ALMA [N ii] 205 μm Imaging Spectroscopy of the Interacting Galaxy System BRI 1202-0725 at Redshift 4.7^*

Star formation rate (SFR) measures the fundamental physical process of transforming gas into stars and is one of the most important drivers of galaxy evolution. For high-z galaxies, Lu et al. (2015, hereafter Lu15) explored a new dual-spectral line approach for estimating both SFR and the far-infrared (FIR) color C(60/100) (thus, T_dust), where C(60/100) refers to the rest-frame f_ν(60 μm)/f_ν(100 μm) ratio. For local (ultra-)luminous infrared galaxies ((U)LIRGs), the luminosity of the CO(7−6) line emission, L_CO(7−6), can be used to infer the SFR of the galaxy with a ∼30% accuracy, irrespective of whether the galaxy hosts an active galactic nucleus (AGN; Lu et al. 2014, 2017; Lu15). Furthermore, by measuring the flux of the [N ii] 205 μm line (1461.13 GHz; hereafter [N ii]), one can use the steep dependence of the [N ii] to CO(7−6) flux ratio on C(60/100) to estimate C(60/100) or T_dust, with an accuracy equivalent to ∼2 K in T_dust if the dust emissivity power-law index β is around 2 (Lu15). C(60/100) is empirically related to Σ_SFR, the average SFR surface density (Liu et al. 2015; Lutz et al. 2016), another fundamental parameter of galaxy SF. This indirect estimate of Σ_SFR is useful at high z, where it is often challenging to spatially resolve a galaxy. This dual-line strategy also allows for estimating additional galaxy physical parameters, including the SF area (≈SFR/Σ_SFR), the molecular gas mass (M_gas) from the continuum flux underlying the CO(7−6) line (Scoville et al. 2016), and the gas depletion time τ_gas (=M_gas/SFR). If the lines are sufficiently resolved spectrally and spatially, insights into the gas dynamics can also be gained.

In a Cycle-3 program with the Atacama Large Millimeter/submillimeter Array (ALMA), we conducted a spectral line snapshot survey of eight ULIRGs and four LIRGs of 4 < z ≲ 5.5 to complete their detections in [N ii] and CO(7−6). All our targets have prior detections in the [C ii] 158 μm line (hereafter [C ii]). In this Letter, we present the results from our [N ii] observation of the interacting galaxy group BRI 1202-0725 at z = 4.7 (Isaak et al. 1994). This un-lensed system consists of two ULIRGs: a QSO at z = 4.695 and an optically obscured, submillimeter galaxy (SMG) at z = 4692 and 38 (∼25 kpc) northwest of the QSO (Hu et al. 1996; Ohta et al. 1996; Omont et al. 1996; Yun et al. 2000; Carilli et al. 2002; Yun & Carilli 2002; Momjian et al. 2005; Iono et al. 2006; Salomé et al. 2012). In addition, two Lyα emitting galaxies (LAEs) have been detected in this system, with the first one (LAE1) located between the QSO and SMG, at ∼23 northwest of the QSO and the second one (LAE2) at 27 southwest of the QSO (Fontana et al. 1996; Hu et al. 1996; Ohta et al. 2000; Ohyama et al. 2004).

The [C ii] emission was detected with ALMA on all the member galaxies, along with a possible extended gas ridge between the QSO and SMG (Wagg et al. 2012; Carilli et al. 2013). Both QSO and SMG were also detected in CO(7−6) and CO(5−4) (Omont et al. 1996; Salomé et al. 2012). Decarli et al. (2014) presented the [N ii] spectra of the system obtained with the IRAM interferometer and suggested possible [N ii] detections for the SMG and LAEs, but a non-detection for the QSO. However, the [N ii] line peak flux densities of both QSO and SMG (see Figure 2) are either near or below the spectral noise in Decarli et al. This might explain the flux discrepancy between ours and theirs. Pavesi et al. (2016) analyzed an archival ALMA observation and derived the [N ii] fluxes of 0.74 (±0.07), 1.5 (±0.2), and 0.30 (±0.06) Jy km s⁻¹ for the QSO, SMG, and LAE2, respectively, and a 3σ flux density upper limit of 0.5 mJy for LAE1.

The [N ii] line is a major cooling line for ionized gas in galaxies. This line has only been detected in a handful of galaxies at z ≳ 4 (e.g., Combes et al. 2012; Decarli et al. 2012; Nagao et al. 2012; Rawle et al. 2014; Béthermin et al. 2016; Pavesi et al. 2016). Our ALMA [N ii] observation of BRI 1202-0725 detected both SMG and QSO at good S/N. In Section 2, we describe our observations and results. In Section 3, we analyze the observed [N ii] and dust continuum emissions, and discuss the SF properties and [N ii]/[C ii] flux ratios of the galaxies in BRI 1202-0725. We use a flat cosmology with Ω_M = 0.27, Ω_Λ = 0.73, and H₀ = 71 km s⁻¹ Mpc⁻¹. At z = 4.694, the luminosity distance is 44,172 Mpc, and 1'' corresponds to 6.6 kpc.

BRI 1202-0725 was observed in ALMA Band 6 in the time division mode. Of the four spectral windows (SPWs), each of 1875 MHz wide, one was targeted at the redshifted [N ii] line (at ∼256.6 GHz) and the other three, centered at 254.6, 240.6, and 238.6 GHz, respectively, were used for continuum measurements. Each SPW has 128 channels with an effective resolution of 31.25 MHz (∼36.5 km s⁻¹). The observation consisted of two independent executions, each with a 20.3 minute on-target integration. The first was executed on 2016 January 8 using 37 antennas covering baselines from 15.1 to 310.2 m and the second on 2016 March 13 using 38 antennas and baselines ranging from 15.1 to 460.0 m. The phase, bandpass, and flux calibrations were based on the quasars J1229+0230 and J1159−0940. The data reduction was carried out with the Common Astronomy Software Applications 4.5.3, and the final images were cleaned using the natural weighting, resulting in a synthesized beam of 10 × 08 (FWHM) at a position angle (PA) of 75° (N to E) for both continuum and line data. The rms noise in the continuum image, which is the average of the 3 continuum SPWs, is ∼34 μJy beam⁻¹. The final [N ii] spectral cube has a velocity channel width of 100 km s⁻¹, with an rms noise of ∼0.3 mJy beam⁻¹ in individual channels.

We compare in Figure 1 the continuum image and the [N ii] image integrated over ν_obs = 256.148–257.090 GHz, which encompasses all and the vast majority of the [N ii] fluxes from QSO and SMG, respectively. While the QSO, SMG, and LAE2 are all detected in the continuum, only the QSO and SMG are clearly detected in [N ii]. There appears to exist some faint (up to 3σ or 0.1 mJy beam⁻¹) dust emission connecting the QSO and SMG, with a morphology similar to what is seen in [C ii] (Carilli et al. 2013). However, its counterpart is not detected in the [N ii] image. The outer [N ii] emission contours (up to 3σ levels) of both QSO and SMG show distortions. To gain a better insight into this, we also cleaned the data using the Briggs weighting (with robust = 0) to lower the sidelobes and to effectively enhance the resolution to 07 × 06 (PA ≈ 78 °). The results are shown in the insert in each panel in Figure 1 for an image section covering our targets. While the [N ii] emission of the SMG remains in a similar morphology, that of the QSO is marginally resolved into three peaks: (i) the brightest one aligns with the peak dust emission; (ii) the second one (12^h05^m2309, −7°42'332) is ∼07 southwest of (i), in the direction of LAE2; (iii) the third one (12^h05^m2306, −7°42'325) is ∼1'' northwest of (i), with a peak surface brightness just under 3σ. While (i) and (iii) have similar line central velocities, (ii) is blueshifted by ∼200 km s⁻¹ relative to (i). Contributing to ∼10% of the total [N ii] flux of the QSO, (iii) is the main cause of the distorted contours seen. The relative positions of (ii) and (iii) are suggestive of tidal interaction or gas connection between the member galaxies.

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In this section, we analyze the [N ii] and continuum emissions and derive the parameters in Table 1 using the data cleaned with the natural weighting.

Table 1.  BRI 1202-0725: Observed and Derived Parameters^a

Parameter	Unit	QSO	SMG	LAE2	LAE1
Continuum:
R.A.b	J2000	12h05m2313	12h05m2298	12h05m2304	...
Decl.b	J2000	−7°42'328	−7°42'297	−7°42'345	...
Sν(256 GHz)c	mJy	7.58 (±0.01)	7.20 (±0.01)	0.83 ± 0.01	<0.10
Gaussian fitc	...	(108 × 089, 73°)	(108 × 087, 75°)	(120 × 098, 122°)	...
[N ii]:
Apertured	...	(3'' × 165, 90°)	(265 × 16, 0°)	(16 × 16, 0°)	(1'' × 1'', 0°)
ze	...	4.695	4.693	...	...
Fluxf	Jy km s−1	1.01 (±0.02)	0.99 (±0.02)	<0.13	<0.02
FWHMg	km s−1	380	794	...	...
Diameterh	kpc	9	14	...	...
Derived parameters:
LCO(7−6)i	108 L⊙	6.6 (±0.6)	8.9 (±1.2)	...	...
L[C ii]j	109 L⊙	6.5 (±1.0)	10.0 (±1.5)	...	...
L[N ii]	108 L⊙	5.3 (±0.1)	5.2 (±0.1)	...	...
Tdustk	K	43 (±2)	43 (±2)	...	...
LFIRl	1013 L⊙	2.3 (±18%)	2.4 (±18%)	...	...
LIRm	1013 L⊙	5.0 (±30%)	6.7 (±30%)	...	...
SFRm	103 M⊙ yr−1	5.1 (±30%)	6.9 (±30%)	...	...
ΣSFRn	102 M⊙ yr−1 kpc−2	16/21	18/21	...	...
SF diametero	kpc	1.3	1.5	...	...
${M}_{{{\rm{H}}}_{2}}$ p	1011 M⊙	5	5	...	...
τgasq	107 years	10	7	...	...

Notes.

^aAll SFRs are consistent with Equation (4) in Kennicutt (1998), but scaled to Chabrier IMF. The ALMA flux uncertainties cited do not include the absolute calibration uncertainty. ^bFrom the Gaussian fit. ^cContinuum flux density from the Gaussian fit of which the major and minor FWHM axes and the major axis PA are given. The 3σ flux upper limit for LAE1 assumes an unresolved case. ^dAperture for the 1d spectrum extraction: major × minor axes, followed by the major axis PA. ^eRedshift (in LSR) from the peak frequency of the Gaussian spectral line fit. ^fFlux from the Gaussian line fit. The upper limit for LAE2 was derived using a Gaussian profile with a peak flux density of 3σ and a FWHM equal to that of the [C ii] emission in Carilli et al. (2013). For LAE1, which has a narrow line width, this is simply 3σ times 100 km s⁻¹. Here, σ is the channel-to-channel noise in the spectrum in Figure 2. ^gFWHM of the Gaussian line fit. ^hMajor axis of the Gaussian fit to the [N ii] image in Figure 1, after deconvolution with the ALMA beam. ⁱBased on the line fluxes in Salomé et al. (2012). ^jBased on the line fluxes in Wagg et al. (2012). ^kAdopted dust temperature. ^lThe 42–122 μm luminosity from the FIR dust SED fit. The uncertainty corresponds to a variation of ±2 K in T_dust. ^mInferred from L_CO(7−6). ⁿTwo estimates on Σ_SFR: the first one from C(60/100) (Liu et al. 2015) and the second one from the 70-to-160 μm color of the SED fit (Lutz et al. 2016). ^oFWHM of the SF region, assuming Σ_SFR = 2000 M_⊙ yr⁻¹ kpc⁻². ^pMolecular gas mass inferred from the rest-frame 850 μm luminosity of the SED fit. ^qGas depletion time.

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3.1. [N ii] Line Emission

In Figure 2, we show the [N ii] spectra extracted within the elliptical apertures defined in Table 1. For the QSO and SMG, we fit to their spectrum a single Gaussian profile plus a constant to derive the line flux. The resulting [N ii] line widths are similar to or slightly wider than those of the corresponding CO and [C ii] lines (Salomé et al. 2012; Carilli et al. 2013). For the LAEs, the frequency location of the [N ii] peak emission is marked in Figure 2, based on the [C ii] redshift (Carilli et al. 2013). Neither LAE is detected here.

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Both SMG and QSO are spatially resolved in our [N ii] image. A 2d Gaussian fit to the [N ii] image of the SMG (Figure 1(b)) yielded 23 × 11 (FWHM along each axis), with the major axis at PA ≈ 12°. For the QSO, these values are (16 × 09, PA ≈ 91°). After a deconvolution with the ALMA beam, they become (21 × 05, PA ≈ 10°) and (13 × 04, PA ≈ 95°) for the SMG and QSO, respectively. The de-convolved major axes correspond to ∼14 and 9 kpc for the SMG and QSO, respectively. Both QSO and SMG are unresolved in the continuum (see Section 3.2) and also appear to be unresolved in [C ii] in Carilli et al. (2013) beam size 12 × 08). Therefore, the [N ii] emission is much more extended (or diffuse) than the dust or [C ii] emission in these galaxies.

For the QSO, the [N ii] channel images reveal that the most blueshifted (redshifted) emission is located at the southwestern (northeastern) side of the galaxy, consistent with a rotation. For the SMG, the [N ii] channel images do not support a position–velocity pattern consistent with a rotation. These are all consistent with what is seen in CO(5−4) and [C ii] (Salomé et al. 2012; Carilli et al. 2013).

3.2. Dust Continuum

We derive the total continuum flux density of a galaxy by fitting a 2d Gaussian to the galaxy image in Figure 1(a) after temporarily masking out the other galaxies. The resulting Gaussian FWHMs (in Table 1) confirm that both QSO and SMG are unresolved. (In fact, the insert in Figure 1(a) shows that, even at the enhanced resolution of ∼07, they are still unresolved with the following fitted Gaussian FWHMs: 073 × 059 (PA = 76°) for the QSO and 075 × 058 (PA = 89°) for the SMG.) Table 1 shows that LAE2 is largely resolved.

In Figure 3, we plot the [C ii]/CO(7−6) and [N ii]/CO(7−6) flux ratios, each as a function of C(60/100), for the local (U)LIRGs and high-z galaxies used in Lu15. The corresponding flux ratios of the QSO and SMG in BRI 1202-0725 are indicated by the horizontal lines. The black line in each plot is a least-squares bisector fit (Isobe et al. 1990) to the local (U)LIRGs of detections only, excluding the AGNs (see Figure 3 caption). For the SMG, the corresponding line flux ratio intercepts the black line at C(60/100) ≈ 1.19 in (a) and ≈1.23 in (b). The difference between these values is smaller than the scatter (0.15–0.2) in these plots. We therefore simply adopt their average value of 1.21. For the QSO, such determined C(60/100) values are 1.22 and 1.15, respectively. Even though there is some apparent segregation between the AGNs and the rest of the local (U)LIRGs in Figure 3(b), the AGN sample size is still too small to draw a firm conclusion on this. We therefore also adopt the average C(60/100) = 1.19 for the QSO. We set the uncertainty of these FIR colors to the horizontal scatter w.r.t. the black line in the [N ii] plot, i.e., ∼0.15. Assuming β = 1.8 (Planck Collaboration et al. 2011), these adopted FIR colors correspond to T_dust ≈ 43 (±2)K.

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The QSO and SMG are only spatially separated by a few existing interferometric observations, all at the long-wavelength side of their SED peak (see Figure 4). The T_dust determined above provides a crucial constraint on the SED shape. Our best modified-blackbody SED fits for the QSO and SMG separately, after fixing T_dust at 43 K and β at 1.8, are also shown in Figure 4. We note that similar SED fits would be obtained by using only the continuum flux from the CO(7−6) observation and our adopted C(60/100). The luminosity of the SED fit integrated over 20–1000 μm is 2.9 (3.0) × 10¹³ L_⊙ for the QSO (SMG). These values are 3–5 times larger than those in Salomé et al. (2012). The L_FIR (over 42−122 μm) from our SED fit is 2.3 $(2.4)\times {10}^{13}\,{L}_{\odot }$, resulting in a $\mathrm{log}{L}_{\mathrm{CO}(7-6)}/{L}_{\mathrm{FIR}}\approx -4.54$ (−4.42) for the QSO (SMG). These values agree with the average of −4.61 for our local (U)LIRG sample to within ∼1.5σ, where σ (≈0.12) is the local sample standard deviation (see Lu15).

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3.3. Star Formation Properties

3.4. On [N ii]/[C ii] Ratios

This Letter benefited from a number of thoughtful comments made by the anonymous referee. This Letter makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00388.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This work is supported in part by the NSFC grants 11673028, 11673057. and 11420101002. T.D.S., G.C.P., and D.R. acknowledge ALMA-CONICYT project 31130005 and FONDECYT project 1151239, FONDECYT fellowship #3150361, and grant ST/N000919/1, respectively.