Discovery of VVV CL001.
Key Words.:globular clusters: general, globular clusters: individual (UKS 1, VVV CL001), surveys
Context:It is not known how many globular clusters may have been left undetected towards the Galactic bulge.
Aims:One of the aims of the VISTA Variables in the Via Lactea (VVV) Survey is to accurately measure the physical parameters of the known globular clusters in the inner regions of the Milky Way and to search for new ones, hidden in regions of large extinction.
Methods:Deep near infrared images give deep -band photometry of a region surrounding the known globular cluster UKS 1 and reveal a new low-mass globular cluster candidate that we name VVV CL001.
Results:We use the horizontal branch red clump in order to measure E(B-V)2.2 mag, mag, and D=15.9 kpc for the globular cluster UKS 1. Based on the near-infrared colour magnitude diagrams, we also measure that VVV CL001 has E(B-V)2.0, and that it is at least as metal-poor as UKS 1, however, its distance remains uncertain.
Conclusions:Our finding confirms the previous projection that the central region of the Milky Way harbors more globular clusters. VVV CL001 and UKS 1 are good candidates for a physical cluster binary, but follow-up observations are needed to decide if they are located at the same distance and have similar radial velocities.
The inner regions of the Milky Way have been mapped thoroughly at all wavelengths. Yet, it is not known if there are still some distant globular clusters awaiting to be discovered, hidden beyond the bulge, due to the high density of stellar sources and the large and inhomogeneous interstellar extinction. Near-IR surveys have an advantage for searching these regions. Indeed, the 2MASS discovered two new globular clusters (Hurt et al. 2000). But the limiting magnitude of 2MASS (=14.3, for 10-detections; Skrutskie et al. 2006) may prevent the discovery of fainter objects, especially if they are located in highly reddened regions.
The asymmetry of the spatial distribution of known globular clusters around the Galactic center indicates that previous observations may have overlooked some additional globular clusters. Ivanov et al. (2005a) recently estimated that there may be about 10 clusters missing towards the inner Milky Way. The recent discoveries (last 10 years) include both faint (low mass) halo clusters as well as reddened globular clusters projected toward the bulge, e.g. 2MASS GC01 and 2MASS GC02 by Hurt et al. (2000) (see also Ivanov et al. 2000), ESO 280 SC06 by Ortolani et al. (2000), GLIMPSE C01 by Kobulnicky et al. (2005; but see also Ivanov et al. 2005; Davies et al. 2010), GLIMPSE C02 by Kurtev et al. (2008), AL-3 by Ortolani et al. (2006), FSR 1735 by Froebrich et al. (2007), Koposov 1 and Koposov 2 by Koposov et al. (2007), FSR 1767 by Bonatto et al. (2007), Whiting 1 (Carraro 2005) and Pfleiderer 2 by Ortolani et al. (2009).
The VISTA Variables in the Via Lactea (VVV) Public Survey has started mapping the inner disk and bulge of our Galaxy with VISTA 4m telescope (Visible and Infrared Survey Telescope for Astronomy) in the near-IR (Minniti et al. 2010; Saito et al. 2010). One of the main scientific goals of the VVV Survey is to study the bulge globular clusters and to search for new clusters. Here we present VVV CL001, the first globular cluster candidate discovered by the VVV Survey.
2 The VVV Survey Data
The VVV Survey data are acquired with the VISTA 4m telescope at ESO Paranal Observatory (Emerson & Sutherland 2010). The VVV field b351 was observed in the bands under subarcsec seeing conditions ( arcsec in ). The YZ- band observations are still pending. Each one of the VVV fields (tiles) covers in total ( in by in ). The bulge field b351 observed here is centered at , , , .
We use here the images processed by CASU VIRCAM pipeline v1.0 (e.g. Irwin et al. 2004). The photometry was obtained with DoPhot (Schechter et al. 1993). Also, the photometry is uniformly calibrated against the 2MASS catalog (Skrutskie et al. 2006). The limiting magnitude of the single epoch VVV images is in the bulge fields (for details on the observing strategy, see Minniti et al. 2010).
The distance probed along the line of sight depends on the reddening of the fields. For example, in zero reddening disk fields we would see horizontal branch red clump beyond 50 kpc. Therefore we can search for distant galactic globular clusters and measure their physical parameters.
Visual inspection of the images of the field b351 led to the serendipitous discovery of a star cluster candidate that we name VVV CL001. This object is located in the vicinity of the known globular cluster UKS 1 (Figure 1). Based on near infrared stellar density maps (see Figure 2) we conclude that this is not a statistical fluctuation of the background, and that the cluster VVV CL001 is centered at , (, ). Applying the procedure by Koposov et al. (2008); Belokurov et al. (2009), the statistical significance of the over-density at the coordinates given above is S= 9.31. There is no source identified at this location based on radio, infrared, and visible data on the SIMBAD, 2MASS and NED databases, or any other star cluster catalog. The second obvious density peak at , is due to a saturated star.
The proximity of VVV CL001 and UKS 1 on the sky (Figure 1) made it appropriate to use the latter cluster as comparison in order to measure the reddening and distance of the new globular cluster candidate. Therefore, we concentrate first on measuring the parameters of UKS 1 using VVV Survey data.
3 The Globular Cluster UKS 1
Previous infrared photometry and spectroscopy of UKS 1 giants revealed that it is a very distant and reddened cluster, with distance modulus , and E(B-V) 3 (Minniti et al. 1995), and moderately metal-rich, with [Fe/H]=-0.78 (Origlia et al. 2005).
In order to determine the distance to UKS 1 we will use the horizontal branch red clump, which is very conspicuous in this cluster. The red clump stars with known parallaxes measured by the Hipparcos satellite are well calibrated standard candles (Paczynski & Stanek 1998; Alves 2000). Alves (2000) obtained a K-band calibration of the horizontal branch red clump luminosity, and applied the calibration to the red clump of the Galactic bulge. For example, this calibration has also been applied to the red clump of the Large Magellanic Cloud (e.g., Alves et al. 2002; Pietrzyński et al. 2003a; Borissova et al. 2009). The uncertainties in the reddening should in all cases be larger than the uncertainties due to the unknown metallicity. Our magnitudes are in the 2MASS magnitude system. Therefore we transform the -band magnitudes of red clump stars of Alves (2000) to magnitudes using (from Grocholski & Sarajedini 2002). The zero point differences should be less than magnitudes (Alves et al. 2002). From the previously cited works, we adopt the following mean values for the red clump stars: , and (López-Corredoira et al. 2002).
The distance modulus to the horizontal branch red clump in the globular cluster would be:
Adopting the mean red clump magnitudes and colors discussed above, and the reddening coefficients from Cardelli et al. (1989), this simplifies to
Using this equation we compute the mean distance modulus (and distance in kpc) for the horizontal branch red clump in UKS 1, which has mean and (Figure 4). This yields a mean , equivalent to , and a distance modulus , equivalent to 15.9 kpc. The uncertainty in the red clump position was estimated to be m=0.012 and =0.08 , based on analysis of the relevant region of the CMD. This results in an error in the distance modulus and corresponding distance of =0.078 and D=0.6 kpc. The distance to UKS 1 was also measured by Ortolani et al. (2007) using deep CST photometry. They find the mean magnitude of the horizontal branch at , and the mean colour or depending on the calibration, and estimated to kpc, placing it beyond the Galactic center. They find a total reddening of to also depending on the calibration. Our values support the lower reddening values of Ortolani et al. (2007). However, our distance measurement favors the largest distance measurement of Ortolani et al. (2007), and it is consistent with the fact that UKS 1 is located well beyond the Galactic center, but not as far as the Sgr dwarf galaxy discovered by Ibata et al. (1994).
4 The Globular Cluster Candidate VVV CL001
Figure 3 shows that the red giant branch of VVV CL001 is well defined, but the precise location of the horizontal branch red clump is not, and we cannot use the same method to estimate the distance. However, Figure 4 shows that the RGB of VVV CL001 is bluer than that of UKS 1. This may be due to a lower extinction or to a lower metallicity, or to a combination of both effects. Within the selected sample, the saturation of the brightest RGB stars prevents us from using the tip of the RGB for an independent distance determination. The comparison between red clump and tip of the RGB distance estimates could otherwise be used to assess the accuracy of the distance. Although the absence of a populated horizontal branch red clump in VVV CL001 may be due to low metallicity, we test the possibility that this is a staticstical effect, due to the small sample. The CMDs of a randomly selected UKS 1 stars, corresponding to the VVV CL001 sample size don’t feature a distinct HB either. This leads us to the conclusion that the missing HB is mostly a statistical effect. Refing the selection criteria and an increased spatial resolution may help to detect some HB stars.
Based on the CMD, and assuming similar reddenings for both clusters, we can conclude that VVV CL001 has similar metallicity or is slightly more metal-poor than UKS 1. Otherwise it would be hard to reconcile the clearly differing colours of the RGBs of these two clusters (see Figure 3), but they do not allow for a large difference in metallicity. The integrated near-infrared luminosity for Milky Way globular clusters are not very well known (Cohen et al. 2007, and references therein), nevertheless by comparing the RGB population in VVV CL001 and UKS 1 we also estimate that the new cluster candidate is 3.7 mag fainter in then UKS 1.
Thus, even though we do not have an accurate distance, the comparison with UKS 1 allows us to conclude that VVV CL001 is located beyond the Galactic center, on the opposite side of the Milky Way. Therefore, this does not appear to be a bulge globular cluster, as defined by Minniti et al. (1995); Barbuy et al. (1998).
For consistency, we can also determine the reddening from the mean location of the VVV CL001 red giants in the J-H vs colour-colour diagram. The mean and =0.4 yields = 6 and , slightly less reddened than UKS 1. On the other hand, the maps of Schlegel et al. (1998) give mean reddening of for this region of VVV field b351, which is larger than our measurements, but the reddening towards the bulge is known to be nonuniform and patchy.
The data are not good enough to measure the distance, but we have an accurate reddening and location, and deeper follow-up observations would allow to define the other structural parameters of VVV CL001. Based on the number of giants we crudely estimate that this cluster has low mass, being about 50-100 times less massive than UKS 1. In that case, VVV CL001 would join the list of very low luminosity globular clusters of the Milky Way (Koposov et al. 2007) (Table 1). Given the uncertainty of the cluster distance the possibility of VVV CL001 being an old, compact open cluster is at this point not strictly ruled out, although the high stellar concentration and the well populated RGB favour the globular cluster interpretation.
The proximity of these two globular clusters in the sky is surprising. There are no other globular cluster pairs in the Milky Way separated by only arcmin. If they are located at the same distance, and have the same radial velocity, these two clusters may be bound: taking the distance of UKS 1 of 0.6 kpc, the separation would be less than pc. There are physical pairs of clusters known in other galaxies (e.g., Bhatia & Hatzidimitriou 1988; Dieball et al. 2002; Bekki et al. 2004; Minniti et al. 2004). However, the present initial results are not good enough to indicate that VVV CL001 is at the same distance as UKS 1, and therefore we cannot claim that they form a physical binary cluster. We still need to measure their radial velocities to test the binary nature of the two clusters.
Alternatively, this may be just a chance alignment, and VVV CL001 could be a globular cluster or, although less likely, an old open cluster (see Section 4), either more distant or closer to the Sun than UKS 1. Clearly, deeper photometric observations are also needed for this object.
The VVV Survey is searching for missing globular clusters in the inner regions of the Milky Way galaxy. We report here the discovery of VVV CL001 (Figure 1), a low-mass globular cluster candidate at , . This is located only 8 arcmin away from the Galactic globular cluster UKS 1 in the sky.
We take advantage of this spatial proximity to use UKS 1 as reference in order to estimate the parameters of this newly discovered cluster. We measure the distance and reddening of UKS 1, finding and kpc.
We present the first colour-magnitude diagrams of the new cluster VVV CL001 (Figure 3), estimating . We cannot define the mean -band magnitude of the horizontal branch red clump for this cluster, and therefore its distance is uncertain. A very crude estimate by comparison with the RGB of UKS 1 gives a similar distance, placing VVV CL001 well beyond the bulge of the Milky Way. Observations in the -band to be acquired in the following seasons by the VVV Survey would allow us to improve this CMD of Figure 3 and define the HB as well as the MSTO, which would allow a photometric age determination. Also, we estimate that the RR Lyrae of VVV CL001 would be within the limit of detection of our VVV Survey.
We cannot definitely conclude that the proximity of UKS 1 and VVV CL001 on the sky imply that they compose a binary cluster because the distance to VVV CL001 is too uncertain. This remains as an exciting possibility that needs to be confirmed not only by means of a more accurate distance determination, but also by measuring their respective radial velocities from spectroscopic measurements. Finally, the present results are very encouraging and we conclude that the VVV Survey can potentially provide the largest and most homogeneous census of globular clusters in the survey area, out to well beyond the Galactic center, even in regions of large extinction.
|VVV CL001||Koposov 1||Koposov 2|
|Position [l,b]||4.99, 1.84||260.98, 70.75||195.11, 25.55|
|Distance [kpc]||uncertain||50kpc||40 kpc|
|Radius||1 pc||3 pc||3 pc|
based on a distance of 16 kpc and a selected radius of 15 arcsec
Acknowledgements.We thank the Cambridge Astronomical Survey Unit (CASU) for processing the VISTA raw data. We acknowledge support by the FONDAP Center for Astrophysics 15010003, BASAL Center for Astrophysics and Associated Technologies PFB-06, MILENIO Milky Way Millennium Nucleus P07-021-F from MIDEPLAN, FONDECYT 1090213 from CONICYT, and the European Southern Observatory. We use data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. We also thank the referee, whose comments helped to improve the paper significantly.
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