Quasi-stellar objects (quasars) locatedbehind nearby galaxies provide anexcellent absolute reference system forastrometric studies, but they are diffi-cult to identify because of fore- andbackground contamination. We haveembarked on a programme to expandthe quasar reference system behindthe Large and Small Magellanic Clouds,the Magellanic Bridge and MagellanicStream. Hundreds of quasar candidateswere selected, based on their near-infrared colours and variability proper-ties from the ESO VISTA MagellanicClouds (VMC) Public Survey. A subsetof 49 objects was followed up with opti-cal spectroscopy with FORS2. We con-firmed the quasar nature of 37 objects(34 new identifications) that span a red-shift range from z ~ 0.5 to 4.1.Fil: Ivanov, Valentin D.. No especifíca;Fil: Cioni, Maria Rosa L.. Universitat Potsdam; Alemania. Leibniz-Institut für Astrophysik Potsdam; Alemania. University of Hertfordshire; Reino UnidoFil: Bekki, Kenji. The University of Western Australia; AustraliaFil: de Grijs, Richard. Peking University; China. International Space Science Institute; ChinaFil: Emerson, Jim. Queen Mary University of London; Reino UnidoFil: Gibson, Brad K.. University of Hull; Reino UnidoFil: Kamath, Devika. Instituut voor Sterrenkunde; BélgicaFil: van Loon, Jacco Th.. Keele University; Reino UnidoFil: Piatti, Andres Eduardo. Universidad Nacional de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: For, Bi Qing. The University of Western Australia; Australi

Bekki, Kenji

Cioni, Maria Rosa L.

de Grijs, Richard

Emerson, Jim

For, Bi Qing

Gibson, Brad K.

Ivanov, Valentin D.

Kamath, Devika

Piatti, Andres Eduardo

van Loon, Jacco Th.

English

CONICET Digital

32 The Messenger 163 – March 2016The VMC is carried out with VISTA ( Emerson et al., 2006), a 4.1­metre tele­scope on Cerro Paranal, equipped with the VISTA InfraRed CAMera (VIRCAM; Dalton et al., 2006), a wide-field near-infrared camera producing ~ 1 by 1.5 degree images across the 0.9–2.4 μm wavelength range. The VISTA data are processed with the VISTA Data Flow  System (VDFS) pipeline (Irwin et al., 2004) at the Cambridge Astronomical Survey Unit1. The data products are available through the ESO Science Archive2 or the specialised VISTA Science Archive (VSA; Cross et al., 2012).Quasar candidate selectionCioni et al. (2013) derived selection cri­teria (Figure 2) to identify candidate  quasars based on both the locus of 117 known quasars in a (Y–J) versus (J–Ks) colour–colour diagram and their Ks­band variability behaviour. The diagram was based on average magnitudes obtained from deep tile images created by the Wide Field Astronomy Unit (WFAU3) as part of the VMC data processing, using version 1.3.0 of the VDFS pipeline.Figure 2 shows the colour–colour dia­gram demonstrating the colour selection of our quasar candidates. The regions (marked with letters) where known qua­sars are found and the locus of the plan­etary nebulae (Cioni et al., 2013) is indi­cated. The blue crosses (×) indicate VMC counterparts to the spectroscopically confirmed quasars (Cioni et al., 2013), selected adopting a maximum matching radius of 1 arcsecond (the average sepa­ration is 0.15 ± 0.26 arcseconds).The selected candidates for our study  are included in Ivanov et al. (2016); for the quantitative description of the selection criteria, see Cioni et al. (2013). Extended sources were included in our search  to ensure that low­redshift quasars with considerable contributions from their  host galaxies were not omitted. Their extended nature is marginal, because they are dominated by their nuclei, and they are still useful for quasar absorption­line studies. The 68 brightest candidates were selected to homogeneously sample seven VMC tiles where quasars had  not yet been found. The total number of Valentin D. Ivanov1Maria-Rosa L. Cioni 2, 3, 4Kenji Bekki 5Richard de Grijs 6, 7, 8Jim Emerson9Brad K. Gibson10Devika Kamath11Jacco Th. van Loon12Andrés E. Piatti13,14Bi-Qing For 51 ESO2  Universität Potsdam, Institut für Physik und Astronomie, Germany3  Leibniz-Institut für Astrophysik Potsdam (AIP), Germany4  University of Hertfordshire, Physics Astronomy and Mathematics, United Kingdom5  ICRAR, The University of Western  Australia, Crawley, Australia6  Kavli Institute for Astronomy and Astro­physics, Peking University, Beijing, China7  Department of Astronomy, Peking  University, Beijing, China8  International Space Science Institute –Beijing, China9  School of Physics and Astronomy, Queen Mary University of London, United Kingdom10  E. A. Milne Centre for Astrophysics, Department of Physics & Mathematics, University of Hull, United Kingdom11  Instituut voor Sterrenkunde, K. U.  Leuven, Belgium12  Lennard­Jones Laboratories, Keele University, United Kingdom13  Observatorio Astronómico, Universidad Nacional de Córdoba, Argentina14  Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, ArgentinaQuasi-stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are diffi-cult to identify because of fore- and background contamination. We have embarked on a programme to expand the quasar reference system behind  the Large and Small Magellanic Clouds, the Magellanic Bridge and Magellanic Stream. Hundreds of quasar candidates were selected, based on their near-infrared colours and variability proper-ties from the ESO VISTA Magellanic Clouds (VMC) Public Survey. A subset of 49 objects was followed up with opti-cal spectroscopy with FORS2. We con-firmed the quasar nature of 37 objects (34 new identifications) that span a red-shift range from z ~ 0.5 to 4.1.Quasi­stellar objects (quasars) are active nuclei of distant galaxies, undergoing episodes of strong accretion. Typically, the contribution from the host galaxy is small, and they appear as point­like ob ­jects with strong emission lines. Quasars are cosmological probes that serve as background “beacons” to explore the intervening interstellar medium, but they also are distant, unmoving objects used to establish an absolute astrometric ref­erence system on the sky. The smaller the measured proper motions (PMs) of foreground objects are, the more useful the quasars become — as is the case  for nearby galaxies. Quasars behind these galaxies are hard to identify because of foreground contamination, the additional (patchy) reddening inside the intervening galaxies themselves, and the galaxies’ relatively large angular areas on the sky. The latter point underscores the need  to carry out dedicated wide-field surveys, sometimes covering hundreds of square degrees, to find a sufficient density of background quasars. The Magellanic Clouds are an extreme case where these obstacles are notably enhanced.The VISTA survey of the Magellanic CloudsThe ESO Public Survey with the VLT Infrared Survey Telescope for Astronomy (VISTA) of the Magellanic Clouds (VMC; Cioni et al., 2011) covers 184 square degrees around the Large and Small Magellanic Clouds (LMC, SMC), the Magellanic Bridge, and the Stream  (Figure 1). The magnitude limit is to Ks = 20.3 mag (signal-to-noise ratio  ~ 10; Vega system) in the YJKs­bands; 12 separate epochs in the Ks­band, spread over at least a year are also  taken. The main survey goals are to  study the star formation history and the geometry of the Magellanic Cloud sys­tem, as well as its cluster and variable­star populations.Astronomical ScienceTowards a Fundamental Astrometric Reference System behind the Magellanic Clouds: Spectroscopic Confirmation of New Quasar Candidates Selected in the Near-infrared33The Messenger 163 – March 2016candidates could increase greatly if fainter objects are considered.Spectroscopic follow­up observationsFollow­up spectra of 49 candidates were obtained with the FOcal Reducer and low­dispersion Spectrograph (FORS2; Appenzeller et al., 1998) on the Very Large Telescope in long­slit mode, with the 300V+10 grism, delivering spectra over λ = 445–865 nm with a spectral resolving power R ~ 440 (1.3­arcsecond slit). Two 450 s exposures were taken for Dec (degrees) 0–5–10RA (degrees)0–10 10 20J–Ks (mag)Y–J (mag)01230 0.5 1DCABFigure 1. VMC footprints on the sky (shown as  contiguous rectangles). The spectroscopically  followed­up quasar candidates are marked in red, and confirmed quasars from Kozlowski et al. (2013) Figure 2. Colour–colour diagram showing the colour selection of our quasar candidates. The dashed black lines iden­tify the regions (marked with letters) where known quasars are found, while the green line marks  the blue border of the planetary  nebulae locus. The spectroscopically followed­up quasars are marked with solid red dots, while the non­ quasars are marked with red triangles. Blue crosses (×) indicate the location of the VMC counterparts to the spectroscopically con­firmed quasars from Kozłowski et al. (2013). Black dots are randomly selected LMC objects (with errors in all three bands of < 0.1 mag), to demonstrate the locus of “normal” stars, as well as background galaxies in regions B and C.most objects, except for some fainter objects, for which the exposure times were 900 s. The signal­to­noise ratio (S/N) varies among the spectra, but it is typically ~ 10–30 at λ ~ 6000–6200 Å. The observing details are given in Ivanov et al. (2016). The data reduction was  carried out using the ESO pipeline, ver­sion 5.0.0. Various IRAF4 tasks from  the onedspec and rv packages were used in the subsequent analysis. Some reduced spectra are shown in Figure 3.Prominent emission lines were identified, from which the quasar redshifts were in blue. The dashed grid shows lines of constant right ascension (spaced by 15°), and constant  declination (spaced by 5°). Coordinates are given with respect to (RA, Dec) = (51°, –69°).measured. For most line centres the  typical formal statistical errors are Δλ ~1 Å, which translates into redshift errors  Δz < 0.001. These are optimistic estimates that neglect wavelength calibration errors. We evaluated the latter by measuring  the wavelengths of 45 strong and isolated sky lines in five randomly selected spec­tra. We did not find any trends with wave­length and a root mean square (rms) error of 1.57 Å was determined. This translates into a redshift uncertainty of Δz ~ 0.0002 for a line at 7000 Å, near the centre of  our spectral coverage. To evaluate the real uncertainties, we compared the red­shifts derived from different lines of the same object. The average difference for 35 pairs of lines, for quasars with multiple lines, is effectively zero: |zi–zj| = 0.006 ± 0.007.For objects with multiple lines we adopted the average difference as redshift error, adding in quadrature the wavelength  calibration error of Δz = 0.0002. This addition only made a difference for a few low­ redshift objects. For quasars for which only a single line was available, we conservatively adopted as redshift errors Δz = 0.005 for objects at z < 1 and Δz = 0.015 for more distant objects. Finally, as external verification, in the Sloan Digital Sky Survey (SDSS) rest frame composite spectrum we re­measured the redshifts of the same lines that were detected  in our spectra, obtaining values below  z = 0.0001, as expected.ResultsThe majority of the observed objects are quasars: 37 objects appear to be bona fide quasars at z ~ 0.47–4.10 (10 are located behind the LMC, 13 behind the SMC and 14 behind the Bridge area), showing some broad emission lines, even though some spectra need smoothing  for display purposes. The spectra of the three highest­redshift quasars exhibit  Lyα absorption systems; a few quasars (e.g., SMC 3_5 22, BRI 2_8 197, etc.) show blue­shifted C IV absorption, per­haps due to winds from active galactic nuclei. We defer a more detailed study of indi vidual objects until the remainder of the sample have been followed up. Our  success rate is ~76 %, testifying to the robustness and reliability of our selection 34 The Messenger 163 – March 20164_3 1029g and LMC 8_8 376g are too noisy for secure classification.ProspectsCioni et al. (2013) estimated that the VMC survey may find a total of approximately 1830 quasars. The success rate of 76 % reached here brings this number down  to some 1390. The spectra of the candi­dates in seven tiles, of the 110 tiles that make up the entire VMC survey, yielded on average ~ 5.3 quasars per tile. Scaling this number up to the full survey area yields ~ 580 quasars. This is a lower limit, because only the brightest candidates  in the seven tiles were followed up, so the larger number is still a viable prediction.This project is still at an early stage, but once spectroscopic confirmations have been achieved, the identified quasars will provide an excellent, independent refer­ence system for detailed astrometric studies of the Magellanic Clouds system, complementing Gaia. In addition, the homogeneous, multi­epoch observations of the VMC survey, combined with the large quasar sample, open up the possi­bility to investigate in detail the mecha­nisms that drive quasar variability, for example, with structure functions in the near­infrared, following the example  of the SDSS quasar variability studies.ReferencesAppenzeller, I. et al. 1998, The Messenger, 94, 1Cioni, M.­R. L. et al. 2011, A&A, 527, 116Cioni, M.­R. L. et al. 2013, A&A, 549, A29Cross, N. J. G. et al. 2012, A&A, 548, 119Dalton, G. B. et al. 2006, SPIE Conf. Ser., 6269, 30Emerson, J. et al. 2006, The Messenger, 126, 41Ivanov, V. et al. 2016, A&A, in press, arXiv:1510.05504Irwin, M. J. et al. 2004, SPIE Conf. Ser., 5493, 411Kozlowski, S. et al. 2013, ApJ, 775, 92Vanden Berk, D. E. et al. 2001, AJ, 122, 549Links1  Cambridge Astronomical Survey Unit (CASU): http://casu.ast.cam.ac.uk/2  ESO Science Archive: http://archive.eso.org3  Wide Field Astronomy Unit (WFAU): http://www.roe.ac.uk/ifa/wfau/4  The Image Reduction and Analysis Facility (IRAF) is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), under a cooperative agreement with the US National Science Foundation.criteria. Many of our quasars are present in the GALEX and SAGE–SMC (Surveying the Agents of Galaxy Evolution – Small Magellanic Cloud) source catalogues. The confirmed quasar SMC 5_2 241 has a candidate radio counterpart: SUMSS J002956–714640 at 2.8 arcseconds  separation, detected in the 843 MHz Sydney University Molonglo Sky Survey.Many quasars with redshifts z ≤ 1 were classified as extended sources by the VDFS pipeline, supporting our decision  to include extended objects in the sam­ple: they are either contaminated by the host galaxy or by chance alignment with foreground objects from either the LMC or the SMC. Four extended objects are contaminating low­redshift galaxies: LMC 9_3 2728g, LMC 8_8 655g, and LMC 8_8 208g show hydrogen, some oxygen and nitrogen in emission, but no obvious broad lines, so we interpret these as indi­cators of ongoing star formation rather than nuclear activity; while LMC 8_8 341g may also show Hβ in absorption. In addi­tion, LMC 8_8 341g has a recession velocity of ~ 300 km s–1, consistent within the uncertainties with LMC membership, making it a possible, moderately young LMC star cluster. After target selection we realised that three of our candidates (SMC 5_2 203, SMC 3_5 24, and SMC 3_5 15) were previously confirmed qua­sars, and two others (LMC 9_3 137 and LMC 4_3 95g) were previously suspected quasars.Three point­source­like objects are most likely emission-line stars: LMC 4_3 95, LMC 4_3 86, and SMC 3_5 29. The spectra of LMC 8_8 422g, LMC 4_3 3314g, and LMC 9_3 3107g do not offer any solid clues as to their nature. Some BL Lacertae (BL Lacs) — active galaxies believed to be seen along a relativistic jet emanating from the active nucleus —  are also featureless, but they usually have bluer continua than the spectra of these three objects. A possible test is to search for rapid variability, typical of BL Lacs, but the VMC cadence is not well­suited for such an exercise, and the light curves of the three objects do not show any peculiarities. Finally, the spectra of LMC Astronomical Science Ivanov V. D. et al., Spectroscopic Confirmation of New Quasar Candidates in the NIRSMC 5_2 2414.098BRI 3_5 1913.297SMC 3_5 823.003BRI 2_8 1362.617BRI 2_8 22.477LMC 9_3 2639g2.311SMC 3_5 332.248SMC 3_5 222.143BRI 2_8 1972.101LMC 4_3 542.0941500 2000 2500 300005101520F λ + const, arbitrary unitsWavelength (Å)Lyα Si IV C IV C III] Mg II [O II]Figure 3. Example spectra of the quasar candidates sorted by redshift, shifted to restframe wavelength. The spectra were normalised to an average value of 1.0, and shifted vertically by offsets of two, four, etc., for display purposes. The SDSS composite quasar spectrum (Vanden Berk et al., 2001) is shown at the top.

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

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