Abstract
We present preliminary results of a submillimeter spectral line survey of a strongly lensed dusty star-forming galaxy (DSFG) at z ∼ 2, near the peak epoch of cosmic star formation. We use data from the Submillimeter Array, which recently completed a systems upgrade that substantially increased the instantaneous bandwidth. Focusing on the brightest spectral lines of CO and atomic carbon, we characterize the interstellar medium properties of this galaxy. We find highly excited CO and [C i] emission, and a large [C i] line ratio implies a higher excitation temperature than other extreme high-redshift sources. Based on this evidence, we infer that the source likely hosts a dust-obscured active galactic nucleus similar to other well-studied DSFGs.
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1. Introduction
High redshift, dusty star-forming galaxies (DSFGs) are a population of luminous, dust-obscured objects undergoing short-lived intense starburst events. Since their discovery, high-redshift DSFGs have challenged models of galaxy evolution (Casey et al. 2014). They are best studied in submillimeter (sub-mm) wavelengths because the majority of UV/optical light from the starbursts is obscured by dust and re-radiated at long wavelengths. DSFGs are most common at 1 < z < 4 (Chapman et al. 2005), and have been discovered out to z = 7 (Marrone et al. 2018).
The intense star formation in DSFGs is fueled by massive reservoirs of molecular gas, but cold molecular hydrogen (the most abundant molecule) is not directly observable. The brightest tracers of the molecular gas are transitions of CO and [C i]. The physical conditions of the gas reservoirs can be probed by sub-mm observations of other molecules such as HCN, HCO+, H2O, and OH+. Given the varying excitation conditions of these molecules, we can piece together a comprehensive view of the temperature, density, heating/cooling, and dynamics of the cold gas. However, this becomes increasingly difficult at higher resdshifts, as detections of molecular lines are fainter.
Díaz-Sánchez et al. (2017) observed the extremely bright source WISE J1329+2243 at z = 2.04. The galaxy is intrinsically very luminous and is additionally lensed by a foreground cluster at z = 0.44, magnifying it by a factor of 11. WISE J1329+2243 exhibits some of the brightest molecular lines of any known object in the distant universe, and has previously been detected in CO(1–0), CO(4–3), and [C i](1–0) (Dannerbauer et al. 2019). Here we present Submillimeter Array (SMA) observations of high-excitation CO lines in this source, as well as atomic carbon. Together with ancillary data, this forms one of the most complete CO spectral line energy distributions (SLEDs) in the distant universe, and we suggest that WISE J1329+2243 can be used as a CO line template for future high-redshift studies.
2. Analysis and Results
We observed WISE J1329+2243 at 0.87, 1.1, and 1.4 mm in 2019 May using the SMA SWARM correlator, which processes 32 GHz of instantaneous bandwidth. The source was not spatially resolved by the lower-frequency tunings at 26–34 spatial resolution, and was marginally resolved in the highest-frequency data. These observations covered the CO transitions from J = 6–5 to 9–8, and will be presented in more detail in future work. The data were reduced and analyzed with MIR and CASA (McMullin et al. 2007). We subtracted the continuum in the visibility plane, and created image cubes with ≈100 km s−1 resolution.
We then extracted spectra for each frequency tuning by fitting a 2D spatial Gaussian to each channel of the image cube, which we verified accurately recovers the spectrum compared to aperture-based methods. The size and position of the profile in each channel is fixed to the best-fit values from the continuum image of each tuning, leaving only the flux at each channel as a free parameter. We choose this method because it is repeatable and allows the uncertainties to be estimated more accurately in resolved data. We fit the extracted CO and [C i] spectra with Gaussian line profiles, in order to extract their integrated fluxes. We find a very similar CO line width as Dannerbauer et al. (2019). The spectra are shown in the right panel of Figure 1.
Figure 1. The CO excitation in WISE J1329+2243 is intermediate between that of two well-studied quasars and higher even than the average of DSFGs at these redshifts, likely indicating that this source also contains an AGN heavily obscured by dust. Left: CO SLED and [C i] line ratios for our source compared to well-studied literature objects. Right: CO and [C i] spectra of WISE J1329+2243 from the SMA data.
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Figure 1 shows the line flux ratio of the available CO and [C i] lines compared to CO(1–0). We compare to several high-redshift objects from the literature, including the well-studied Cloverleaf and APM 08279+5255 quasars as well as an average SLED of z = 2–6 DSFGs that show no evidence for active galactic nucleus (AGN) activity (Wagg et al. 2006; Bradford et al. 2009; Walter et al. 2011; Spilker et al. 2014; Uzgil et al. 2016; Dannerbauer et al. 2019).
3. Discussion
As Figure 1 shows, the CO emission of the source is thermalized up to J = 6. The CO is more excited than the DSFG average, but between that of two well-studied quasars, the Cloverleaf quasar and APM 08279+5255. DSFGs usually show highly excited CO SLEDs as they undergo intense starburst events. However, our source is significantly more excited than the average DSFG model created by Spilker et al. (2014) and is instead much more similar to the obscured quasars. Therefore, we infer that our source also likely contains an AGN responsible for the higher CO excitation, supported by the strong radio detection in the Very Large Array survey FIRST (Becker et al. 1994). As there is no evidence for an AGN from current rest-optical spectroscopy (Díaz-Sánchez et al. 2017), we speculate that the AGN is highly obscured by dust. This conclusion is also supported by the high [C i] line ratio in this source, which is unusual even among other extreme high-redshift objects.
The [C i] line ratio can be directly translated to a carbon excitation temperature and carbon mass. Combined with an assumed carbon abundance, this can be used to estimate the gas mass of galaxies. Dannerbauer et al. (2019) assumed an excitation temperature Tex = 35 K and a carbon abundance of M[C i]/6 × 
= 5.4 × 10−5 and find a gas mass of 9.3 × 1010 M⊙. With our new detection of [C i](2–1), we find that Tex = 
K, which is significantly higher than typically assumed in the literature. This nevertheless has only a minor impact on the gas mass derived from the [C i] data, because excitation temperature has little influence on the derived mass for Tex ≳ 20 K. Hence, we find a very similar gas mass of (9.3 ± 1.1) × 1010 M⊙ to that of Dannerbauer et al. (2019) under the same assumptions.
The wide bandwidth of the SMA enables us to constrain many atomic and molecular features simultaneously. We picked an extremely bright galaxy as a showcase of this capability of SMA. In this Note, we focused on the bright CO and [C i] lines, but many more lines are contained within the SMA data. Full results from this sub-mm line survey will be presented in future work. Combined, all the available sub-mm spectroscopy can present a very detailed view of the ISM conditions in this extreme starburst galaxy.
We acknowledge the support of the John W. Cox Endowment for advanced studies in astronomy by The Department of Astronomy at The University of Texas at Austin. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. J.S.S. is supported by NASA Hubble Fellowship grant #HF2-51446 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555.