Molecules at Z=0.89 Be Observed in Emission Because of Distance Dilution

Astronomy & Astrophysics manuscript no. paper-pks-ATCA7mm-final c ESO 2018 September 20, 2018 Molecules at z=0.89: A 4-mm-rest-frame absorption-line survey toward PKS 1830−211 S. Muller1, A. Beelen2, M. Guélin3,4, S. Aalto1, J. H. Black1, F. Combes5, S. J. Curran6, P. Theule7, and S. N. Longmore8 1 Onsala Space Observatory, SE 439-92, Onsala, Sweden 2 Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France 3 Institut de Radioastronomie Millimétrique, 300, rue de la piscine, 38406 St Martin d’Hères, France 4 Ecole Normale Supérieure/LERMA, 24 rue Lhomond, 75005 Paris, France 5 Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France 6 School of Physics, University of New South Wales, Sydney NSW 2052, Australia 7 Physique des interactions ioniques et moléculaires, Universitéde Provence, Centre de Saint Jérôme, 13397 Marseille Cedex 20, France 8 ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany Received / Accepted ABSTRACT We present the results of a 7 mm spectral survey of molecular absorption lines originating in the disk of a z=0.89 spiral galaxy located in front of the quasar PKS 1830−211. Our survey was performed with the Australia Telescope Compact Array and covers the frequency interval 30–50 GHz, corresponding to the rest-frame frequency interval 57–94 GHz. A total of 28 different species, plus 8 isotopic variants, were detected toward the south-west absorption region, located about 2 kpc from the center of the z=0.89 galaxy, which therefore has the largest number of detected molecular species of any extragalactic object so far. The results of our rotation diagram analysis show that the rotation temperatures are close to the cosmic microwave background temperature of 5.14 K that we expect to measure at z=0.89, whereas the kinetic temperature is one order of magnitude higher, indicating that the gas is subthermally excited. The molecular fractional abundances are found to be in-between those in typical Galactic diffuse and translucent clouds, and clearly deviate from those observed in the dark cloud TMC 1 or in the Galactic center giant molecular cloud Sgr B2. The isotopic ratios of carbon, nitrogen, oxygen, and silicon deviate significantly from the solar values, which can be linked to the young age of the z=0.89 galaxy and a release of nucleosynthesis products dominated by massive stars. Toward the north-east absorption region, where the extinction and column density of gas is roughly one order of magnitude lower than toward the SW absorption region, only a handful of molecules are detected. Their relative abundances are comparable to those in Galactic diffuse clouds. We also report the discovery of several new absorption components, −1 with velocities spanning between −300 and +170 km s . Finally, the line centroids of several species (e.g., CH3OH, NH3) are found to be significantly offset from the average velocity. If caused by a variation in the proton-to-electron ∆µ −6 mass ratio µ with redshift, these offsets yield an upper limit | µ |<4×10 , which takes into account the kinematical noise produced by the velocity dispersion measured from a large number of molecular species. Key words. quasars: absorption lines - quasars: individual: PKS 1830−211 - galaxies: ISM - galaxies: abundances - ISM: molecules 1. Introduction As the sensitivity of millimeter instruments has in- creased, a large number of molecules have become observ- arXiv:1104.3361v2 [astro-ph.CO] 20 Sep 2011 able in galaxies in the Local Universe (see e.g. Wang et al. More than 150 molecules have been discovered in space, 2004; Mart´ınet al. 2006 for NGC4945 and NGC253, respectively) and molecules are still detected at high redshifts as a result of targeted investigations or spectral surveys (currently up to z=6.42), though only a few species such toward Galactic sources such as circumstellar envelopes + of evolved stars (e.g. IRC+10216, Cernicharo et al. 2000; as CO, HCN, HCO , and HNC (e.g. Walter et al. 2003; Wagg et al. 2005; Riechers et al. 2006; Guélin et al. 2007). Patel et al. 2011), massive star-forming clouds (e.g. Sgr B2, Nummelin et al. 2000), or cold molecular clouds (e.g. TMC- Those detections are mostly related to ultra-luminous 1, Ohishi et al. 1992; Kaifu et al. 2004). The study of the galaxies, which are potentially unrepresentative of all galaxies at these epochs. Moreover, line emission from these high- rotational and vibrational spectra of these molecules is par- ticularly useful to probe the physical conditions and chem- z galaxies is dominated by the warmest and densest regions ical content of the interstellar gas. Most importantly, the of the molecular component. large number of molecules found in various environments Observations of molecular absorption lines toward a demonstrates the richness and variety of interstellar chem- bright radio continuum source allow rare molecular species istry. to be detected even in distant galaxies, where they cannot 1 Muller et al. 2011: Molecules at z=0.89 be observed in emission because of distance dilution. This sarily totally, coupled with the CMB. Rotation tempera- technique can thus be used to investigate the chemistry tures derived for several molecules are consistent with the and its complexity at high redshifts and trace the chemi- value TCMB=5.14 K at z=0.89 (Combes & Wiklind 1997; cal enrichment history of the Universe. In addition, high- Menten et al. 1999; Henkel et al. 2009), as predicted for an z molecular absorbers can serve as cosmological probes of adiabatic expansion of the Universe. the cosmic microwave background (CMB) temperature and The absorption toward the NE component (hereafter have attracted much interest in constraining variations in FG0.89NE) is located ∼147 kms−1 blueward and is nar- the fundamental constants of physics (see e.g., Henkel et al. rower (FWHM∼15 kms−1), so that both velocity compo- 2009 and references therein). nents can easily be resolved kinematically. Nevertheless, the Unfortunately, only a handful of high-redshift radio column of molecular gas toward the NE image is about two molecular absorbers 1 have been discovered to date (see orders of magnitude less than in FG0.89SW, and the corre- the review by Combes 2008), despite numerous searches sponding absorption lines are significantly weaker. Little is (e.g. Wiklind & Combes 1996a; Drinkwater et al. 1996; known about the excitation of and the physical conditions Murphy et al. 2003; Curran et al. 2004, 2006, see also toward FG0.89NE. However, based on the analysis of time Curran et al. 2011). The molecular absorption originat- variations in the NE absorption, Muller & Guélin (2008) ing in the z=0.89 and z=0.68 intervening galaxies lo- argued that it should be caused by a small number (≤5) of cated in front of the quasars PKS 1830−211 (z=2.5) clouds of size ∼1 pc and of density a few 100 cm−3, hence and B 0218+357 (z=0.94) respectively, were discovered by resembling Galactic diffuse clouds. Wiklind & Combes (1995, 1996b) and have many similar- Molecular absorption has also been detected in the ities: both absorbers appear to be (faint) nearly face-on galaxies hosting the radio sources B3 1504+377 at z=0.67 spiral galaxies (Winn et al. 2002; York et al. 2005); both (in CO, HCO+, HCN, and HNC; Wiklind & Combes galaxies act as gravitational lenses and split the image of 1996b), PKS 1413+135 at z=0.25 (in CO, HCO+, their background quasar into two main compact compo- HCN, HNC, and CN; Wiklind & Combes 1997), and nents and an Einstein ring, as seen at radio wavelengths PKS 0132−097 at z=0.76 (although only in OH; (Jauncey et al. 1991; Patnaik et al. 1993); they both har- Kanekar et al. 2005). However, the weakness of the back- bor large column densities of absorbing molecular gas ground radio continuum sources, narrower linewidths and 22 −2 (N(H2) > 10 cm ); and in both cases the radio contin- smaller column densities of absorbing gas than the two ab- uum of the background quasar is bright enough to permit sorbers toward PKS 1830−211 and B 0218+357, ensures sensitive observations with current millimeter instruments. that it is difficult to perform a deep inventory of their molec- A dozen molecular species have indeed been successfully ular constituents with current instruments. detected in these absorbers, as well as absorption by their In this paper, we present the first unbiased spectral- rare isotopologues (cf e.g., Wiklind & Combes 1995, 1996b, line survey toward a high-z molecular absorber, located at 1998; Menten et al. 1999; Muller et al. 2006). z=0.89 in front of the quasar PKS 1830−211. Observations, The multiplicity of lensed continuum images of the conducted with the Australian Compact Array and cov- background quasar makes it possible to explore as many ering the whole 7 mm band from 30 GHz to 50 GHz, different lines of sight through the intervening galaxy. In are presented in §.2. Our results are presented in §.3, and the case of B 0218+357, molecular absorption is detected then discussed in §.4, in terms of isotopic ratios (§.4.1), only toward one image of the quasar (Menten & Reid 1996; the chemical composition relative to other objects (§.4.2), Muller et al. 2007). Toward PKS 1830−211, molecular ab- and constraints on the variation in fundamental constants sorption is detected, however, for both of the bright and of physics (§.4.3). Notes for the different molecular species compact images, located one arcsecond apart on the north- detected in the survey are given in appendix.

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