Copious Amounts of Dust and Gas in a z = 7.5 Quasar Host Galaxy

[C ii]_3/2–1/2 158 μm (hereafter [C ii]), CO(7–6), CO(10–9), H₂O, and [C i]_2-1 observations of J1342+0928 were performed with the IRAM NOrthern Extended Millimeter Array (NOEMA). Observations were done with the array in compact configuration, using 7–8 antennas. All of the NOEMA data have been reduced using the latest version of the GILDAS software.¹¹

The observations were gathered between 2017 March 15 and May 21 in various visits. For the [C ii] observations, the NOEMA receiver 3 (1.2 mm) was tuned to 224.121 GHz in the first execution, and to 222.500 GHz in all the other visits, in order to better encompass the line within the WideX 3.6 GHz bandwidth. The CO(10–9) line and the H₂O 3(2, 1)–3(1, 2) line at rest-frequency 1162.91 GHz were observed in a single frequency setting, with NOEMA receiver 2 (2 mm) tuned to 135.495 GHz. The CO(7–6) and [C i]_2–1 lines were observed with the 3 mm receivers tuned to 94.587 GHz. The radio quasar 1345+125 served as amplitude and phase calibrator. Additional calibrators used in the bandpass calibration included 3C273 and 3C454.3. The star MWC 349 was used to set the absolute flux scale. Measured line fluxes and continuum flux densities in Section 3 and Table 1 only include statistical errors and do not take the systematic flux calibration uncertainties of ∼10% into account. The total integration time on-source was 13.6, 3.8, and 11.1 hr (8 antenna equivalent) in the 1 mm, 2 mm, and 3 mm bands, respectively. Imaging was performed using natural weighting, in order to maximize sensitivity. The resulting synthesized beams are 24 × 15, 36 × 25, and 58 × 34 and the final 1 mm, 2 mm, and 3 mm cubes reach a sensitivity of 0.47 mJy beam⁻¹, 0.41 mJy beam⁻¹, and 0.17 mJy beam⁻¹ per 100 km s⁻¹ channel (1-σ), respectively. In the 1 mm cube, both the [C ii] emission and the underlying dust continuum are significantly detected (Figure 1 and Section 3), while no emission was detected in the other two cubes.

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Table 1.  Observed and Derived Properties of J1342+0928

R.A. (J2000)	13h42m08097
Decl. (J2000)	+09°28'3828
${z}_{[{\rm{C}}{\rm{II}}]}$	7.5413 ± 0.0007
${F}_{[{\rm{C}}{\rm{II}}]}$ (Jy km s−1)	1.25 ± 0.17
FWHM${}_{[{\rm{C}}{\rm{II}}]}$ (km s−1)	383 ± 56
${\mathrm{EW}}_{[{\rm{C}}{\rm{II}}]}$ (μm)	1.73 ± 0.43
${S}_{223.5\mathrm{GHz}}$ (μJy)	415 ± 73
${S}_{135.5\mathrm{GHz}}$ (μJy)	<139
${S}_{95\mathrm{GHz}}$ (μJy)	<48
${S}_{41\mathrm{GHz}}$ (μJy)	15.0 ± 5.7
${S}_{1.4\mathrm{GHz}}$ (μJy)	<432
${F}_{\mathrm{CO}(3-2)}$ (Jy km s−1)	<0.081
${F}_{\mathrm{CO}(7-6)}$ (Jy km s−1)	<0.13
${F}_{\mathrm{CO}(10-9)}$ (Jy km s−1)	<0.32
${F}_{[{\rm{C}}{\rm{I}}]}$ (Jy km s−1)	<0.14
${F}_{{{\rm{H}}}_{2}{\rm{O}},1172\mathrm{GHz}}$ (Jy km s−1)	<0.30
${F}_{{{\rm{H}}}_{2}{\rm{O}},1918\mathrm{GHz}}$ (Jy km s−1)	<0.33
${L}_{\mathrm{FIR}}$ (${L}_{\odot }$)	$(0.5\mbox{--}1.4)\times {10}^{12}$
${L}_{\mathrm{TIR}}$ (${L}_{\odot }$)	$(0.8\mbox{--}2.0)\times {10}^{12}$
${L}_{[{\rm{C}}{\rm{II}}]}$ (${L}_{\odot }$)	$(1.6\pm 0.2)\times {10}^{9}$
${L}_{[{\rm{C}}{\rm{I}}]}$ (${L}_{\odot }$)	$\lt 7.8\times {10}^{7}$
${L}_{\mathrm{CO}(3-2)}^{{\prime} }$ (K km s−1 pc2)	$\lt 1.5\times {10}^{10}$
SFRTIR (${M}_{\odot }$ yr−1)	120–300
SFR${}_{[{\rm{C}}{\rm{II}}]}$ (${M}_{\odot }$ yr−1)	85–545
Md (${M}_{\odot }$)	$(0.6\mbox{--}4.3)\times {10}^{8}$
${M}_{{{\rm{C}}}^{+}}$ (${M}_{\odot }$)	$4.9\times {10}^{6}$
${M}_{{{\rm{H}}}_{2}}$ (${M}_{\odot }$)	$\lt 1.2\times {10}^{10}$

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In the continuum images, an additional source is located ∼10'' northeast of the quasar (see Figure 2) with flux densities of ${S}_{223.5\mathrm{GHz}}=434\pm 73$ μJy, ${S}_{135.5\mathrm{GHz}}=197\pm 46$ μJy, and ${S}_{95\mathrm{GHz}}=41\pm 16$ μJy. The spectrum of this object does not show emission lines. While the redshift remains unknown, the lack of line emission in the 1 mm datacube, which covers a [C ii] redshift of ${\rm{\Delta }}z\approx 0.1$ around that of the quasar, makes it unlikely that this source is physically associated with J1342+0928.

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We searched for CO(3–2) emission from J1342+0928 with the VLA in 2017 April. The redshift of the source places the line at 40.4852 GHz. The data also provided a deep continuum observation at 41 GHz. A total of 9 hr (7 hr on-source), was spent using the 8 bit, 2 GHz bandwidth correlator mode for highest line sensitivity. An additional 3 hr was spent using 3 bit, 8 GHz bandwidth from 40 to 48 GHz for an additional continuum measurement.

Standard phase and amplitude calibration was performed, using J1331+305 to set the absolute gain scale and bandpass, and J1347+122 to determine complex gains as a function of time. Phase stability was excellent.

The line data were imaged using natural weighting and smoothed to a velocity resolution of 44.5 km s⁻¹. The synthesized beam is 22 × 20, and the rms noise per channel was 0.10 mJy beam⁻¹. We also created a 41.0 GHz continuum image by suitably combining all the data. The rms noise of this continuum image is 5.7 μJy beam⁻¹. No line was found, but a potential continuum source is reported (Section 3.1).

The dust continuum around the redshifted [C ii] emission (rest-frame wavelength of ∼158 μm) has been detected at a signal-to-noise ratio (S/N) ∼ 6 and a strength of ${S}_{223.5\mathrm{GHz}}=415\pm 73$ μJy (Figures 1 and 2). The source is not resolved with the 24 × 15 (12.1 × 7.3 kpc²) beam. We also estimated the source size in the uv plane and derive a source radius <05, which is consistent with the size measurement of the continuum image. The position of the quasar host, R.A. = 13^h42^m08097; decl. = +09°28'3828, is consistent with the near-infrared location of the quasar (Bañados et al. 2017). The host galaxy is not detected in continuum in the other NOEMA setups down to 3σ continuum limits of ${S}_{135.5\mathrm{GHz}}\lt 139$ μJy and ${S}_{95\mathrm{GHz}}\lt 48$ μJy. The VLA continuum map shows a potential source (${S}_{41\mathrm{GHz}}\,=15.0\pm 5.7$ μJy beam⁻¹ and S/N ∼ 2.6; Figure 2), located ∼07 from the [C ii] emission of J1342+0928.

To compute the far-infrared (rest-frame 42.5–122.5 μm) and total infrared (TIR; 8–1000 μm) luminosities, ${L}_{\mathrm{FIR}}$ and ${L}_{\mathrm{TIR}}$, and the dust mass M_d in J1342+0928, we follow the same procedure as outlined in Venemans et al. (2016). In summary, we utilize three different models to estimate dust emission: a modified black body (MBB) with a dust temperature T_d = 47 K and an emissivity index of $\beta =1.6$ (e.g., Beelen et al. 2006) and two templates of local star-forming galaxies (Arp220 and M82) from Silva et al. (1998). We also take the effect of the cosmic microwave background (CMB) on the dust emission into account (e.g., da Cunha et al. 2013; Venemans et al. 2016). The mass of dust is derived both by assuming an opacity index of ${\kappa }_{\lambda }=0.77{(850\mu {\rm{m}}/\lambda )}^{\beta }$ cm² g⁻¹ (Dunne et al. 2000) and from scaling the Arp220 and M82 templates (Silva et al. 1998). We stress that due to the unknown shape of the dust continuum, the FIR and TIR luminosities remain highly uncertain, while the SFR and dust mass we derive crucially depend on the applicability of local correlations to this high-redshift source.

Scaling the NOEMA continuum detection of ${S}_{223.5\mathrm{GHz}}\,=415\pm 73$ μJy to the three dust spectral energy distribution (SED) models results in luminosities of ${L}_{\mathrm{FIR}}=(0.5\mbox{--}1.4)\,\times {10}^{12}$ ${L}_{\odot }$ and ${L}_{\mathrm{TIR}}=(0.8\mbox{--}2.0)\times {10}^{12}$ ${L}_{\odot }$. The derived dust mass is ${M}_{d}=(0.6\mbox{--}4.3)\times {10}^{8}$ ${M}_{\odot }$. Applying the local scaling relation between ${L}_{\mathrm{TIR}}$ and SFR from Murphy et al. (2011) and assuming the infrared luminosity is dominated by star formation (e.g., Leipski et al. 2014) results in an SFR of 120–300 ${M}_{\odot }$ yr⁻¹. This is significantly lower than the SFR derived for some of the quasar hosts at $z\sim 6$ (e.g., Walter et al. 2009), but very similar to the SFR in J1120+0641 at z = 7.1 (Venemans et al. 2017).

We now look into the origin of the potential VLA continuum detection. The first possibility is that the source is spurious. The S/N is only 2.6, and as shown in Figure 2, several positive noise peaks are visible close to the location of the quasar. It is therefore plausible that the 41 GHz detection will disappear when adding more data. On the other hand, if the source is real, then the question is whether it is due to dust emission, free–free emission, or non-thermal processes, e.g., synchrotron radiation. The typical quasar dust SED, the MBB with T_d = 47 K and $\beta =1.6$ predicts flux densities of 90, 28, and 1.5 μJy at 135.5, 95, and 41.0 GHz, respectively (Figure 3). The limits in the NOEMA 2 mm and 3 mm bands are consistent with these expected flux densities, but the flux density measured in the VLA image is significantly (∼10×) higher than expected from the dust emission. A much shallower emissivity index ($\beta \ll 1.5$) and/or a lower dust temperature, which would result in a higher flux density at 41 GHz, can be ruled out by the nondetections at 135.5 and 95 GHz (Figure 3). Based on the derived SFR in the host galaxy (SFR = 85–545 ${M}_{\odot }$ yr⁻¹; Table 1), the strength of free–free emission at 41.0 GHz is negligible (${S}_{\mathrm{ff}}\ll 1$ μJy; e.g., Yun & Carilli 2002). Alternatively, the flux density could be due to synchrotron radiation. We can estimate the radio loudness of the quasar using the radio-to-optical flux density ratio $R={S}_{5\mathrm{GHz},\mathrm{rest}}/{S}_{4400\mathring{\rm A} ,\mathrm{rest}}$ with ${S}_{5\mathrm{GHz},\mathrm{rest}}$ and ${S}_{4400\mathring{\rm A} ,\mathrm{rest}}$ the flux densities at rest-frame 5 GHz and 4400 Å, respectively (Kellermann et al. 1989). Assuming a radio continuum can be described by a power law (${f}_{\nu }\propto {\nu }^{\alpha }$) with $\alpha =-0.75$ (e.g., Bañados et al. 2015), we derive ${S}_{5\mathrm{GHz},\mathrm{rest}}=363$ μJy. Following Bañados et al. (2015), we derive ${S}_{4400\mathring{\rm A} ,\mathrm{rest}}=29$ μJy from the WISE W1 magnitude (W1 = 20.17). We obtain R = 12.4, making J1342+0928 a radio-loud quasar (where radio-loud is defined as $R\gt 10$). Note that this is still consistent with the nondetection in the FIRST survey, with a 3σ upper limit of ${S}_{1.4\mathrm{GHz}}\lt 432$ μJy, as the expected flux density for J1342+0928 is ${S}_{1.4\mathrm{GHz}}\approx 190$ μJy (Figure 3). Deeper imaging at radio frequencies will provide a definitive answer.

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We detect the [C ii] emission line in J1342+0928 in the continuum-subtracted [C ii] map (Figure 2) with an S/N ∼ 10. The spectrum, extracted from the peak pixel in the datacube, is shown in Figure 1. From a Gaussian fit to the line, we derive a redshift of ${z}_{[{\rm{C}}{\rm{II}}]}=7.5413\pm 0.0007$, a line flux of ${F}_{[{\rm{C}}{\rm{II}}]}\,=1.25\pm 0.17$ Jy km s⁻¹, and a dispersion of ${\sigma }_{[{\rm{C}}{\rm{II}}]}=163\,\pm 24$ km s⁻¹ (FWHM${}_{[{\rm{C}}{\rm{II}}]}=383\pm 56$ km s⁻¹); see Table 1. This corresponds to a [C ii] luminosity in this quasar of ${L}_{[{\rm{C}}{\rm{II}}]}\,=(1.6\pm 0.2)\times {10}^{9}$ ${L}_{\odot }$, which is roughly ∼15% brighter than J1120+0641 at z = 7.1 (Venemans et al. 2017) and a factor 3–5 fainter than the most [C ii] luminous quasar at $z\sim 6$ (e.g., Maiolino et al. 2005; Wang et al. 2013).

The redshift derived from the [C ii] line is higher than that derived from the UV emission lines of the quasar. The C iv and Mg ii lines are blueshifted by 6580 ± 270 km s⁻¹ and 500 ± 140 km s⁻¹ with respect to the [C ii] line. The Mg ii shift is close to the mean blueshift of the Mg ii line of 480 km s⁻¹ found in a sample of $z\sim 6\mbox{--}7$ quasars (e.g., Venemans et al. 2016). This could indicate the presence of an outflow (e.g., Mazzucchelli et al. 2017).

We measure a rest-frame [C ii] equivalent width of ${\mathrm{EW}}_{[{\rm{C}}{\rm{II}}]}\,=1.73\pm 0.43$ μm, which is consistent with the mean ${\mathrm{EW}}_{[{\rm{C}}{\rm{II}}]}$ of local starburst galaxies (which have $\langle {\mathrm{EW}}_{[{\rm{C}}{\rm{II}}]}\rangle =1.27\,\pm 0.53$ μm; see e.g., Díaz-Santos et al. 2013; Sargsyan et al. 2014) and higher than those of luminous (${M}_{1450}\lt -27$) quasars at $z\sim 6$ (e.g., Wang et al. 2013). The [C ii]-to-FIR luminosity ratio is ${L}_{[{\rm{C}}{\rm{II}}]}$/${L}_{\mathrm{FIR}}=(0.6\mbox{--}2.6)\times {10}^{-3}$ (Figure 4), again consistent within the large uncertainties with the ${L}_{[{\rm{C}}{\rm{II}}]}$/${L}_{\mathrm{FIR}}$ ratio of local star-forming galaxies that have a median ${L}_{[{\rm{C}}{\rm{II}}]}/{L}_{\mathrm{FIR}}=2.5\times {10}^{-3}$ (e.g., Díaz-Santos et al. 2013).

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We can estimate the SFR from the [C ii] emission using the SFR–${L}_{[{\rm{C}}{\rm{II}}]}$ relations for high-redshift ($z\gt 0.5$) galaxies of De Looze et al. (2014):

$$\begin{eqnarray}&&{\mathrm{SFR}}_{[{\rm{C}}{\rm{II}}]}/{M}_{\odot }\,{\mathrm{yr}}^{-1}=3.0\times {10}^{-9}{({L}_{[{\rm{C}}{\rm{II}}]}/{L}_{\odot })}^{1.18},\end{eqnarray} \tag{ 1 }$$

with a systematic uncertainty of a factor of ∼2.5. With ${L}_{[{\rm{C}}{\rm{II}}]}\,=(1.6\pm 0.2)\times {10}^{9}$ ${L}_{\odot }$ we derive ${\mathrm{SFR}}_{[{\rm{C}}{\rm{II}}]}=85\mbox{--}545$ ${M}_{\odot }$ yr⁻¹, which is similar to the SFR based on the TIR luminosity (Section 3.1).

The [C ii] emission is not resolved in the 25 × 15 beam (Figure 2). We fitted a 2D Gaussian to the [C ii] map using the CASA task "imfit" and we derive a 1σ upper limit on the size of 17 × 12 (FWHM). A similar limit on the source diameter of $D\lt 1\buildrel{\prime\prime}\over{.} 0$ is found when fitting a 1D Gaussian to the uv data. This translates to an upper limit on the size of the [C ii]-emitting region of 8.4 × 5.9 kpc² or a diameter of D ≲ 7 kpc. Approved observations with the Atacama Large Millimeter/submillimeter Array (ALMA) at higher spatial resolution will put tighter constraints on the size of the host galaxy.

From the strength of the [C ii] emission line, we can derive the mass of singly ionized carbon. In analogy to the formula to compute the mass of neutral carbon provided in Weiß et al. (2005) and assuming optically thin [C ii] emission, the mass of singly ionized carbon can be calculated using

$$\begin{eqnarray}{M}_{{{\rm{C}}}^{+}}/{M}_{\odot } & = & {{Cm}}_{{\rm{C}}}\displaystyle \frac{8\pi k{\nu }_{0}^{2}}{{{hc}}^{3}A}Q({T}_{\mathrm{ex}})\displaystyle \frac{1}{4}{e}^{91.2/{T}_{\mathrm{ex}}}{L}_{[{\rm{C}}\,{\rm{II}}]}^{{\prime} }\\ & = & 2.92\times {10}^{-4}Q({T}_{\mathrm{ex}})\displaystyle \frac{1}{4}{{\rm{e}}}^{91.2/{T}_{\mathrm{ex}}}{L}_{[{\rm{C}}\,{\rm{II}}]}^{{\prime} },\end{eqnarray} \tag{ 2 }$$

with C the conversion between pc² and cm², m_C the mass of a carbon atom, $A=2.29\times {10}^{-6}$ s⁻¹ the Einstein coefficient (Nussbaumer & Storey 1981), $Q({T}_{\mathrm{ex}})=2+4{{\rm{e}}}^{-91.2/{T}_{\mathrm{ex}}}$ the C ii partition function, and T_ex the excitation temperature. As [C ii] is emitted from the outer layers of photon-dominated region (PDR) clouds, ${T}_{\mathrm{ex}}\gtrsim 100$ K is a good assumption (see, e.g., Meijerink et al. 2007). Setting ${T}_{\mathrm{ex}}=100$ K we derive ${M}_{{{\rm{C}}}^{+}}=4.9\times {10}^{6}$ ${M}_{\odot }$. For ${T}_{\mathrm{ex}}=200$ K (75 K), the mass would be ∼20% lower (higher).

We do not detect any of the other targeted emission lines in J1342+0928. To derive upper limits on the line fluxes, we averaged the datacubes over $2.8\times {\sigma }_{[{\rm{C}}{\rm{II}}]}$ (460 km s⁻¹). We measured the following 3σ upper limits: ${F}_{\mathrm{CO}(10-9)}\lt 0.32$ Jy km s⁻¹, ${F}_{\mathrm{CO}(7-6)}\lt 0.13$ Jy km s⁻¹, ${F}_{[{\rm{C}}{\rm{I}}]}\lt 0.14$ Jy km s⁻¹, ${F}_{\mathrm{CO}(3-2)}\lt 0.081$ Jy km s⁻¹, and ${F}_{{{\rm{H}}}_{2}{\rm{O}},1172\mathrm{GHz}}\lt 0.30$ Jy km s⁻¹.

The limits on the CO luminosity are ${L}_{\mathrm{CO}(10-9)}^{{\prime} }\lt 5.2\,\times {10}^{9}$ K km s⁻¹ pc², ${L}_{\mathrm{CO}(7-6)}^{{\prime} }\lt 4.3\times {10}^{9}$ K km s⁻¹ pc², and ${L}_{\mathrm{CO}(3-2)}^{{\prime} }\lt 1.5\times {10}^{10}$ K km s⁻¹ pc². We can estimate a limit on the molecular gas mass ${M}_{{{\rm{H}}}_{2}}$ by utilizing ${M}_{{{\rm{H}}}_{2}}=\alpha {L}_{\mathrm{CO}(1-0)}^{{\prime} }$ with α the CO luminosity-to-gas mass conversion factor. Assuming the CO(3–2) emission is thermalized (e.g., Riechers et al. 2009), the CO(1–0) luminosity is given by ${L}_{\mathrm{CO}(1-0)}^{{\prime} }={L}_{\mathrm{CO}(3-2)}^{{\prime} }$. Adopting $\alpha =0.8$ (e.g., Downes & Solomon 1998), we set an upper limit on the molecular gas mass of ${M}_{{{\rm{H}}}_{2}}\lt 1.2\times {10}^{10}$ ${M}_{\odot }$.

The limiting luminosity of the [C i] line is ${L}_{[{\rm{C}}{\rm{I}}]}\lt 7.8\,\times {10}^{7}$ ${L}_{\odot }$. With a measured [C ii] luminosity of ${L}_{[{\rm{C}}{\rm{II}}]}=(1.6\,\pm 0.2)\times {10}^{9}$ ${L}_{\odot }$ (Section 3.3), we can set a lower limit to the [C ii]-to-[C i] luminosity ratio of ${L}_{[{\rm{C}}{\rm{II}}]}/{L}_{[{\rm{C}}{\rm{I}}]}\gt 18$. Following Venemans et al. (2017), we can compare this luminosity ratio to those predicted by the ISM models of Meijerink et al. (2007). From the measured luminosity ratio we can exclude that the line emission originates from a region where the X-ray radiation from the accreting black hole is dominating the emission.

From the velocity dispersion σ of the [C ii] emission and the radius R of the line emitting region, we can estimate a dynamical mass of the quasar host galaxy by utilizing the virial theorem: ${M}_{\mathrm{dyn}}=3R{\sigma }^{2}/2G$ with G as the gravitational constant. Assuming that the velocity dispersion can be derived from the Gaussian fit to the [C ii] emission (Figure 1), and adopting a maximum radius of the [C ii] emission of $R\lt 3.5\,\mathrm{kpc}$ (Section 3.3), we infer a dynamical mass ${M}_{\mathrm{dyn}}\lt 3.2\times {10}^{10}$ ${M}_{\odot }$. If instead we assume that the [C ii] emission is in a rotating disk with inclination angle i (e.g., Wang et al. 2013; Willott et al. 2015; Venemans et al. 2016), we derive a higher dynamical mass of ${M}_{\mathrm{dyn}}\lt 1.0\times {10}^{11}/{\sin }^{2}(i)$ ${M}_{\odot }$. Adopting $i=55^\circ$, the median inclination angle of $z\sim 6$ quasar hosts (Wang et al. 2013), the dynamical mass of J1342+0928 becomes ${M}_{\mathrm{dyn}}\lt 1.5\,\times {10}^{11}$ ${M}_{\odot }$, which is ≲190× higher that of the black hole (Bañados et al. 2017). To more accurately constrain the dynamical mass, high spatially resolved observations of the [C ii] emission are necessary.