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Towards a Fundamental Astrometric Reference System Behind the Magellanic Clouds: Spectroscopic Confirmation of New Quasar Candidates Selected in the Near-Infrared

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Towards a Fundamental Astrometric Reference System Behind the Magellanic Clouds: Spectroscopic Confirmation of New Quasar Candidates Selected in the Near-Infrared Astronomical Science Towards a Fundamental Astrometric Reference System behind the Magellanic Clouds: Spectroscopic Confirmation of New Quasar Candidates Selected in the Near-infrared Valentin D. Ivanov1 ties from the ESO VISTA Magellanic The VMC is carried out with VISTA Maria-Rosa L. Cioni 2,3,4 Clouds (VMC) Public Survey. A subset ( Emerson et al., 2006), a 4.1­metre tele­ Kenji Bekki 5 of 49 objects was followed up with opti- scope on Cerro Paranal, equipped with Richard de Grijs 6,7,8 cal spectroscopy with FORS2. We con- the VISTA InfraRed CAMera (VIRCAM; Jim Emerson 9 firmed the quasar nature of 37 objects Dalton et al., 2006), a wide-field near- Brad K. Gibson10 (34 new identifications) that span a red- infrared camera producing ~ 1 by Devika Kamath11 shift range from z ~ 0.5 to 4.1. 1.5 degree images across the 0.9–2.4 μm Jacco Th. van Loon12 wavelength range. The VISTA data are Andrés E. Piatti13,14 processed with the VISTA Data Flow Bi-Qing For 5 Quasi­stellar objects (quasars) are active System (VDFS) pipeline (Irwin et al., 2004) nuclei of distant galaxies, undergoing at the Cambridge Astronomical Survey episodes of strong accretion. Typically, Unit1. The data products are available 1 ESO the contribution from the host galaxy is through the ESO Science Archive2 or the 2 Universität Potsdam, Institut für Physik small, and they appear as point­like ob ­ specialised VISTA Science Archive (VSA; und Astronomie, Germany jects with strong emission lines. Quasars Cross et al., 2012). 3 Leibniz-Institut für Astrophysik Potsdam are cosmological probes that serve as (AIP), Germany background “beacons” to explore the 4 University of Hertfordshire, Physics intervening interstellar medium, but they Quasar candidate selection Astronomy and Mathematics, United also are distant, unmoving objects used Kingdom to establish an absolute astrometric ref­ Cioni et al. (2013) derived selection cri­ 5 ICRAR, The University of Western erence system on the sky. The smaller teria (Figure 2) to identify candidate Australia, Crawley, Australia the measured proper motions (PMs) of quasars based on both the locus of 117 6 Kavli Institute for Astronomy and Astro­ foreground objects are, the more useful known quasars in a (Y–J) versus (J–Ks) physics, Peking University, Beijing, the quasars become — as is the case colour–colour diagram and their Ks­band China for nearby galaxies. Quasars behind these variability behaviour. The diagram was 7 Department of Astronomy, Peking galaxies are hard to identify because of based on average magnitudes obtained University, Beijing, China foreground contamination, the additional from deep tile images created by the 8 International Space Science Institute – (patchy) reddening inside the intervening Wide Field Astronomy Unit (WFAU3) as Beijing, China galaxies themselves, and the galaxies’ part of the VMC data processing, using 9 School of Physics and Astronomy, relatively large angular areas on the sky. version 1.3.0 of the VDFS pipeline. Queen Mary University of London, The latter point underscores the need United Kingdom to carry out dedicated wide-field surveys, Figure 2 shows the colour–colour dia­ 10 E. A. Milne Centre for Astrophysics, sometimes covering hundreds of square gram demonstrating the colour selection Department of Physics & Mathematics, degrees, to find a sufficient density of of our quasar candidates. The regions University of Hull, United Kingdom background quasars. The Magellanic (marked with letters) where known qua­ 11 Instituut voor Sterrenkunde, K. U. Clouds are an extreme case where these sars are found and the locus of the plan­ Leuven, Belgium obstacles are notably enhanced. etary nebulae (Cioni et al., 2013) is indi­ 12 Lennard­Jones Laboratories, Keele cated. The blue crosses (×) indicate VMC University, United Kingdom counterparts to the spectroscopically 13 Observatorio Astronómico, Universidad The VISTA survey of the Magellanic confirmed quasars (Cioni et al., 2013), Nacional de Córdoba, Argentina Clouds selected adopting a maximum matching 14 Consejo Nacional de Investigaciones radius of 1 arcsecond (the average sepa­ Científicas y Técnicas, Buenos Aires, The ESO Public Survey with the VLT ration is 0.15 ± 0.26 arcseconds). Argentina Infrared Survey Telescope for Astronomy (VISTA) of the Magellanic Clouds (VMC; The selected candidates for our study Cioni et al., 2011) covers 184 square are included in Ivanov et al. (2016); for the Quasi-stellar objects (quasars) located degrees around the Large and Small quantitative description of the selection behind nearby galaxies provide an Magellanic Clouds (LMC, SMC), the criteria, see Cioni et al. (2013). Extended excellent absolute reference system for Magellanic Bridge, and the Stream sources were included in our search astrometric studies, but they are diffi- (Figure 1). The magnitude limit is to to ensure that low­redshift quasars with cult to identify because of fore- and Ks = 20.3 mag (signal-to-noise ratio considerable contributions from their background contamination. We have ~ 10; Vega system) in the YJKs­bands; host galaxies were not omitted. Their embarked on a programme to expand 12 separate epochs in the Ks­band, extended nature is marginal, because the quasar reference system behind spread over at least a year are also they are dominated by their nuclei, and the Large and Small Magellanic Clouds, taken. The main survey goals are to they are still useful for quasar absorption­ the Magellanic Bridge and Magellanic study the star formation history and the line studies. The 68 brightest candidates Stream. Hundreds of quasar candidates geometry of the Magellanic Cloud sys­ were selected to homogeneously sample were selected, based on their near- tem, as well as its cluster and variable­ seven VMC tiles where quasars had infrared colours and variability proper- star populations. not yet been found. The total number of 32 The Messenger 163 – March 2016 measured. For most line centres the typical formal statistical errors are Δλ ~1 Å, which translates into redshift errors ) 0 Δz < 0.001. These are optimistic estimates ees that neglect wavelength calibration errors. egr We evaluated the latter by measuring (d –5 the wavelengths of 45 strong and isolated c sky lines in five randomly selected spec­ De tra. We did not find any trends with wave­ –10 length and a root mean square (rms) error of 1.57 Å was determined. This translates –1010 020 into a redshift uncertainty of Δz ~ 0.0002 RA (degrees) for a line at 7000 Å, near the centre of our spectral coverage. To evaluate the Figure 1. VMC footprints on the sky (shown as in blue. The dashed grid shows lines of constant real uncertainties, we compared the red­ contiguous rectangles). The spectroscopically right ascension (spaced by 15°), and constant shifts derived from different lines of the followed­up quasar candidates are marked in red, declination (spaced by 5°). Coordinates are given and confirmed quasars from Kozlowski et al. (2013) with respect to (RA, Dec) = (51°, –69°). same object. The average difference for 35 pairs of lines, for quasars with multiple Figure 2. Colour–colour lines, is effectively zero: |zi–zj| = 0.006 ± 3 diagram showing the 0.0 07. colour selection of our quasar candidates. The dashed black lines iden­ For objects with multiple lines we adopted tify the regions (marked the average difference as redshift error, with letters) where known adding in quadrature the wavelength quasars are found, while the green line marks calibration error of Δz = 0.0002. This 2 B the blue border of the addition only made a difference for a few planetary nebulae locus. low-­redshift objects. For quasars for The spectroscopically which only a single line was available, we followed­up quasars are ) marked with solid red conservatively adopted as redshift errors ag dots, while the non­ Δz = 0.005 for objects at z < 1 and Δz = (m A quasars are marked with 0.015 for more distant objects. Finally, as Ks red triangles. Blue C external verification, in the Sloan Digital J– 1 crosses (×) indicate the location of the VMC Sky Survey (SDSS) rest frame composite counterparts to the spectrum we re­measured the redshifts spectroscopically con­ of the same lines that were detected firmed quasars from in our spectra, obtaining values below Kozłowski et al. (2013). Black dots are randomly z = 0.0001, as expected. selected LMC objects 0 D (with errors in all three bands of < 0.1 mag), to Results demonstrate the locus of 00.5 1 “normal” stars, as well as Y–J (mag) background galaxies in The majority of the observed objects are regions B and C. quasars: 37 objects appear to be bona fide quasars at z ~ 0.47–4.10 (10 are candidates could increase greatly if most objects, except for some fainter located behind the LMC, 13 behind the fainter objects are considered. objects, for which the exposure times SMC and 14 behind the Bridge area), were 900 s. The signal­to­noise ratio showing some broad emission lines, even (S/N) varies among the spectra, but it is though some spectra need smoothing Spectroscopic follow­up observations typically ~ 10–30 at λ ~ 6000–6200 Å. for display purposes. The spectra of the The observing details are given in Ivanov three highest­redshift quasars exhibit Follow­up spectra of 49 candidates were et al. (2016). The data reduction was Lyα absorption systems; a few quasars obtained with the FOcal Reducer and carried out using the ESO pipeline, ver­ (e.g., SMC 3_5 22, BRI 2_8 197, etc.) low­dispersion Spectrograph (FORS2; sion 5.0.0. Various IRAF4 tasks from show blue­shifted C IV absorption, per­ Appenzeller et al., 1998) on the Very the onedspec and rv packages were haps due to winds from active galactic Large Telescope in long­slit mode, with used in the subsequent analysis.
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