---
author:
- 'M. Pereira-Santaella'
- 'E. González-Alfonso'
- 'A. Usero'
- 'S. García-Burillo'
- 'J. Martín-Pintado'
- 'L. Colina'
- 'A. Alonso-Herrero'
- 'S. Arribas'
- 'S. Cazzoli'
- 'F. Rico'
- 'D. Rigopoulou'
- 'T. Storchi Bergmann'
title: 'First detection of the 448GHz H$_2$O transition in space'
---

Introduction {#sec:intro}
============

Water is a molecule of astrophysical interest because it not only plays a central role in the Oxygen chemistry of the interstellar medium (e.g., @Hollenbach2009 [@vanDishoeck2013]) but it is also one of main coolants of shocked gas (e.g., @Flower2010). In addition, thanks to its energy level structure, water couples very well to the far-infrared (far-IR) radiation field providing an effective probe of the far-IR continuum in the warm compact regions found in active galactic nuclei (AGN) and young star-forming regions (e.g., @GonzalezAlfonso2014, hereafter ).

Water excitation models have long predicted the maser nature of the  transition pumped by collisions when the kinetic temperature is $T_{\rm kin}\sim1000\,{\rm K}$ and the hydrogen density $n_{\rm H_2}\sim10^5$cm$^{-3}$ (e.g., @Deguchi1977 [@Cooke1985; @Neufeld1991; @Yates1997; @Daniel2013; @Gray2016]). This transition can also be excited by radiative pumping through the absorption of far-IR photons (see Section \[s:model\] and Figure \[fig:levels\]). Therefore, the determination of the dominant excitation mechanism, which might vary from source to source, is required to properly interpret the  emission as a tracer of dense hot molecular gas or as a tracer of intense IR radiation fields in compact regions.

In this letter, we present the first detection of the ortho-H$_2$O  448.001GHz transition in space[^1]. No previous detections of this transition in Galactic objects have been reported, probably because of the high atmospheric opacity due to the terrestrial water vapor. Only recently, thanks to the sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA), it became possible to observe this transition in nearby galaxies red-shifted into more accessible frequencies.

We observed the H$_2$O  transition in ESO 320-G030 (IRAS F11506-3851; $D = 48$Mpc; 235 pcarcsec$^{-1}$). This object is an isolated spiral galaxy with a regular velocity field [@Bellocchi2016] and an IR luminosity (log /= 11.3) in the lower end of the luminous IR galaxy (LIRGs) range (11 $<$ log /$<$ 12). It is a starburst object with no evidence of an AGN based on X-ray and mid-IR diagnostics (@Pereira2010 [@Pereira2011]) hosting an extremely obscured nucleus ($A_{\rm V}\sim40$mag) and a massive outflow powered by the presumed nuclear starburst detected in the ionized, neutral atomic and molecular phases (@Arribas2014; @Cazzoli2014 [@Cazzoli2016]; @Pereira2016b, hereafter ). In addition, a molecular gas inflow is suggested by the inverse P-Cygni profile observed in the far-IR OH absorptions [@GonzalezAlfonso2017]. It is an OH megamaser source [@Norris1986], but no 22GHz H$_2$O maser emission has been detected [@Wiggins2016]. This is consistent with the starburst activity of the nucleus of ESO 320-G030 (see @Lo2005).

ALMA data reduction {#s:data}
===================

We obtained band 8 ALMA observations of ESO 320-G030 on 2016 November 16 using 42 antennas of the 12-m array as part of the project \#2016.1.00263.S. The total on-source integration time was 10.5min. The baselines ranged from 15m to 920m that correspond to a maximum recoverable scale of $\sim$2 based on the ALMA Cycle4 Technical Handbook equations. A three pointing pattern was used to obtain a mosaic with uniform sensitivity over a $\sim$8$\times$8 field of view.

In this letter, we only use data from a spectral window centered at 443.0GHz (1.875GHz/1270kms$^{-1}$ bandwidth and 1.95MHz/1.3kms$^{-1}$ channels) were the redshifted H$_2$O  448.001GHz transition is detected. The remaining ALMA data will be analyzed in a future paper (Pereira-Santaella et al., in prep.) The data were reduced and calibrated using the ALMA reduction software CASA (v4.7.0; ). For the flux calibration we used J1229+0203 (3C 273) assuming a flux density of 2.815Jy at 449.6GHz and a spectral index $\alpha=-0.78$ ($f_\nu\propto\nu^\alpha$). The final data-cube has 300$\times$300 pixels of 005 and 31.2MHz (20km/s) channels. For the cleaning, we used the Briggs weighting with $R=0.5$ [@Briggs1995PhDT] which provides a beam with a full-width half-maximum (FWHM) of 026$\times$024 ($\sim$60pc) and a position angle (PA) of 58[$^\circ$]{}. The 1$\sigma$ sensitivity is $\sim$4.8mJybeam$^{-1}$ per channel. We corrected the data-cube for the primary beam pattern of the mosaic.

![Partial energy level diagram of ortho-H$_2$O. The  448GHz transition is indicated in red. The 78.7 and 132.4 transitions, which populate the 4$_{23}$ level radiatively through the absorption of far-IR photons (see Section \[s:model\]), are marked in blue. \[fig:levels\]](fig_levels.pdf){width="42.00000%"}

Data analysis {#s:analysis}
=============

![Map of the 448GHz (rest frequency) continuum (top panel) and zeroth moment of the H$_2$O  emission (bottom panel) of ESO 320-G030. The dashed line contour marks the 3$\sigma$ level (7mJybeam$^{-1}$ and 2.5Jykms$^{-1}$beam$^{-1}$, respectively). The solid contour lines indicate the peak$\times$(0.5, 0.9) levels. The red hatched ellipses indicate the beam size (026$\times$024, PA$=58{\ensuremath{^\circ}}$). The coordinates are relative to 11 53 11.7192 +39 07 49.105 (J2000). \[fig:alma\_maps\]](h2o_fig1.pdf){width="44.00000%"}

![Continuum subtracted profile of the H$_2$O  448GHz emission in ESO 320-G030 extracted using a circular aperture with $d=0\farcs8$ centered at the nucleus (see Figure \[fig:alma\_maps\]). The dotted green line is the normalized CO(2–1) profile extracted from the same region. The black vertical line indicates the systemic velocity derived from the CO(2–1) global kinematic model . The red solid line is the best Gaussian fit to the water profile (see Section \[s:analysis\]). \[fig:h2o\_profile\]](fig_sp.pdf){width="38.00000%"}

We detect continuum and line emission only in the central $\sim$200pc ($\sim$1). This is consistent with the extent of the 233GHz (1.3mm) continuum emission in this object (see ). We estimated the continuum level in each pixel from the median flux density in the line-free channels of the spectral window. The resulting continuum map is shown in Figure \[fig:alma\_maps\]. The measured total continuum emission in the central 200pc is 183 $\pm$ 4mJy.

From the continuum subtracted data cube, we extracted the nuclear spectrum using a $d=0\farcs8$ aperture (Figure \[fig:h2o\_profile\]). A line is detected at 443451 $\pm$ 2MHz. This corresponds to a rest frame frequency of 448007 $\pm$ 4MHz (using the systemic velocity $v_{\rm radio} = 3049\pm2$kms$^{-1}$, derived from CO(2–1); see ) which agrees with the frequency expected for the ortho-H$_2$O  transition (448001MHz; @Pickett1998). This line identification is also supported by the detection of strong far-IR and sub-mm water transitions in the *Herschel* observations of this object (see Section \[s:model\]). We also detect a weaker emission line (3.7$\pm$0.6Jykms$^{-1}$) which we tentatively identify as two CH$_2$NH transitions at $\sim$446.8GHz ($E_{\rm up}$=96 and 117K; @Pickett1998). Another two CH$_2$NH transitions at 447.9 and 448.1GHz might contribute to the  flux. But they have higher $E_{\rm up}$, $\geq 280$K, so their contributions are likely negligible.

We fitted a Gaussian to the H$_2$O  profile and the result is shown in Figure \[fig:h2o\_profile\]. We obtained a total flux of 37.0 $\pm$ 0.7Jykms$^{-1}$, a velocity of 3045 $\pm$ 1kms$^{-1}$, and a FWHM of 161 $\pm$ 2kms$^{-1}$. The  profile is symmetric and it is centered at the systemic velocity. By contrast, the nuclear CO(2–1) profile has a higher FWHM and presents a more complex asymmetric profile (see Figure\[fig:h2o\_profile\] and figure6 of ).

From the 448GHz continuum and the zeroth moment water emission maps (Figure \[fig:alma\_maps\]), we measured the sizes of the emitting regions by fitting a 2D Gaussian. Both the continuum and the water emission are spatially resolved in the ALMA observations with the continuum being more extended. The continuum size (FWHM) is 038$\times$032, which, deconvolved by the beam size, corresponds to 60pc$\times$50pc at the distance of ESO 320-G030. The size of the water emission is 030$\pm$002, which is equivalent to a deconvolved FWHM of 40$\pm$3pc. For a uniform-brightness disk, the equivalent radius is $0.8\times{\rm FWHM}$ [@Sakamoto2008], i.e., $R\sim45$ and $30-35$pc for the 448 GHz continuum and the H$_2$O line, respectively.

![Velocity field of the H$_2$O  emission. The black cross marks the position of the water emission peak (see Figure \[fig:alma\_maps\]). The dashed line is the minor kinematic axis derived from the kinematic analysis of the CO(2–1) emission (see ). \[fig:h2o\_kinematics\]](h2o_fig2b.pdf){width="43.00000%"}

We also determined the spatially resolved kinematics of the water emission by fitting a Gaussian profile pixel by pixel. The velocity field of the water line is shown in Figure \[fig:h2o\_kinematics\] for the pixels where the line is detected at $>$3$\sigma$. It shows a clear rotating pattern whose kinematic axes are approximately aligned with the large-scale kinematic axes derived from both the CO(2–1) and H$\alpha$ emissions (; @Bellocchi2013). The slight angular deviation, $\sim$25[$^\circ$]{}, is similar to that observed in the nuclear CO(2–1) kinematics and it might be related to the secondary stellar bar and the elongated molecular structure associated with this bar . The FWHM line widths ranges from $\sim$100–170kms$^{-1}$ with the maximum value close to the water emission peak.

Based on the measured continuum fluxes at 448GHz and 244GHz (), and on the emitting region size, we estimated the dust temperature and optical depth. First, we subtracted the free-free contribution at these frequencies ($\sim$7mJy; ). Then, we solved the gray-body equation assuming $1.5<\beta<1.85$ and using a Monte Carlo bootstrapping method to estimate the confidence intervals. We find that $T_{\rm dust}=25-80$K and $\tau_{\rm 448\,GHz}=0.2-1.3$. These values may be significantly higher in the more compact region sampled by the H$_2$O 448GHz emission.

![a) Model predictions showing the luminosity of the H$_2$O 448GHz line as a function of the continuum optical depth at 448GHz ($\tau_{\rm 448}$, lower axis) and at 100$\mu$m ($\tau_{\rm 100}$, upper axis), for uniform $T_{\mathrm{dust}}=50$, 65, and 80K. The models assume spherical symmetry with a radius $R=35$ pc. The assumed H$_2$O abundance is $X(\mathrm{H_2O})=1.5\times10^{-6}$ (solid lines) and $X(\mathrm{H_2O})=6\times10^{-6}$ (dashed red line). The shadowed regions mark the favored ranges inferred from the ESO 320-G030 observations. b) Comparison between the predicted continua at 448GHz (squares) and 244GHz (starred symbols) and the observed values (after subtracting the free-free emission; horizontal stripes). c) Comparison between the predicted absorbing flux of the pumping H$_2$O $4_{23}-3_{12}$ line at 79$\mu$m and the observed value ($-920$ Jykms$^{-1}$ within $\pm150$ kms$^{-1}$; horizontal stripe). The insert compares the observed H$_2$O $4_{23}-3_{12}$ absorption at 78.7$\mu$m with the predictions of the three models encircled in the three panels. The width of the horizontal stripes assume uncertainties of $\pm10$% for $L_{\rm{H_2O\,448}}$, and $\pm20$% for the continuum flux densities and for the flux of the H$_2$O 79$\mu$m line. \[fig:model\]](h2o448ghz_letter_revised.pdf){width="43.00000%"}

Modeling the H$_2$O 448GHz emission {#s:model}
===================================

Figure  shows the model predictions for the H$_2$O448GHz luminosity as a function of the continuum optical depth for different dust temperatures ($T_{\mathrm{dust}}=50$, 65, and 80K). The models, based on those reported in , use the observed size ($R=35$pc) and assume a H$_2$O column density of $N_{\mathrm{H_2O}}=2\times10^{18}\times\tau_{100}$ (solid lines) and $8\times10^{18}\times\tau_{100}$ (dashed red line). These values correspond to H$_2$O abundances relative to H nuclei of $X_{\mathrm{H_2O}}=1.5\times10^{-6}$ and $6\times10^{-6}$, respectively, for a standard gas-to-dust ratio of 100 by mass. The horizontal shaded rectangle indicates the measured value of $3.8\times10^4$ $L_{\odot}$, and the vertical shaded rectangle highlights the observationally favored $\tau_{\rm 448}\gtrsim0.2$, corresponding to $\tau_{\rm 100}\gtrsim8$.

At low column densities, $L_{\rm{H_2O\,448}}$ increases sharply with $\tau_{448}$ due to the enhancement of the far-IR radiation field, responsible for the H$_2$O excitation, and to the increase of $N_{\mathrm{H_2O}}$. The H$_2$O448GHz line is not masing, but usually shows suprathermal excitation ($T_{\mathrm{EX}}>T_{\mathrm{dust}}$) in some shells.

The excitation is dominated in all cases by radiative pumping through the $4_{23}-3_{12}$ and $4_{23}-4_{14}$ lines at $78.7$ and $132.4$$\mu$m (Figure \[fig:levels\]). Collisional excitation (included in the models with $n_{\mathrm{H2}}=3\times10^4$cm$^{-3}$ and $T_{\mathrm{gas}}=150$K) has the effect of increasing the population of the low-lying levels from which the radiative pumping cycle works (see ) thus still having an overall effect on line fluxes. As $\tau_{100}$ increases above unity, the increase in $\tau_{100}$ does hardly enhance the far-IR radiation field and $L_{\rm{H_2O\,448}}$ flattens. It is just in this regime where $L_{\rm{H_2O\,448}}$ approaches the observed value for high enough $T_{\mathrm{dust}}\gtrsim65$ K or $N_{\mathrm{H_2O}}=8\times10^{18}\times\tau_{100}$, indicating that [*the H$_2$O 448GHz line is an excellent probe of buried galaxy nuclei*]{}. At higher $\tau_{100}$, line opacity effects and extinction effects at 448GHz (for $\tau_{448}$ approaching unity) decrease $L_{\rm{H_2O\,448}}$.

With an adopted H$_2$O abundance of $1.5\times10^{-6}$ and $T_{\mathrm{dust}}\sim65$ K (green lines and symbols), we can approximately match the observed H$_2$O448GHz emission (Figure ), and the 448 and 244GHz continuum emission (Figure ) for $\tau_{448}\approx0.3$ and the observed size. However, the same observables can also be fitted, for $\tau_{448}=0.4-0.6$, with a higher $X_{\mathrm{H_2O}}=6\times10^{-6}$ and a more moderate $T_{\mathrm{dust}}=50$ K (red-dashed lines). We can discriminate between both solutions by noting that the dust opacity conditions required for the H$_2$O448GHz line to emit efficiently, $\tau_{100}>1$, are similar to the conditions required to have strong absorption in the high-lying H$_2$O lines at far-IR wavelengths (e.g., @GonzalezAlfonso2012 [@Falstad2017]), [*strongly suggesting that both the 448 GHz emission line and the far-IR absorption lines arise in similar regions*]{}. One of the main H$_2$O lines responsible for the pumping of the H$_2$O448GHz transition, the $4_{23}-3_{12}$ line at $\approx79$$\mu$m (Figure \[fig:levels\]), was observed with [*Herschel*]{}/PACS [@Pilbratt2010Herschel; @Poglitsch2010PACS] within the open time program HerMoLIRG (PI: E. González-Alfonso; OBSID=1342248549). We compare in Figure  the predicted absorbing flux in this line and the observed value ($-920$ Jykms$^{-1}$ between $-150$ and $+150$ kms$^{-1}$, the observed velocity range of the H$_2$O448GHz line at zero intensity; see Figure \[fig:h2o\_profile\]). While the $T_{\mathrm{dust}}\sim50$ K model underpredicts the pumping H$_2$O 79$\mu$m absorption, the $T_{\mathrm{dust}}\sim65$ K model better accounts for it, with still some unmatched redshifted absorption (see insert in Figure ). We thus conclude that [*the H$_2$O 448GHz line originates in warm regions ($T_{\mathrm{dust}}\gtrsim60$ K)*]{}.

Our favored models indicate that the luminosity of the nuclear region where the H$_2$O 448GHz arises is $(4-6)\times10^{10}$, i.e. $\sim25$% of the total galaxy luminosity. While approximately accounting for the observables reported in this [*Letter*]{} ($L_{\rm{H_2O\,448}}$, allowed $\tau_{448}$ range, $f_{448}$, $f_{244}$, and $4_{23}-3_{12}$ absorption strength for the observed size), we advance the [*Herschel*]{} detection of very-high lying H$_2$O absorption lines indicating the presence of an additional warmer component in the nuclear region of ESO 320-G030. The full set of H$_2$O (and OH) lines will be studied in a future work.

Conclusions
===========

We detected the ortho-H$_2$O  transition at 448GHz using ALMA observations of the local spiral LIRG ESO 320-G030. The H$_2$O 448GHz emission arises from the highly obscured nucleus of this galaxy and is spatially resolved ($r\sim30$pc). The H$_2$O 448GHz velocity field is compatible with the global regular rotation pattern of the molecular and ionized gas in ESO 320-G030. Our radiative transfer modeling shows that it is mainly excited by the intense far-IR radiation field present in the nucleus of this source. The conditions for the excitation of the 448GHz water transition indicate that it can probe deeply buried, warm environments both locally and at high redshifts.

We thank the anonymous referee for useful comments and suggestions. We thank M.Villar-Martín and S.Motta for useful comments and careful reading of the manuscript. MPS acknowledges support from STFC through grant ST/N000919/1, the John Fell Oxford University Press (OUP) Research Fund and the University of Oxford. EGS, AU, SGB, JMP, LC, AAH, SA, SC, and FRV acknowledge financial support by the Spanish MEC under grants ESP2015-65597-C4-1-R, AYA2012-32295, ESP2015-68694, AYA2013-42227-P and AYA2015-64346-C2-1-P, which is partly funded by the FEDER programme. EGA a Research Associate at the Harvard-Smithsonian CfA and acknowledges support by NASA grant ADAP NNX15AE56G. This paper makes use of the following ALMA data: ADS/JAO.ALMA\#2016.1.00263.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and 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.

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[^1]: @Persson2007 reported a tentative detection of the water isotopologue H$_2^{18}$O  transition at 489.054GHz in Orion, although it is blended with a much stronger methanol transition.
