the dense gas by the IR field is parameterized by an IR delution factor. This filling factor represents the solid angle fraction of the gas exposed to the IR field.
Figure 10 (left) exemplifies the variations of the HCN line SED as a function of the IR radiation temperature. We find that for dust temperatures below 100 K the excitation through vibrational pumping is small compared to the collisional excitation for H2 densities typical for HCN emitting regions (104-5 cm-3). This is in line with the finding that IR pumping is not an important mechanism for local IR bright galaxies and most high-z sources as their dust SEDs show a much smaller contribution of dust with temperatures in excess of 50 K compared to APM 08279 (e.g. Lisenfeld etal. 2000, Beelen etal. 2006). For increasing dust temperatures, however, the IR pumping becomes very efficient. At a dust temperature of about 200 K the HCN excitation is completely dominated by IR pumping as long as the IR delution factor is above a few percent.
In Fig. 10 (middle) we show the effect of the IR dust field on the HCN excitation for the 1-component CO model ( K, cm-3, K, r0=790 pc). For this model the collisional excitation of the HCN(5-4) line is negligible. But already an IR delution factor of boosts the HCN luminosity by almost 2 orders of magnitude. For a filling factor of 10% or higher the IR pumping is strong enough to explain the observed HCN(5-4) luminosity in APM 08279. Thus the exceptional high dust temperature in APM 08279 provides a straightforward explanation for the strong HCN luminosity in this source without any ad hoc changes of the HCN abundance or extreme H2 densities. This most likely also holds for the HCO+ emission recently detected in APM 08279 (see Garcia-Burillo etal. 2006 and there discussion on the IR pumping of HCO+).
Although the single component model with IR pumping yields a good fit to the CO and HCN line intensities as well as to the dust continuum it is presumably too simple as the HCN emission is known to arise from cloud cores with are typically an order of magnitude denser than those traced by the bulk of the CO emission. We therefore also show the effect of IR pumping for the 2-component CO model (Fig. 10, right). Since the IR filling factors for both components can not be derived from the available data, we consider here only the extreme case where the IR filling factor for the warm component, which is responsible for the dust emission at 220 K, is unity. From the Figure it can be seen that the IR pumping of the warm component, even in this extreme case, is not sufficient to explain the observed HCN(5-4) luminosity. Its contribution to the HCN(5-4) flux is similar to that of the pure collisional excitation from the cold dense medium. But even the sum of both mechanisms still underpredicts the HCN(5-4) luminosity so that also a small part of the cold dense gas needs to be exposed to the 220 K dust field. The required IR filling factor for the cold dense gas, however, is only 0.25%. The HCN(1-0) line luminosity from this model is dominated by the emission from the cold, dense gas and yields K kms-1 pc2. The contribution due to collisional excitation is K kms-1 pc2. We note that the IR filling factor for the cold gas remains low even if the IR filling for the warm gas is smaller than unity. That is, in the absence of the warm gas component the IR filling for the cold gas required to match the observed HCN(5-4) luminosity is still below 1%.
In the following we use our effective (magnified) radius
r0 derived from the LVG and dust models in combination with the
radial dependence of the magnification from the lens model
of Egami etal. (2000, Fig. 9)
to re-estimate the magnification and intrinsic size of the CO
and dust emission regions. By definition,
the apparent CO luminosity,
is related to
the effective radius r0 by
|Figure 11: Radial dependence of calculated using the differential triple-image lens model from Egami etal. (2000, their Fig. 9) for a filled, face-on disk (top) and a disk seen at an inclination of with an area filling factor for the CO emission of (bottom). Curves are shown for selected LVG models. For the cold component has been set based on the radius of the warm component to 50 and 100 pc for the face-on and inclined case respectively. For all other models we used pc. The horizontal lines show the total, observed CO(1-0) line luminosity (Riechers etal. 2006, solid line) and the contributions to from the cold (dashed line) and the warm (dotted line) gas components derived in Sect. 3.2.3. The relevant intersections between these curves are marked by the arrows for the four models. Resulting source radii and effective magnifications are given in the plot.|
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The true source radii, however, will be larger if the CO disk is inclined
and the gas area filling factor is smaller than unity.
For this more realistic geometry Eq. (11) becomes
Table 3: Summary of best-fit model.
Hence the true source size, which dominates the observed continuum and line emission, is very compact with pc and has a large effective magnification of for a large range of geometries. A caveat is the unknown structure of the underlying CO distribution (expressed above as the filling factor of the CO emitting gas and the inclination). An arbitrary CO distribution with a low area filling factor could extend well beyond the strongly magnified region. Thus the above estimates do not rule out that the compact, strongly magnified source is surrounded by additional, unlensed gas at larger radii. E.g. a molecular reservoir similar to that seen around M 82's starburst disk (Walter etal. 2002) or the spiral arms surrounding the nucleus of NGC 1068 (e.g. Schinnerer etal. 2000) would remain undetected as its contribution to the observed CO line intensities would be negligible. We note, however, that our analysis excludes an additional gas reservoir of several kpc size (as proposed by Papadopoulos etal. 2001) as it would have a significant contribution to the low-J CO lines (see also Riechers etal. 2006).
At a true radius of 100-250 pc, the lens image in the Egami etal. model is an Einstein ring or filled circle, which may explain the observed symmetric CO(9-8) distribution. Egami etal. predict a diameter for this ring - somewhat smaller than that derived from the CO(9-8) u,v fits. Thus our high resolution data also hint to a overall CO size out to 300 pc. A possible contradiction to this prediction is the morphology of the CO(1-0) distribution observed by Lewis etal. (2002a), which is an incomplete ring. New high-resolution VLA CO(1-0) observations with better signal-to-noise ratio, however, show that the CO is very similar to the IR distribution (Riechers etal. in prep.), supporting the idea that the CO magnification is higher than the values derived in other studies. The results of the 2-component modeling for CO, HCN, and the dust continuum are summarized in Table 3 and in the schematic diagram in Fig. 12.
|Figure 12: Schematic diagram of our 2-component model for the CO, HCN, and mm-FIR dust emission from APM 08279+5255. There are two constraints on the geometry: 1) the large CO and HCN linewidths of 480 kms-1 imply the molecular rings are being viewed at high inclination. 2) Our line of sight to the black hole must intersect the BAL outflow cone, as in the model by Elvis (2000), so that UV broad absorption lines are seen against the UV continuum and UV emission lines of the accretion disk. In the molecular rings, the HCN and high-J CO lines mainly come from the "cold'', dense component at >100 pc (outer disk), and some of the mid-J CO emission comes from the "warm'', lower-density component at 50-100 pc (inner disk). The NIR radiation (Egami etal. 2000; Soifer etal. 2004) comes from the 1500 K-dust sublimation radius at 1 pc. The absorbers responsible for the X-ray BALs are located at radii of <0.1 pc from the black hole (Chartas etal. 2002; Hasinger etal. 2002).|
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The radial temperature profile expected from the AGN heating also implies that it is unlikely that the cold and the warm gas are randomly mixed but it supports the interpretation that the cold gas arises from regions at larger distances from the AGN. As the cold gas dominates the HCN(5-4) line luminosity this is also supported by the somewhat narrower linewidth observed in the HCN(5-4) line. Further support for this picture comes from the analysis of the IR pumping of HCN which shows that the cold gas is only weakly exposed to the 220 K IR dust field (IR ). Such a low filling factor of the IR field contradicts the view that the cold gas is embedded in the warm gas phase.
For high-z objects with high FIR luminosities, a ULIRG conversion factor of 0.8 (K kms-1 pc2)-1 (Downes & Solomon 1998) is typically adopted to convert the lower-J CO line luminosities to the total molecular gas mass (see e.g. Solomon & Vanden Bout 2005). The original argument for using this factor is that much of the CO emission in the rapidly-rotating circumnuclear disks of ULIRGs comes from a spread-out intercloud medium, not from self-gravitating clouds (Downes etal. 1993). As we have seen from our two component gas model, even the warm component has a H2 density typical for star forming galaxies (n(H2) ) and about 70% of the CO(1-0) emission comes from extremely dense gas. Unlike typical CO lines from ULIRGS, the global CO emission in APM 08279 is therefore not dominated by a diffuse intercloud medium, but by very dense gas, with cm-3. The conversion factor scales as , both for self-gravitating molecular clouds (see the typical ranges of the factor listed by Radford etal. (1991), and for a more distributed medium that is not self-gravitating (Downes etal. 1993). For the cold gas component of APM 08279, the CO brightness temperatures is similar to those in ULIRGS ( K), but the gas density is about forty times higher than in ULIRGS. This suggests the conversion factor should be 6 (K kms-1 pc2)-1. This gives us a first estimate of the molecular gas mass, based on CO alone:
If, however, we accept the mass estimate from the optically thin part of the dust spectrum as a good working estimate, we can compare this mass directly with the CO and HCN luminosities, to "derive'' the conversion factor for the cold dense gas. This directly gives = 5.2 (K kms-1 pc2)-1 and (K kms-1 pc2)-1. agrees well with our value estimated by scaling the ULIRG factor. The conversion factor for HCN is a factor of 2 higher than the values given in Gao & Solomon (2004) by in agreement for those derived in Arp 220 and NGC 6240 (Greve etal. 2006).
The bolometric luminosity of the quasar, after correcting for the
factor-of-100 amplification by gravitational lensing, is still an
From the argument originally due
to Zel'dovich & Novikov (1964), the lower limit to the black hole
mass, if electron scattering produces the main opacity, is
The conclusion is that for this QSO at z=3.9, the super-massive black hole is already in place, but the assembly of the stellar bulge is still in progress. Similar conclusions have been gained by Walter etal. (2004) for J1148+5251 at z=6.4 and by Shields etal. (2006) on a larger sample of high-z QSOs. A caveat arising from the special lens configuration in APM 08279 is, that the emission lines only trace a very compact region surrounding the AGN which implies that this conclusion only holds if the central density profile of the stellar bulge follows a profile to the central 100 pc.
|Figure 13: Black hole, molecular, stellar and dynamical mass in the central region of APM 08279 as a function of assumed inclination of the molecular disk. Radii as a function of the inclination have been calculated from Eq. (13) for the cold, dense gas (r0=995 pc, pc) using a filling factor of unity. The stellar mass has been calculated using a scalelength of the stellar bulge of 5 and 10 kpc and assuming the relation were to hold. Variations of the stellar bulge contribution with inclination are only due to changes of the size of the region considered. The corresponding radii are given at the top axis of the plot. The inclination for which the total mass (gas + stellar + BH) matches the dynamical mass are given for both bulge geometries together with the corresponding numbers for the true CO radius, effective magnification of the molecular gas, and the CO rotation velocity.|
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|Figure 14: Comparison of the CO line SEDs of selected local and high-z galaxies. The SEDs are shown for APM 08279 (this paper, Fig. 7), BR1202-0725 (z=4.7, Carilli etal. 2002; Riechers etal. 2006), J1148+5251 (z=6.4, open squares, Bertoldi etal. 2003; Walter etal. 2003) the high-excitation component in the center of M82 (Weiß etal. 2005b), NGC 253 center (Güsten etal. 2006) SMM 16359 (z=2.5, Weiß etal. 2005a) and the Galactic Center (solid circles, Fixsen etal. 1999). The CO line SEDs are normalized by their CO(1-0) flux density.|
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The CO and dust size measurement, combined with the lensing model of Egami etal. (2000) shows that the source region is very compact (radius 60-300 pc) and that its magnification is similar to the optical/IR magnification (60-110). The high CO excitation and the dust continuum emission are best modeled with a 2-component gas model. The warm (220 K) gas component is most likely directly heated by the AGN and arises from radii between 60-150 pc. This component dominates the FIR luminosity (). The cooler gas (65 K), which carries of the total gas mass of arises from somewhat larger radii. Its main characteristic is a high H2 density of cm-3 - about 10 times higher than the gas in other high-z QSOs and local starburst galaxies. The high gas density implies that the standard ULIRG conversion factor, usually applied to high-z galaxies, does not provide a good estimate of the molecular gas in APM 08279. Our study suggests that conversion factor for CO, in this special case, is (K kms-1 pc2)-1.
Although the gas density for the cold gas is typical for the HCN emitting volume in local ULIRGs it is not high enough to explain the observed HCN(5-4) luminosity in APM 08279. From LVG models including IR pumping via the 14 m bending mode of HCN we conclude that the hot dust heated by the AGN efficiently boosts the HCN excitation and that this excitation channel is more important than the collisional excitation for this particular source. The star formation rate associated with the dense gas component is only .
An estimate of the mass budget in the central region of APM 08279 suggests that the black hole and gas mass does not leave sufficient room for a stellar mass contribution following the local relation for reasonable inclinations of the molecular disk. Hence we conclude that the super-massive black hole in APM 08279 is already in place, before the buildup of the stellar bulge is complete.
We thank the IRAM receiver engineers D. John and S. Navarro for their great help in optimizing receiver tunings as well as the 30 m telescope operators and the Plateau de Bure Interferometer operators for their all-around assistance with the observing in general. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).