Draft version September 4, 2015Preprint typeset using LATEX style emulateapj v. 01/23/15

TOWARDS PRECISION PHOTOMETRY WITH EXTREMELY LARGE TELESCOPES:THE DOUBLE SUBGIANT BRANCH OF NGC 1851

P. Turri1

Department of Physics and Astronomy, University of Victoria,3800 Finnerty Road, Victoria, BC V8P 5C2, Canada

A. W. McConnachie, P. B. StetsonHerzberg Astronomy and Astrophysics, National Research Council Canada

5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

G. FiorentinoOsservatorio Astronomico di Bologna, Istituto Nazionale di Astrofisica

Via Ranzani 1, 40127 Bologna, Italy

D. R. Andersen, J.-P. VeranHerzberg Astronomy and Astrophysics, National Research Council Canada

5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

and

G. Bono2

Dipartimento di Fisica, Universita di Roma Tor Vergata,Via della Ricerca Scientifica 1, 00133 Roma, Italy

Draft version September 4, 2015

ABSTRACT

The Extremely Large Telescopes currently under construction have a collecting area that is an orderof magnitude larger than the present largest optical telescopes. For seeing-limited observations theperformance will scale as the collecting area but, with the successful use of adaptive optics, formany applications it will scale as D4 (where D is the diameter of the primary mirror). Central tothe success of the ELTs, therefore, is the successful use of multi-conjugate adaptive optics (MCAO)that applies a high degree correction over a field of view larger than the few arcseconds that limitsclassical adaptive optics systems. In this letter, we report on the analysis of crowded field imagestaken on the central region of the Galactic globular cluster NGC 1851 in Ks band using GeMS at theGemini South telescope, the only science-grade MCAO system in operation. We use this cluster as abenchmark to verify the ability to achieve precise near-infrared photometry by presenting the deepestKs photometry in crowded fields ever obtained from the ground. We construct a colour-magnitudediagram in combination with the F606W band from HST/ACS. As well as detecting the “knee” inthe lower main sequence at Ks '20.5, we also detect the double subgiant branch of NGC 1851, thatdemonstrates the high photometric accuracy of GeMS in crowded fields.Keywords: globular clusters: individual (NGC 1851) — instrumentation: adaptive optics — tech-

niques: photometric

1. INTRODUCTION

In classical adaptive optics (AO), the aberrated wave-front of a guide star is measured by a wavefront sensorand corrected by a deformable mirror (DM) to deliver adiffraction-limited image inside a field of view only sev-eral arcseconds wide, outside of which the point spreadfunction (PSF) rapidly widens and deforms. This scaleis set by the limiting angle inside of which the turbu-lence is correlated. A larger and more uniform field ofview can be obtained with multi-conjugate adaptive op-

turri@uvic.ca1 Herzberg Astronomy and Astrophysics, National Research

Council Canada, 5071 West Saanich Road, Victoria, BC V9E2E7, Canada

2 Osservatorio Astronomico di Roma, Istituto Nazionale di As-trofisica, Via Frascati 33, 00040 Monte Porzio Catone (RM),Italy

tics (MCAO), by using an asterism of guide stars withmultiple DMs optically conjugated at different heights ofthe atmosphere. Typically, AO works best in the near-infrared (NIR) because the spatial and temporal coher-ence of the aberrated wavefront is larger at longer wave-lengths, resulting in better corrections (for a comprehen-sive review, see Davies & Kasper (2012)).The Gemini Multi-Conjugate Adaptive Optics System(GeMS) (Rigaut et al. 2014; Neichel et al. 2014) on theGemini South telescope on Cerro Pachon in Chile is thefirst facility-class MCAO instrument in operation, al-though the concept has previously been proven with theMulti-Conjugate Adaptive Optics Demonstrator (MAD)on VLT (Marchetti et al. 2007). GeMS uses two DMs andfive sodium laser guide stars (LGS) to deliver a correctedfield of view of 83′′ × 83′′ to the Gemini South AdaptiveOptics Imager (GSAOI) (McGregor et al. 2004; Carrasco

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Figure 1. Left: Mosaic image of NGC 1851 taken with GeMS/GSAOI with a field of view of 83′′ × 83′′. The positions of the five sodiumLGSs are shown with orange markers and indicated by red circles are the three NGSs (clockwise from the left, mR = 13.3, 14.4, 13.7).Right: a zoom-in of the region in the left panel contained within the blue rectangle observed with GSAOI in Ks band (top) and the sameregion observed with HST/ACS in the optical by Sarajedini et al. (2007) (bottom). The spatial resolution of Gemini in the NIR and HSTin the optical is comparable.

et al. 2012), a mosaic of 2 × 2 HAWAII-2RG chips witha pixel scale of 0.01962′′ px-1.Galactic globular clusters (GGCs) are an ideal target forMCAO systems. From an instrumentation perspective,their moderately large expanse on the sky and their highdegree of crowding means that they are well-suited toMCAO systems that can potentially resolve faint stars intheir central regions, while still possessing a large num-ber of bright red natural guide stars (NGS) that areneeded for the determination of the tip-tilt and defo-cus aberrations (Rigaut & Gendron 1992; Herriot et al.2006). While deep and accurate NIR stellar photome-try of GGCs has been a prerogative of the Hubble SpaceTelescope (Milone et al. 2012, 2014), in the future AO-assisted ELTs could get an edge over space telescopes inthe NIR due to their much smaller diffraction limit.From an astronomical perspective, the last decade haswitnessed a revolution in our understanding of GCs. Thecurrent spectroscopic evidence indicates that all GGCsshow star-to-star variations of C and N abundances to-gether with well defined O-Na and Mg-Al anticorrela-tions in both evolved (RGB, HB) and unevolved stars.The fraction of GGCs showing also multiple sequences islimited and probably correlated with the total mass ofthe cluster (Gratton et al. 2012). This confirmation ofmultiple stellar populations is in contrast to the classi-cal view of a coeval cluster with homogeneous chemicalabundances. In the case of NGC 1851, a double subgiantbranch (SGB) has famously been observed by Milone etal. (2008) using HST/ACS images. This feature has beeninterpreted as evidence of a second generation of star for-mation ∼ 1 Gyr older than the first; alternatively, Cassisiet al. (2008) suggested that the double SGB can be ex-plained as two coeval populations, where one is a factortwo more enhanced in CNO than the other.Here, we present first results from a Gemini/GSAOI cam-paign targeting Galactic GCs using GeMS. In particular,we present observations of NGC 1851 taken in the Ks

band that represent the deepest Ks photometry of a GCtaken from the ground. We use this GC as a bench-mark to demonstrate the precision of the MCAO databy producing a CMD of the stellar population with anHST/ACS optical band.

2. DATA

During the GeMS/GSAOI Science Verification phasein Fall 2012, we obtained images of NGC 1851 in the J(λ = 1.235 µm) and Ks (λ = 2.159 µm) bands. Subse-quent observing runs targeted five other globular clustersin the same bands. For this first paper in this series, wedescribe analysis of the Ks band data for NGC 1851; afull analysis of the J and Ks band discussing the prop-erties of NGC 1851 will be published in a forthcomingarticle, and the other GCs will be discussed in subse-quent papers. The observation with MCAO of globularclusters in NIR will prove particularly useful to studylow mass stars, giving information on the total clustermass, the lower end of the mass function and the colour-temperature relation for cold atmospheres. We will alsodiscuss the use of these data for astrometric analyses.The median seeing of our observations was recorded byRoboDIMM on Cerro Tololo to be of 0.77′′ and the in-strument delivered in Ks band an average corrected fullwidth at half maximum (FWHM) of 0.09′′, comparableto the 0.06′′ FWHM of the diffraction limit case. Theintegration consisted of 12 dithered sub-exposures of 160s each, and the time from the first to last science imagewas 75 minutes. Two short exposures of 21 s and 90s were observed to obtain photometry for the brightersources that were saturated in our longer exposures.Image reduction has been performed using the GeminiGSAOI IRAF package that contains a bad pixel map andappropriate non-linearity correction. A super-flat wasmade that combined the twilight and dome flat fields,utilising the uniform spatial illumination of the formerand the high signal-to-noise ratio of the latter. The lowdark current of the H2RG detectors means we did not

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Figure 2. Left: (Ks, F606W-Ks) colour-magnitude diagram of NGC 1851; the detail of the double SGB is shown in the inset. Right: sameas the left panel with average photometric (random) uncertainties indicated. Overlaid is the fiducial line with the approximate locationsof the main sequence turnoff and main sequence knee highlighted by red dots.

need to apply a dark frame. Local sky estimates wereused for the point source photometry (see below), and sowe did not need to apply a sky frame either. However, itis worth noting that there is a substantial number of badpixels in each chip, and that one of the chips has a ∼ 0.2mag higher efficiency than the other. Figure 1 shows anexample of a processed image.The DAOPHOT suite of programs (Stetson 1987; Stetson& Harris 1988; Stetson 1994) has been used to performthe photometric analysis on the individual frames, hav-ing already been proven to work with MCAO data (Bonoet al. 2010; Fiorentino et al. 2011; Davidge 2014; Sara-cino et al. 2015). To remove cosmic rays and bad pixelsfrom our catalogue, we keep only objects that are foundwithin a radius of one pixel in at least three ditheredexposures out of twelve. Particular care has been paidto the PSF fitting routine due to the expected shapeof the MCAO PSF. AO concentrates the light into thecore of the PSF, but only up to a certain scale given bythe density of actuators in the DM, beyond which thePSF remains uncorrected (Jolissaint et al. 2006). Thus,even though the fraction of light in the core of the PSFis substantially greater than in the seeing-limited case,the size of the halo relative to the core is much largerin the AO case, and the adopted PSF model should re-flect this. In addition, even although MCAO provides amore uniform correction over a larger field than classi-cal AO, some significant spatial and temporal variationin the PSF structure may still be expected. Therefore,each exposure has ∼ 300 manually selected bright andisolated stars used to model the PSF as an analytic pro-file plus a spatially-varying look-up table, where eachpixel of the PSF is allowed to spatially vary as a cubicfunction, independent of every other pixel of the PSF.While we conduct our analysis independently on every

exposure of every chip, we also perform an analysis onthe stacked median image of each chip in order to detectand photometer the faintest stars. We ensure a high fi-delity for our stellar catalogue by cross-matching everydetection to the catalogue of NGC 1851 from the ACSSurvey of Galactic Globular Clusters (Sarajedini et al.2007). Noise features and artefacts that are incorrectlyidentified as stellar objects are removed by this process,while real stars are kept. The full HST/ACS catalogueis considerably deeper than the Ks catalogue (by ∼4.5magnitudes), and so this does not limit the depth of theKs band data presented.Each frame of each chip is calibrated independently tothe Ks band magnitude scale of the 2MASS Point SourceCatalog through an intermediate catalogue generated byone of the authors (PBS) from 256 seeing-limited archivalimages obtained with CTIO/NEWFIRM. These latterexposures are both wide and shallow enough to contain alarge number (∼ 4000) of stars present also in the 2MASScatalogue. These were used to calibrate the magnitudesof fainter stars in the same images, which could thenbe used to determine the photometric zero points of theGeMS images.

We note that the extended halos of the MCAO PSF,combined with the spatial variation in the shape of thePSF, can lead to a photometric zero-point that changesbetween sub-exposures. This will be discussed and ex-plored in more detail in Turri et al. (2015, in prepara-tion). The data presented in this letter have been cor-rected for this variable zero-point, but no color correctionis applied, however we estimate it to be small. Note thatthis makes no difference to the analysis presented herein.For each star, the final magnitude was calculated by com-bining the magnitudes measured in each exposure usingartificial skepticism (Stetson 1989, chap. 3). The red-

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Figure 3. Left: As Figure 2, but enlarged in the region of the double SGB. Overlaid is the selection box orientated parallel with the linearsegment of the SGB. Right: histogram of the distribution of offsets relative to the median of the left panel box. Overlaid is the best-fitdouble Gaussian to the bimodal distribution.

dening correction in the direction of NGC 1851 is small(E(B − V ) = 0.02, Harris (1996, 2010 edition)) and hasnot been applied.

3. RESULTS

Figure 2 shows the (Ks, F606W-Ks) CMD for NGC1851 obtained by our analysis, and contains ∼ 16000stars. The error bars are a combination of random uncer-tainty (photon and readout noise), PSF fitting residualsand scatter of the measurements between exposures. TheF606W data are taken from Sarajedini et al. (2007), andthe Ks data have additionally had quality control criteriain sharpness and χ2 applied. In addition, a coarse propermotion cut has been applied to reject foreground stars byrestricting the apparent difference in the positions of thestars between the two epochs to 8 mas (the HST/ACSdata were observed in 2006). More details and astromet-ric analysis will be presented in Turri et al. (2015, inpreparation).Several notable features are visible in Figure 2. Firstly,the extended horizontal branch is clearly visible and thered HB is particularly well defined. The asymptotic gi-ant branch can be seen departing from the red HB justprior to the point where our data are saturated. In ad-dition, Saviane et al. (1998) have observed the red gi-ant branch bump in NGC 1851 at a magnitude similarto the red HB. This feature occurs when the H-burningshell moving outwards approaches the chemical discon-tinuity caused by the first dredge-up of the convectiveenvelope, and it can be observed in Figure 2 on the RGBat Ks=13.8.The depth of the Ks is of particular note. Figure 2reaches ∼3.5 magnitudes deeper than the main sequenceturnoff with a magnitude error of 0.15 at Ks=22 usinga total exposure of 1920 s. This depth is impossible toachieve for a crowded target like NGC 1851 without theuse of MCAO. A similar depth in Ks has been obtainedby Monelli et al. (2015) in M 15 using FLAO on LBTbut 3′ from the center of the cluster.A bend in the main sequence is visible towards the limitsof our data, and starts at Ks ' 20.5. This feature, knownas the main sequence knee (MSK) is a typical feature inNIR colour-magnitude diagrams of GCs and is caused bycollisionally induced absorption of molecular hydrogen incold stellar atmospheres (Bono et al. 2010; Monelli et al.2015). It has been proposed that the relative distancebetween the MSK and the main sequence turnoff can be

used as a distance and reddening independent cluster agedetermination (Calamida et al. 2009), being the positionof the former point independent of age. We will revisitthis claim in future papers in this series.Of particular interest, the subgiant branch in Figure 3is clearly split into two sequences. This feature was firstidentified for NGC 1851 by Milone et al. (2008) usingHST/ACS data. Previous observations of the core of aGC with double SGBs from the ground have revealedat most a broadened SGB with a bimodal distribution(specifically, the excellent study of NGC 6656 by Li-bralato et al. (2014)); only observations of the uncrowdedoutskirts have been able to discern multiple SGBs fromseeing-limited images (Milone et al. 2009; Han et al. 2009;Cummings et al. 2014). The fact that this feature is vis-ible in the centre of NGC 1851 using the Ks band datafrom GeMS is a verification of our small photometricuncertainties which demonstrates significant promise forthe successful exploitation of MCAO for precision pho-tometry of crowded fields in the ELT era.The right panel of Figure 3 shows a histogram of theoffsets of the stars contained within the box in the leftpanel. Two separate peaks are clearly identifiable andthese correspond to the faint and bright SGB (fSGB,bSGB) as described by Milone et al. (2008). We modelthis distribution as two Gaussian curves that are over-laid on the histogram. By integrating these curves, wefind NfSGB/NbSGB = 0.47, entirely consistent with thefindings of Milone et al. (2009).

4. CONCLUSION

We present first results from a study of the core of NGC1851 using the GeMS MCAO system on Gemini South.We demonstrate the ability of this system to achieve pre-cise near-infrared photometry of crowded regions by pre-senting the deepest Ks observations ever obtained fromthe ground. In the colour-magnitude diagram createdwith the addition of the HST F606W band, we detectthe “knee” in the lower main sequence and the doublesubgiant branch of the globular cluster. A subsequentpaper will present a more detailed analysis of NGC 1851using these data, as well as an analysis of the photometricand astrometric performance and stability of the GeMSsystem. Here, we have used this cluster as a benchmarkof the quality of the MCAO photometry that has beenobtained. This work is part of a larger project withGeMS to obtain J, Ks photometry for several GCs in

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the southern hemisphere some of which have NIR datafrom HST/WFC3, e.g. NGC 2808 (Milone et al. 2012).MCAO systems are an integral part of the two largestELT projects as they tackle science programs that re-quire an extended field of view, with NFIRAOS as a firstlight system for the Thirty Meter Telescope (Boyer et al.2014), and MAORY on the European-ELT (Diolaiti et al.2014). As the only science-grade MCAO system, GeMSis an important new facility instrument that can serve asa scientific testbed for developing a practical understand-ing of the challenges of obtaining accurate photometryand astrometry using such a system. This pilot studyimplies that high precision photometry can be achievedfrom the ground using MCAO, and can inform the de-velopment of appropriate data processing techniques andobserving strategies to ensure the ELTs deliver their fullscientific promise over extended fields of view.

Based on observations obtained at the Gemini Obser-vatory, which is operated by the Association of Univer-sities for Research in Astronomy, Inc., under a cooper-ative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (UnitedStates), the National Research Council (Canada), CON-ICYT (Chile), the Australian Research Council (Aus-tralia), Ministerio da Ciencia, Tecnologia e Inovacao(Brazil) and Ministerio de Ciencia, Tecnologıa e Inno-vacion Productiva (Argentina). Acquired through theGemini Science Archive and processed using the GeminiIRAF package.G. Fiorentino has been supported by the FIRB 2013(MIUR grant RBFR13J716).We appreciate the prompt support received from theGeMS staff for the image reduction. We are grateful tothe adaptive optics group at NRC Herzberg for feedbackon the system performance and to INAF - OsservatorioAstronomico di Bologna for insights on stellar popula-tions in globular clusters.Facilities: Gemini:South (GeMS, GSAOI).

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