IONIZED NITROGEN AT HIGH REDSHIFT

Forbidden atomic fine-structure transitions are important cooling lines of the interstellar medium (ISM). They provide effective cooling in cold regions where allowed atomic transitions cannot be excited, and thus are critical diagnostic tools to study the star-forming ISM. Perhaps the most important cooling line is the forbidden ²P_3/2 → ²P_1/2 fine-structure line of ionized carbon ([C ii]) at 158 μm, which alone accounts for 0.1%–1% of the total continuum far-infrared (FIR) luminosity in local, star-forming galaxies (see, e.g., Malhotra et al. 2001). Other main cooling atomic lines are the oxygen [O i] (63 μm and 145 μm) and [O iii] (52 μm and 88 μm) lines, as well as the nitrogen [N ii] lines at 122 μm and 205 μm.

As the ionization potential of carbon is 11.3 eV (hydrogen: 13.6 eV), [C ii] is a tracer for both the neutral atomic and ionized medium, predominantly of photon-dominated regions. The ionization potentials for oxygen and nitrogen, on the other hand, are 13.6 eV and 14.5 eV, respectively, implying that their ions trace the ionized medium. The [N ii]_205 μm transition is of particular interest as it has a critical density in the ionized medium that is very close to that of [C ii], thus potentially providing complementary information on the origin of the [C ii] emission (e.g., Oberst et al. 2006; Walter et al. 2009b).

In studies of star formation at high-z, the need for diagnostics other than [C ii] is also motivated by two other reasons: (1) the relative intensities of [C ii] and [N ii]_205 μm are susceptible to N/C abundance variations (e.g., Matteucci & Padovani 1993), thus acting as diagnostics of metal enrichment in the first galaxies. (2) The [C ii] line falls out of the 1.3 mm window at z ≈ 8 and will not be observable at z up to 10.2 with ALMA, as no 2 mm receiver (ALMA "band 5") will be available for a large number of antennas. This limits [C ii] studies in the first galaxies observable at the beginning of cosmic reionization.

Whereas the [C ii] line has now been abundantly detected in the local universe (e.g., Stacey et al. 1991; Madden et al. 1997; Luhman et al. 1998; Malhotra et al. 2001; Beirão et al. 2010; Edge et al. 2010; Fischer et al. 2010; Ivison et al. 2010; Loenen et al. 2010) and is now (almost) routinely detected at high redshift (Maiolino et al. 2005, 2009; Iono et al. 2006; Walter et al. 2009a; Wagg et al. 2010; Stacey et al. 2010; Cox et al. 2011; De Breuck et al. 2011), measurements of the [N ii]_205 μm line are scarce. The [N ii]_205 μm line was first detected by FIRAS aboard COBE in the Milky Way (Wright et al. 1991), and later in the Galactic H ii regions G333.6–0.2 Colgan et al. (1993) and DR21 (White et al. 2010) and in the Carina Nebula (Oberst et al. 2006). The [N ii]_205 μm line has also been recently detected in some local galaxies, e.g., NGC 1097 (Beirão et al. 2010; see also Garciá-Carpio et al. 2011). On the other hand, little is known about nitrogen at high redshift. Multiple attempts were performed to detect this line at high redshift (4C41.17 and PC2047+0123: Ivison & Harrison 1996; Cloverleaf: Benford 1999; APM 08279: Krips et al. 2007; J1148+5251: Walter et al. 2009b; SDP.81: Valtchanov et al. 2011). However, all these measurements, except the one on the z = 6.42 quasar J1148+5251, were too insensitive by about an order of magnitude to potentially detect the [N ii]_205 μm line. Very recently, Ferkinhoff et al. (2011) reported the first detection of the second line of ionized nitrogen, [N ii]_122 μm, in two z ∼ 2.7 targets, SMMJ02399-0136 (z = 2.81) and the Cloverleaf QSO (z = 2.56). Bradford et al. (2011) published a tentative (1.5σ) detection of [N ii]_205 μm in the lensed quasar APM 08279+5255 (z = 3.911), and Combes et al. (2012) reported the detection of the [N ii]_205 μm line in a lensed submillimeter galaxy (SMG), HLSJ091828.6+514223, at z = 5.2.

Here we present secure detections of the [N ii]_205 μm fine-structure line in two high-z sources, the lensed quasar APM 08279+5255 (z = 3.911) and the SMG MM 18423+5938 (z = 3.930). These sources are strongly magnified by gravitational lensing (magnification factor μ = 4–100 for APM 08279+5255 and 12–20 for MM 18423+5938; see Egami et al. 2000; Riechers et al. 2009; Lestrade et al. 2011), and represent two of the brightest molecular emitters at this redshift, with CO(6–5) peak flux densities of 7.3 mJy (Weiß et al. 2007) and 33 mJy (Lestrade et al. 2010), respectively. Observations were carried out at the IRAM Plateau de Bure Interferometer (PdBI) and are summarized in Section 2. Results are presented in Section 3.

Throughout the paper we will assume a standard cosmology with H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_m = 0.3, and Ω_Λ = 0.7.

In our observations we exploited the capabilities of the new "band 4" receiver at PdBI. Covering the frequency range between 277 and 371 GHz (with a small gap due to atmospheric absorption at roughly 320–330 GHz), this band opens up the opportunity to search for the [N ii]_205 μm line in a wide redshift range (2.85 < z < 4.27).

APM 08279+5255 was observed in compact array configuration with five antennas (6Cq-E10) on 2011 March 8. Baselines ranged between 20 and 140 m. 3C273, 3C84, and MWC349 were used as amplitude calibrators. The quasar 0917+624 was observed every 30 minutes for phase calibration. The time on sources was 4.5 hr (five-antenna equivalent). MM 18423+5938 was observed in both compact and extended configurations (6Cq-E10 and 6Bq) between 2011 January 3 and 2011 March 9. Baselines ranged between 17 and 446 m. 3C273, 3C345, MWC349, and 3C84 were used as amplitude calibrators, while 1849+670 was observed as phase calibrator. The total time on source was 10 hr (six-antenna equivalent).

The tuning frequencies were 297.522 and 296.400 GHz, respectively, based on the CO redshift of the sources (Weiß et al. 2007; Lestrade et al. 2010, 2011). The receiver worked in lower-side band. System temperature ranged between 150 and 350 K. Data reduction and analysis were performed using the most recent version of the GILDAS package. Maps were extracted using natural weighting. This allows us to fully recover the flux of our sources, given that their spatial extent (≲ 2'') is comparable with or smaller than the angular scale filtered in by the smallest baselines in our observations (∼2''). The resulting synthesized beams are 12 × 09 for APM 08279+5255 and 10 × 09 for MM 18423+5938. In order to take full advantage of the high spatial resolution observations of MM 18423+5938, we also extracted a pure continuum map of this source using uniform weighting. This high-resolution map has a synthetic beam size of 06 × 05, at the price of filtering out a significant fraction (∼66%) of the flux from the extended emission of the object. Therefore, in the remainder of the analysis, all the flux measurements will refer only to the map obtained with natural weighting. The 1σ noise per 20 MHz wide channel (≈20.2 km s⁻¹) is 3.4 mJy for APM 08279+5255 and 2.4 mJy for MM 18423+5938, corresponding to a 1σ sensitivity of 0.66 mJy beam⁻¹ and 0.82 mJy beam⁻¹ over the line width (see Section 3).

3.1. [N ii]_205 μm and Continuum Emission

3.1.1. APM 08279+5255

Figure 1 shows the observed spectrum and the (continuum-subtracted) line map of APM 08279+5255. The [N ii]_205 μm line is detected at modest significance. We fitted the spectrum with a flat continuum plus a Gaussian profile for the [N ii]_205 μm emission. The fitted [N ii]_205 μm flux is S([N ii]_205 μm) = 4.8 ± 0.8 Jy km s⁻¹, consistent with the tentative detection (7.6 ± 5.4 Jy km s⁻¹) reported in Bradford et al. (2011). We measure a line width of 570 ± 110 km s⁻¹, consistent with the weighted average of the line width values from CO transitions reported in Weiß et al. (2007). Line luminosities L_c and L' are derived following Solomon et al. (1992), and reported in Table 1 together with all the relevant numbers and fitted parameters. We measure a continuum flux of 33 ± 3 mJy, in agreement with the extrapolation between the SCUBA and PdBI observations at 850 μm and 1.4 mm, respectively (Weiß et al. 2007).

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Table 1. Line and Continuum Properties in APM 08279+5255 and MM 18423+5938

Quantity	Units	APM 08279+5255	MM 18423+5938	References
Redshift		3.911	3.930	1,2
DL	(Gpc)	34.897	35.097
μ		4–100	12–20	3,4,2
νobs	(GHz)	297.522	296.400	0
1σ rms (20 MHz)	(mJy beam−1)	3.4	2.4	0
[N ii]205 μm
S([N ii]205 μm)	(Jy km s−1)	4.8 ± 0.8	7.4 ± 0.5	0
FWHM	(km s−1)	570 ± 110	230 ± 20	0
Lc([N ii]205 μm)	(109μ−1 L☉)	1.8 ± 0.3	2.8 ± 0.2	0
L'([N ii]205 μm)	(1010μ−1 K km s−1 pc2)	1.81 ± 0.3	2.8 ± 0.2	0
Lc([N ii]205 μm)/LFIR	(10−6)	9.0 ± 1.5	5.8 ± 0.4	0
M(H ii)	(109μ−1 M☉)	≳ 4.1	≳ 6.4	0
M(H ii)/M(H2)		≳ 0.8%	≳ 2.9%	0
Other lines
Lc([C i]1 − 0)	(109μ−1 L☉)	0.118 ± 0.016	0.29 ± 0.06	5,2
Lc([C i]2 − 1)	(109μ−1 L☉)	<0.23	0.88 ± 0.17	6,2
Lc(CO(1–0))	(109μ−1 L☉)	0.0050 ± 0.0004	0.0129 ± 0.0017	3,7
Lc(CO(2–1))	(109μ−1 L☉)	0.048 ± 0.011	0.179 ± 0.018	3,7
Lc(CO(3–2))	(109μ−1 L☉)	0.196 ± 0.018	...	8
Lc(CO(4–3))	(109μ−1 L☉)	0.44 ± 0.02	0.59 ± 0.06	1,2
Lc(CO(6–5))	(109μ−1 L☉)	1.2 ± 0.2	1.19 ± 0.11	1,2
Lc(CO(7–6))	(109μ−1 L☉)	...	0.82 ± 0.10	2
Lc(CO(9–8))	(109μ−1 L☉)	3.16 ± 0.16	...	1
Lc(CO(10–9))	(109μ−1 L☉)	3.5 ± 0.6	...	1
Lc(CO(11–10))	(109μ−1 L☉)	3.7 ± 0.6	...	1
Lc(CO(12–11))	(109μ−1 L☉)	3.5 ± 1.8	...	9
Lc(SO2)	(109μ−1 L☉)	...	0.60 ± 0.16	0
M(H2)	(1011μ−1 M☉)	5.3 ± 0.5	2.2 ± 0.3	3,7
Continuum
S(850 μm)	(mJy)	75 ± 4	...	1
S(1 mm)	(mJy)	33 ± 3	53 ± 2	0
S(1.3 mm)	(mJy)	16.9 ± 2.5	30 ± 2	1,2
LFIR	(1013μ−1 L☉)	20	48	1,2

References. (0) This work; (1) Weiß et al. (2007); (2) Lestrade et al. (2010); (3) Riechers et al. (2009); (4) Egami et al. (2000); (5) Wagg et al. (2006); (6) Walter et al. (2011); (7) Lestrade et al. (2011); (8) Downes et al. (1999); (9) Bradford et al. (2011).

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3.1.2. MM 18423+5938

In Figure 2, we show the integrated spectrum of the [N ii]_205 μm emission in MM 18423+5938. The line is detected at very high significance. The Gaussian fit gives an integrated [N ii]_205 μm flux of S([N ii]_205 μm) = 7.4 ± 0.5 Jy km s⁻¹ and a line width of 230 ± 20 km s⁻¹ (for a comparison, the CO(1–0) and CO(2–1) lines reported in Lestrade et al. 2011 have widths of 160 ± 30 and 240 ± 30 km s⁻¹, respectively). Another line is tentatively detected at 298.23 ± 0.02 GHz, i.e., at rest frequency 1470.22 ± 0.11 GHz, consistent with three different transitions of sulfur dioxide, SO₂ (at 1470.225, 1470.327, and 1470.342 GHz, respectively). Given the low signal-to-noise ratio (S/N) of this line (∼3.5σ), we fitted it with a Gaussian by imposing the same line width as observed for [N ii]_205 μm. We find a continuum flux (integrated over the spatial extension of the emission) of 53 ± 2 mJy, consistent with the 1.2 mm MAMBO flux reported in Lestrade et al. (2010), assuming a graybody dust with opacity index β = 1.

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Our PdBI observations clearly resolved the 1 mm continuum emission of MM 18423+5938. The Einstein ring reported in Lestrade et al. (2011) is clearly seen in the pure-continuum, high-resolution map shown in Figure 3 (top-left panel). Also the [N ii]_205 μm emission appears clearly extended even in the lower resolution maps shown in the top-right panel of Figure 3. The [N ii]_205 μm line emission shows a clear velocity gradient from northeast (redshifted) to southwest (blueshifted). This is highlighted in Figure 3 (bottom-left panel) where we overplot the maps of the red and blue wings of the [N ii]_205 μm line. We measure peak-to-peak velocity differences of ∼180 km s⁻¹ and a velocity dispersion ∼80 km s⁻¹. The v/σ > 1 value indicates ordered kinematics for the ionized gas in the lensed source. A detailed model of the lens is required to reconstruct the intrinsic brightness of the source from these images and to properly constrain the dynamics of the system. This is beyond the scope of this paper.

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3.2. [N ii]_205 μm and CO

Here, we compare our [N ii]_205 μm observations with the available literature data on CO emission in the two targets. In particular, the [N ii]_205 μm to CO(6–5) luminosity ratio in our sources ([N ii]_205 μm/CO(6–5)=1.5 ± 0.4 in APM 08279+5255 and 2.3 ± 0.3 in MM 18423+5938) is similar to the values reported by White et al. (2010) in the Galactic region DR21 ([N ii]_205 μm/CO(6–5) = 1.26 ± 0.35), by Panuzzo et al. (2010) in M82 ([N ii]_205 μm/CO(6–5) = 1.68  ±  0.05) and by van der Werf et al. (2010) in Mrk231 ([N ii]_205 μm/CO(6–5) ∼ 1.2).

In Figure 3 (bottom-right panel) we compare our [N ii]_205 μm map of MM 18423+5938 with our map of the 1 mm continuum emission and the CO(2–1) observations presented in Lestrade et al. (2011), which have similar S/N and spatial resolution as the [N ii]_205 μm data presented here. Since nitrogen emission is tracing the ionized gas, while the CO emission maps the molecular gas (which is the fuel of star formation) and the FIR continuum traces the distribution of dust heated by young stars, this comparison allows for a direct, spatially resolved study of three important components of the ISM that are related to star formation. From the figure, it is apparent that the emission from the three tracers shows different morphologies (e.g., most of [N ii]_205 μm and CO(2–1) emission arises in two blobs in the east and southwest parts of the Einstein ring, while a bright continuum emission is observed also on the northern side of the ring).

More quantitatively, in Figure 4 we perform a pixel-by-pixel comparison of the emission of the FIR continuum with respect to the [N ii]_205 μm and CO(2–1) lines.¹⁶ In order to avoid oversampling, we consistently rebinned all the maps into 05 × 05 pixels, and considered only pixels with >1σ flux in each axis. We find that the surface brightness of the continuum, Σ_FIR, shows a steep correlation with the surface brightnesses of [N ii]_205 μm and CO(2–1), Σ_[N ii] and Σ_CO: Σ_FIR ∝ Σ^1.6±0.1_[N ii] and Σ_FIR∝Σ^{1.4 ± 0.2}_CO. The latter relation is of particular interest, as Σ_FIR can be used as a proxy for the star formation surface density, Σ_SFR (Kennicutt 1998), while Σ_CO traces the surface density of the molecular gas, $\Sigma _{\rm H_2}$, which is the fuel for star formation. The right-hand panel of Figure 4 represents therefore the first spatially resolved study of the star formation law in a high-z galaxy.

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Locally, star formation surface density scales, to first order, linearly with the molecular gas surface density (Σ_SFR∝Σ^N_H2, with N ≈ 1; see, e.g., Bigiel et al. 2008; Leroy et al. 2008; Schruba et al. 2011). The relation steepens if one considers high-density environments and molecule-rich galaxies (e.g., Kennicutt 1998; Wong & Blitz 2002; Daddi et al. 2010; Genzel et al. 2011). This appears to be the case for MM 18423+5938, where we measure a slope significantly larger than 1 (N = 1.4 ± 0.2). The steep relation observed between Σ_FIR and Σ_[N ii] may indicate that, as star formation rate (SFR) increases, the ionization state of nitrogen changes, with an increasing fraction of multiply ionized N in the regions of most intense star formation. Moreover, if we divide the Einstein ring of MM 18423+5938 into two parts, north and south, with a cut at declination +59:38:29.3 (roughly corresponding to the center of the ring), we find that each part of the ring follows different power laws. The northern part of the ring has a ∼2 times brighter continuum for a given [N ii]_205 μm or CO(2–1) emission. The Σ_FIR∝Σ^N_CO relation shows a marginally flatter slope (N = 1.2) in the northern part than in the southern part (N = 1.6), in agreement with the relatively higher molecular content in the southern part of the ring.

3.3. Ionized and Molecular Gas Masses

Following Ferkinhoff et al. (2011), we can compute the minimum mass of ionized hydrogen in the high-density, high-temperature limit, assuming that all nitrogen in the H ii regions is singly ionized:

$$\begin{eqnarray} M_{\rm min}({\rm H\,\mathsc{ii}})&=&\frac{L({\rm [{\rm N}\,\mathsc{ii}]_{205 \,\mu {\rm m}}}) \, m_{\rm H}}{(g_1 /g) \, A_{10} \, h \nu _{10} \, \chi ({\rm N}^+)}\nonumber\\ \ms &\approx & 2.27 \, \frac{L({[{\rm N}\,\mathsc{ii}]_{205 \,\mu {\rm m}}})}{{L_\odot }}\, {M_\odot }, \end{eqnarray} \tag{ 1 }$$

where A₁₀ is the Einstein A coefficient of the ³P₁ → ³P₀ transition of nitrogen (2.08 × 10⁻⁶ s⁻¹); g₁ = 3 is the statistical weight of the J = 1 level; g = Σ_ig_iexp (− ΔE_i/k_bT) is the partition function, with ΔE_i being the energy difference between the fundamental and the ith level, k_b being the Boltzmann constant, and T  being the gas temperature; h is Planck's constant; ν₁₀ = 1461.1318 GHz is the rest-frame frequency of the transition; m_H is the mass of a hydrogen atom; and χ(N⁺) is the N⁺/H⁺ abundance ratio. In our working assumption, χ(N⁺) = χ(N) ≈9.3 × 10⁻⁵ (Savage & Sembach 1996). This gives M_min(H ii) = 4.1 × 10⁹ μ⁻¹ M_☉ and 6.4 × 10⁹ μ⁻¹ M_☉ for APM 08279+5255 and MM 18423+5938, respectively. It is interesting to compare these numbers with the molecular gas mass estimated from the CO(1–0) luminosity (Riechers et al. 2009; Lestrade et al. 2011): the minimum mass of the ionized gas is only a tiny fraction (0.8% and 2.9%, respectively) of the molecular reservoir in the two targets. Alternatively, the actual N⁺/H⁺ abundance ratio may be significantly lower than the adopted value. This is likely to happen if these systems are metal poor (but this scenario is ruled out by the bright CO/FIR luminosity ratios observed in our targets), or if the ISM is enshrouded in a hard radiation field (i.e., nitrogen is multiply ionized). This is likely happening in APM 08279+5255, which hosts a quasar (while no obvious signature of nuclear activity is observed in MM 18423+5938). Indeed, Ferkinhoff et al. (2010) estimated that an ionized gas mass of ∼3 × 10⁹ M_☉ is present in APM 08279+5255, based on the [O iii]_88 μm line emission. These observations are complementary to ours, as N is likely to be multiply ionized in the regions where most of the [O iii] emission takes place.

3.4. [N ii]_205 μm Contribution to ISM Cooling

Finally, we evaluate the role of [N ii]_205 μm in the ISM cooling by comparing the [N ii]_205 μm luminosity to that of the FIR continuum. The latter is taken from the spectral energy distribution fits by Weiß et al. (2007) for APM 08279+5255 (L_FIR = 20 × 10¹³μ⁻¹ L_☉),¹⁷ and by Lestrade et al. (2010) for MM 18423+5938 (L_FIR = 48 × 10¹³μ⁻¹ L_☉). The observed [N ii]_205 μm/FIR luminosity ratio is 9.0 × 10⁻⁶ and 5.8 × 10⁻⁶ in the two sources. In Figure 5, we compare these values with the measurements available in the literature. Since the majority of data refer to the [N ii]_122 μm transition, we converted our estimates assuming a [N ii] 122-to-205 μm luminosity ratio of 5 (see, e.g., Beirão et al. 2010). Our observations confirm and extend the decreasing trend of the [N ii]/FIR ratio as a function of the continuum luminosity, toward high luminosities from local galaxies with [N ii]_122 μm/FIR ∼ 3 ×  10⁻⁴ (Malhotra et al. 2001; Panuzzo et al. 2010; Fischer et al. 2010; Edge et al. 2010; Beirão et al. 2010; Vasta et al. 2010; Garciá-Carpio et al. 2011) to FIR-luminous sources with [N ii]_122 μm/FIR ∼ 3–5 ×  10⁻⁵. Even in the extreme case of a 122-to-205 μm ratio of 10, our data would populate a very different region of the plot with respect to the values found by Ferkinhoff et al. (2011), who reported high [N ii]_122 μm/FIR ratios (∼10⁻³) in two FIR-bright sources.

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We present a study of the ionized ISM in two high-z sources based on the forbidden ionized nitrogen emission line at 205 μm. The FIR transitions of ionized nitrogen represent extremely powerful tools to study the properties of purely ionized gas in distant galaxies. A larger number of [N ii]_205 μm detections in distant galaxies is now mandatory in order to build a suitable sample for statistical analysis. This is now possible thanks to the technological upgrades in the field. In particular, the unparalleled sensitivities reached by ALMA will open new possibilities for the studies of high-z objects.

We thank the anonymous referee for his/her expert comments that increased the quality of the manuscript. We thank B. Groves for useful discussions. This work is based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). R.D. acknowledges funding from Germany's National Research Centre for Aeronautics and Space (DLR, project FKZ 50 OR 1104). D.R. acknowledges support from NASA through a Spitzer Space Telescope grant.