On the remote Galactic Globular Cluster NGC 2419 11affiliation: Based on data collected at the 3.5 m Telescopio Nazionale Galileo, operated by INAF
We present a new, deep (V 26) study of the Galactic globular cluster NGC 2419 based on time-series CCD photometry over about 10 years and extending beyond the cluster published tidal radius. We have identified 101 variable stars of which 60 are new discoveries, doubling the known RR Lyrae stars and including 12 SX Phoenicis stars. The average period of the RR Lyrae stars (=0.662 d, and =0.366 d, for fundamental-mode RRab and first-overtone pulsators, respectively), and the position in the period-amplitude diagram both confirm that NGC 2419 is an Oosterhoff II cluster. The average apparent magnitude of the RR Lyrae stars is =20.31 (=0.06, 67 stars) and leads to the distance modulus =19.60 0.05. The Color-Magnitude Diagram, reaching about 2.6 mag below the cluster turn-off, does not show clear evidence of multiple stellar populations. Cluster stars are found until , and possibly as far as , suggesting that the literature tidal radius might be underestimated. No extra-tidal structures are clearly detected in the data. NGC 2419 has many blue stragglers and a well populated horizontal branch extending from the RR Lyrae stars down to an extremely blue tail ending with the “blue-hook”, for the first time recognized in this cluster. The red giant branch is narrow ruling out significant metallicity spreads. Our results seem to disfavor the interpretation of NGC 2419 as either having an extragalactic origin or being the relict of a dwarf galaxy tidally disrupted by the Milky Way.
NGC 2419 is one of the most distant and luminous globular clusters (GCs) in the Milky Way (MW). Although both the distance ( 90 kpc, Harris et al. 1997) and the dynamical parameters (core radius pc, and half-mass radius 19 pc, Harris et al. 1997) put NGC 2419 among the outer halo Galactic globulars, the cluster has several unusual properties for an outer halo GC. It is much more luminous and metal-poor than the other outer halo clusters: with mag (Harris 1996) NGC 2419 is among the five brightest clusters in the MW; and with [Fe/H]2.1 dex (Suntzeff et al. 1988) it belongs to the most metal-poor group of MW GCs that all, except AM-4, are located within 20 kpc. The cluster horizontal branch (HB) also resembles that of much closer “canonical” metal-poor clusters like M15 or M68, and previous investigations show that NGC 2419 has the same age of M92, within 1 Gyr (Harris et al., 1997). However, NGC 2419 is not an inner halo cluster migrated out on an elliptical orbit, since its dynamical parameters and orbital properties (van den Bergh 1993; van den Bergh 1995, and references therein) are typical of an outer halo cluster. NGC 2419 is also anomalous in the half-light radius () vs plane (see Fig. 11 of Mackey & van den Bergh 2005). Among the MW GCs only Cen and M54 have similar properties in this plane, and they both are “peculiar”, since Cen hosts multiple stellar populations (see e.g. Bedin et al. 2004; Rey et al. 2004; Sollima et al. 2005) and likely is the stripped core of a defunct dwarf galaxy (Villanova et al. 2007, and references therein); and M54 is thought to be the core of the Sagittarius (Sgr) dwarf spheroidal galaxy (dSph) that is currently merging with the MW (see e.g. Layden & Sarajedini 2000). All these peculiarities and the similarity with Cen and M54 suggest that NGC 2419 could have an extragalactic origin and be the relict of a dwarf galaxy tidally disrupted by the MW (Mackey & van den Bergh, 2004). Newberg et al. (2003) find that the cluster appears to lie within an overdensity of A-type stars connected to previously discovered tidal tails of the Sgr dSph, and conclude that the cluster might once have been associated to Sgr. In addition, the cluster has a central velocity dispersion () much lower than the dSphs for which this quantity has been measured, and, in the vs plane (Faber & Jackson 1976), lies 3 and 6 apart from the “fundamental plane” relations for GCs and elliptical galaxies, respectively (see de Grijs et al. 2005). For a comparison, in this plane Cen lies at the intersection of these two lines.
Color-Magnitude Diagrams (CMDs) of NGC 2419 published so far either do not go fainter than the main sequence turn-off (TO) or cover small portions of the cluster (Christian & Heasley 1988, Harris et al. 1997, Stetson 1998, 2005; Saha et al. 2005; Sirianni et al. 2005). We also lack a modern study of the cluster variable stars based on accurate CCD photometry, the most recent variability survey being the photographic work by Pinto & Rosino (1977, hereafter PR) who detected 41 variables in the external regions of NGC 2419 and found the average period of the fundamental-mode RR Lyrae stars (RRab) to be consistent with NGC 2419 being an Oosterhoff II (OoII) cluster (Oosterhoff 1939).
In this Letter we present a CMD reaching about 2.6 mag below the NGC 2419 TO and a new study of the variable stars based on image subtraction techniques (Alard 2000), using time-series CCD photometry covering an area that extends well beyond the cluster published tidal radius (, according to Trager et al. 1995). The new data are used to verify whether multiple stellar populations and tidal tails exist in the cluster and to check whether the properties of the RR Lyrae stars support an extragalactic origin for NGC 2419.
2 Observations and data analysis
Time-series photometry of NGC 2419 (RA=07:38:24.0, DEC=38:54:00, J2000) was collected between 2003 September and 2004 February with DOLORES at the 3.5m TNG telescope111http://www.tng.iac.es/instruments/lrs/. The TNG data were complemented by WFPC2@HST F555W and F814W archival photometry spanning 7 years from 1994 to 2000, and by images of the cluster obtained with the Suprime-Cam of the SUBARU 8.2m telescope222http://www.subarutelescope.org/ along four nights in 2002. The SUBARU dataset covers a total area of 50 43 arcmin centered on NGC 2419 and includes both the TNG and HST fields. Results presented in this Letter refer to a region extending in North-South and in East-West from the cluster center. The total number of phase points of the combined datasets reaches 20, 205 and 48 in the , and bands, respectively, with optimal sampling of the light curves of RR Lyrae stars, acceptable coverage in , and rather poor sampling in , since the -band images were taken much more closely spaced in time.
Images were pre-reduced following standard techniques (bias subtraction and flat-field correction) with IRAF333IRAF is distributed by the National Optical Astronomical Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. We measured the star magnitudes by PSF photometry running the DAOPHOTII/ALLSTAR/ALLFRAME packages (Stetson, 1987, 1994) on the TNG, HST and SUBARU datasets, separately. Typical internal errors of the band photometry for single phase points at the level of the HB are in the range from 0.01 to 0.02 mag. The absolute photometric calibration was obtained by using local standards in NGC 2419 from P.B. Stetson’s list444Available at http://cadcwww.dao.nrc.ca/standards/. Zero point uncertainties are of 0.022, 0.014 and 0.014 mag in , and , respectively. Further details on the data reductions can be found in Di Criscienzo et al. (2007, in preparation).
Candidate variable stars were identified using two independent methods: the Optimal Image Subtraction Technique and the package ISIS 2.1 (Alard 2000), applied to the TNG time-series; and an ad hoc procedure applied to the SUBARU data that included calculation of the Fourier transform (in the Schwarzenberg-Czerny 1996 formulation) for each star with more than 25 epoch data; evaluation of the signal-to-noise ratio and then analysis of the stars with S/N 6 and magnitude 23.7 mag (excluding TO and sub giant branch stars). The two procedures returned a catalogue of 101 confirmed variables. Periods (and type classification) were derived using GRaTiS (Graphical Analyzer of Time Series), a custom software developed at the Bologna Observatory (see Di Fabrizio 1999; Clementini et al. 2000). Precision of the period determinations is of 4-5 decimal places (for variables with periods shorter than 2 d, 95 objects) and increases up to 6 digits for stars with the three datasets (TNG, HST and SUBARU) available. The good sampling of the light curves allowed a very accurate definition of the star’s visual mean magnitudes and amplitudes. Coverage of the and light curves is generally much poorer, and we often estimated average and magnitudes by scaling down in amplitude the star light curve to fit observations in the other bands (see Di Criscienzo et al. 2007).
Fig. 1 shows the , CMDs (based on the SUBARU dataset) of objects in four annular regions at increasing distance from the cluster center: 50 r 4.5 (panel a, 36262 objects, 91 variable stars); (panel b, 6116 objects, 7 variables); (panel c, 1592 objects, zero variables); and 16.5 r 18 (panel d, 594 objects, 1 variable: a field Scuti star), whose areas are in the ratio 1:3:2:1. Only objects with 1.2, and 0.2 mag, are displayed. Panel (d) shows the CMD of an external field devoid of cluster stars with same area as the cluster region in panel (a) and thus provides an indication of the contamination by field stars in panel (a).
3 The Variable Star Population
PR identified 41 variable stars in their photographic study of NGC 2419: 25 ab- and 7 c-type RR Lyrae stars, 1 Population II Cepheid, 4 red irregular/semiregular variables, and further 4 variables for which they did not provide period and classification. PR V39 was later recognized as double-mode RR Lyrae star (Clement & Nemec, 1990). We recovered and derived reliable periods for all the previously known variable stars in NGC 2419, and detected 60 new variables that are mainly located in the cluster central regions. The new variables include: 11 and 28 type RR Lyrae stars, 12 SX Phoenicis stars, 3 binaries, 1 long period variable (LPV) near the red giant tip, 2 field Scuti stars, and 3 variables of unknown type. Light curves for different types of variables are shown in Fig. 2. The high performance of the image subtraction to detecting small amplitude variables in very crowded fields allowed us the discovery of a remarkably large number (28 objects) of c-type RR Lyrae stars increasing fivefold the statistics of the RRc stars previously known in NGC 2419. Most of the new RRc’s as well as the new RRab stars with longer periods were missed by PR due to their small amplitude and the location towards the cluster center. Addition of the new discoveries, brings the number of RR Lyrae stars in NGC 2419 to 38 RRab, 36 RRc and 1 RRd star, and changes the ratio of number of RRc over number of RRc+RRab stars from 0.28 (PR) to 0.49, in much better agreement with typical values of the OoII clusters. We also revise the cluster HB morphology parameter HBR (Lee, Demarque & Zinn 1990, and reference therein) from 0.86 (Harris 1996), to about 0.76 (after decontamination by field stars, see Di Criscienzo et al. 2007). Average periods are: =0.662 d (=0.055, average on 38 stars), and =0.366 d, (=0.038, average on 36 stars), for fundamental-mode and first-overtone RR Lyrae stars, respectively, to compare with =0.654 d and =0.384 d from PR. The new values confirm and strengthen the classification of NGC 2419 as OoII cluster. The minimum period of the RRab stars, =0.576 d (star V40, for which PR could not derive period information), is in perfect agreement with values of prototype OoII clusters like M15 and M68. Fig. 3 shows the -band period-amplitude distribution (Bailey diagram) of the NGC 2419 RR Lyrae stars. The bulk of RRab variables lies in the region occupied by the OoII GGCs (dot-dashed line, from Clement & Rowe 2000), confirming the OoII nature of NGC 2419, the few stars between Oo I and II lines being entirely accounted for by crowded objects and possible undetected Blazkho variables (Blazhko 1907; see discussion in Di Criscienzo et al. 2007). For a comparison, we also show in Fig. 3 the period-amplitude distributions of the bona fide regular (solid curve) and well-evolved (dashed curve) ab RR Lyrae stars in M3 from Cacciari et al. (2005). The average apparent magnitude of the NGC 2419 RR Lyrae stars is = 20.31 (=0.06, average on 67 stars) excluding objects contaminated by companions. This value, combined with the cluster metallicity ([Fe/H]=2.1 dex, Suntzeff et al. 1988) and reddening, is used to estimate the cluster distance. Harris (1996) reports mag (from the average of various sources), while a lower value is derived on the basis of the Schlegel et al. (1998) maps. We find mag by matching the edges of the RR Lyrae instability strip of NGC 2419 to those of M68 (Walker 1994, ) and M5 (Reid 1996, mag), in good agreement with Schlegel et al. (1998). Assuming for the absolute luminosity of the RR Lyrae stars at [Fe/H]=, =0.590.03 (Cacciari & Clementini 2003), =0.214 ( 0.047) mag/dex (Clementini et al., 2003) for the slope of the luminosity metallicity relation, =0.08 mag, and [Fe/H]= dex, the distance modulus of NGC 2419 derived from the mean luminosity of its RR Lyrae stars is: =19.60 0.05 (D= 83.2 1.9 kpc).
NGC 2419 hosts many blue straggler stars (BSS). Among them we detected 1 binary system and 12 pulsating variables with periods in the range from 0.041 to 0.140 d and several secondary periodicities. They are likely cluster SX Phoenicis stars (see Di Criscienzo et al. 2007). One of them is located at from the cluster center.
4 The Color-Magnitude Diagram
Fig. 1 shows that the bulk of the NGC 2419 stars and variables is within 4.5 from the center (panel a). Cluster stars are found beyond the published tidal radius (8.74, Trager et al. 1995) until (see panel b) and possibly as far as . The CMD appears to be dominated by field stars and contaminating galaxies for . The main features of the NGC 2419 CMD are (panels a, b): a well defined Main-Sequence reaching about 2.6 mag below the cluster TO at 23.4 mag, with no clear evidence of multiple substructures. The red giant branch (RGB) is narrow, ruling out significant differences in composition among cluster stars. The HB is well populated to the blue and has very few stars redder than the RR Lyrae. Most striking features are the well defined BSS sequence outlined by many SX Phoenicis stars, and the extremely prolonged HB blue tail, extending down to 24.7 mag, (i.e. ) with a gap between 23.4 and 23.8 mag. This group of extremely hot stars, cannot be fitted by theoretical models of extreme HB stars (e.g. Pietrinferni et al. 2006 post-Helium flash theoretical models555Available at http://www.te.astro.it/BASTI/index.php for the proper composition of NGC 2419) since “blue-hook” stars experience the Helium flash after leaving the RGB. First detected by Whitney et al. (1998) and D’Cruz et al. (1998) in their analysis of the Cen HB, “blue-hook” stars have been found so far only in very few GGCs ( Cen, M54, NGC 2808, NGC6388, NGC6273, see Rosenberg et al. 2004, Momany et al. 2007).
5 Discussion and Conclusions
The results presented in this Letter provide some constraints on the hypotheses put forward for the origin of NGC 2419. The finding that the cluster confirms to be of OoII type, like “normal” low metallicity GGCs as M15 and M68, makes its extra-galactic origin unlikely, since GCs in external galaxies generally have properties intermediate between OoI and II types (Catelan 2005). The lack of multiple populations or metallicity spreads does not corroborate the hypothesis that NGC 2419 might be the core of a defunct galaxy either. In addition, no cluster extra-tidal structures are clearly seen in our data beyond to support Newberg et al. (2003) suggestion of a past association with the Sgr dSph, a claim also disfavored by the Sgr field and cluster (M54) RR Lyrae stars having properties on the long-period tail of the OoI group and intermediate between Oo types, respectively (Cseresnjes 2001; Cacciari et al. 2002). From our data NGC 2419 appears indeed a very normal, low metallicity Galactic GC, the only exception being the HB “blue-hook”, a feature detected so far only in very few globulars and, most noteworthy, in those showing multiple main sequences and/or of likely extragalactic origin: NGC 2808 (Piotto et al. 2007); Cen (Villanova et al. 2007); and M54 (Layden & Sarajedini 2000). However, the only property these clusters seem to share is a large integrated luminosity (Rosenberg et al. 2004). On the other hand, NGC 2419 peculiar position on the vs and vs planes remains unexplained. “Clearly there are still all kinds of mysteries that we do not yet understand at the intercept between dwarf spheroidal galaxies and globular clusters” (Sidney van den Bergh 2007, private communication).
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