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Five New Globular Clusters Discovered in the Galactic Bulge

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This Letter reports the discovery of five new globular clusters (GCs) in the Galactic bulge (Camargo 1102, 1103,
1104, 1105, and 1106) using Wide-field Infrared Survey Explorer (WISE) images. Their natures are established by
using 2MASS and Gaia second data release (DR2) photometry. The new findings are old and metal-poor GCs
located less than 4 kpc from the Galactic center. Camargo 1102 seems to be located over the Galactic bar on the far
side of the Milky Way and at a vertical distance lower than 1 kpc. The other four clusters lie even closer to the
Milky Way mid-plane. The old ages and low metallicities suggest that the newly discovered GCs may have the
potential of providing important clues on the early inner Galaxy formation and its subsequent evolution, as well as
the current bulge structure and kinematics.

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Five New Globular Clusters Discovered in the Galactic Bulge

  1. 1. Five New Globular Clusters Discovered in the Galactic Bulge Denilso Camargo Colégio Militar de Porto Alegre, Ministério da Defesa—Exército Brasileiro Av. José Bonifácio 363, Porto Alegre, 90040-130, RS, Brazil; denilso.camargo@gmail.com Received 2018 May 14; revised 2018 June 5; accepted 2018 June 12; published 2018 June 25 Abstract This Letter reports the discovery of five new globular clusters (GCs) in the Galactic bulge (Camargo 1102, 1103, 1104, 1105, and 1106) using Wide-field Infrared Survey Explorer (WISE) images. Their natures are established by using 2MASS and Gaia second data release (DR2) photometry. The new findings are old and metal-poor GCs located less than 4 kpc from the Galactic center. Camargo 1102 seems to be located over the Galactic bar on the far side of the Milky Way and at a vertical distance lower than 1 kpc. The other four clusters lie even closer to the Milky Way mid-plane. The old ages and low metallicities suggest that the newly discovered GCs may have the potential of providing important clues on the early inner Galaxy formation and its subsequent evolution, as well as the current bulge structure and kinematics. Key words: catalogs – Galaxy: bulge – globular clusters: general – surveys 1. Introduction Globular clusters (GCs) were the first stellar systems formed in the early universe and are often considered to be living fossils of galaxy formation. Thus, they have become a powerful tool for improving our understanding of the Milky Way formation and early evolution. GCs also provide a means for tracing the Galaxy’s properties at the present time, for example, the bulge structure and kinematics. The Galactic bulge formation and evolution is one of the most important unsolved problems in the present epoch and, thus, remains the subject of intense debate (Di Matteo et al. 2015). Therefore, the discovery of new GCs in the bulge, as well as deriving accurate parameters for known ones, are essential for improving our knowledge of the inner Milky Way history as well as characterizing its current evolutionary stage. As the infrared wavelengths are less affected by dust than the visible ones, advances in the near- and mid-IR instrumentation in recent decades has made it possible to conduct wide-field sky surveys such as 2MASS, Wide-field Infrared Survey Explorer (WISE), and the European Southern Observatory-VISTA Variables in the Via Lactea (ESO-VVV), which have boosted the number of known star clusters contributing significantly to improve our knowledge about our Galaxy. In addition, the most recent surveys, including Gaia’s second data release (DR2) and the VVV EXtended Survey (VVVX), may provide answers for many of the still- unanswered questions. In this respect, the Harris (1996) GC catalog1 provides 157 objects. Additionally, advances in GC research, such as large sky surveys, have updated the list of Galactic GCs (Minniti et al. 2010, 2011; Fadely et al. 2011; Moni-Bidin et al. 2011; Kim et al. 2015, 2016). Recently, the ESO-VVV survey provided a list of 84 GC candidates toward the bulge (Minniti et al. 2017a, 2017b, 2017c), and Minniti et al. (2017c) suggested an amount of 50 unknown GCs in the Galactic bulge. Piatti (2018) established the nature of 22 GCs from Minniti et al. (2017c). Bica et al. (2016) provided a list of bulge clusters containing 43 entries. Valenti et al. (2007) derived parameters for a sample of 24 bulge GCs, and Cohen et al. (2017) analyzed another 16. This study presents five new GCs in the bulge. Following the recent star cluster catalog (Camargo et al. 2015a, 2016a) the new findings are named Camargo 1102, 1103, 1104, 1105, and 1106. The discovery were made by eye on WISE images. 2. Methods The cluster analysis is based on the 2MASS and Gaia-DR2 photometry. The 2MASS color–magnitude diagrams (CMDs), built with photometry extracted in circular concentric regions centered in the coordinates of each cluster, provide the basic parameters. However, in addition to the high extinction the newly discovered GCs suffer from crowding (strong disk and bulge contamination) toward the Galactic center. Thus, to uncover the cluster-intrinsic CMDs a field decontamination procedure is employed that has been successfully applied in the analysis of star clusters in previous works (Camargo et al. 2015a, 2015c, 2016a). The current algorithm is based on Bonatto & Bica (2007). The basic parameters are derived by fitting PARSEC isochrones (Bressan et al. 2012) on the field-star decontaminated CMDs. Such a procedure is guided by the direct comparison (red- giant-branch (RGB)-fitting) with the CMD of a reference GC (Bica et al. 2006; Valenti et al. 2007). The Gaia-DR2 proper motion (PM) distribution (Gaia Collaboration et al. 2018) is used to reinforce the cluster nature of the new findings. 3. Present Discoveries The present discoveries are part of a survey developed using WISE images (Camargo et al. 2015a, 2015b, 2015c, 2016a, 2016b). Figure 1 displays the WISE multicolor images centered on the coordinates of the GCs discovered in this work (Table 1). These images show only a relatively prominent central stellar concentration, which is expected for GCs in crowded fields that remained unknown until now. The decontaminated CMDs are shown in the upper panels of the Figure 2. As the photometry is limited by the crowding The Astrophysical Journal Letters, 860:L27 (5pp), 2018 June 20 https://doi.org/10.3847/2041-8213/aacc68 © 2018. The American Astronomical Society. All rights reserved. 1 http://physwww.mcmaster.ca/~harris/mwgc.dat 1
  2. 2. toward the Galactic center, the 2MASS CMDs of bulge GCs are typically populated by RGB+asymptotic giant branch (AGB) stars (Anguiano et al. 2016). Table 1 provides the coordinates and the basic parameters derived for the new findings. Such parameters suggest that these objects are old and metal-poor GCs. The middle panels of Figure 2 show the Gaia-DR2 PM distribution for stars located in the central region of the newly identified GCs (black circles) and in a comparison field (brown circles). The proper motion diagrams (PMDs) are built after applying a filter that discards stars with PM uncertainties larger than 0.5 mas yr−1 . The cluster nature of the new findings is evident in the PMDs, which present a clear concentration in the PM distribution of stars within the cluster central region. The 2MASS radial density profiles (RDPs) of the new findings (Figure 2, bottom panels) are built by using color– magnitude filters that select the RGB stars fitted by the isochrones, except for Camargo 1102, which has a filter that was extended to encompass the bright stars. The RDPs of the new findings reinforce their GC nature, but deeper photometry is required to derive structural parameters. Figure 2 (top-left panel) shows the 2MASS J×(J − H) decontaminated CMD of Camargo 1102 superimposed on the decontaminated CMD of NGC 6355 (Harris 1996), which is located on the far side of the Milky Way’s center. To avoid noise in the derived parameters, the CMD of Camargo 1102 is built with stars located in the central region of the cluster, reducing the contribution of possible non-member stars from the cluster outskirts. The comparison between the CMDs of Camargo 1102 and NGC 6355 suggests that the new finding may also be located on the far side of the Galactic center, but Camargo 1102 appears to be more metal-poor than NGC 6355. The best-fitting isochrone (Bressan et al. 2012) for Camargo 1102 suggests an age of 13.3±1.0 Gyr and a metallicity of [Fe/H]=−1.7±0.2 dex. The CMD of the new GC also fit the RGB of M19 that is used as reference cluster for Camargo 1103 (Figure 2). With an extinction of AV=3.6±1 mag, Camargo 1102 seems to be located in a region with extinction lower than the bulge average value (Dutra et al. 2002). This cluster present an absolute magnitude of MV=−6.3± 0.6 mag. The derived heliocentric distance is de=8.23± 1.15 kpc and the Galactocentric distance RGC= 0.85± 0.28 kpc, adopting a distance of the Sun to the Galactic center of Re=8.0 kpc. The rectangular coordinates are xGC= 0.19±1.15 kpc, yGC=−0.12±0.02 kpc, and a vertical dis- tance from the Galactic plane of zGC=822.5±115 pc, which locate Camargo 1102 over the central bar on the far side of the Milky Way. The Gaia PM distribution (Figure 2) also suggests its cluster nature. The brighter stars in the CMD of Camargo 1102 are located in the cluster central region and are present in the Gaia PMD; however, most of the stars forming a possible horizontal branch (HB) present colors and magnitudes consistent with Galactic disk MS stars and are located on the outer parts of the PM distribution. On the other hand, if these stars are cluster members, the heliocentric distance may be lower than the derived value, but the uncertainties in the basic parameters of Camargo 1102 present a solution considering such an HB. The decontaminated CMD of Camargo 1103 superimposed on the M19 (Harris 1996) decontaminated photometry are shown in Figure 2. The brighter stars in the CMD of Camargo 1103 might be foreground stars that survived a non-100% efficient subtraction. The statistical field-star decontamination is affected by the crowding toward the bulge. The best-fitting isochrone and direct comparison with M19 provide the basic parameters that are shown in Table 1. The GC M9 is used as reference cluster to establish the nature of Camargo 1104 (Figure 2). The decontaminated CMD of Camargo 1104 seems to be strongly affected by differential reddening. The isochrone fitting reveals an old and metal-poor GC (Table 1). Most bright stars in the CMD of Camargo 1104, and those consistent with HB stars, are present on the PMD of stars within the cluster central region, but with a relatively peripheral position. The comparison with the RGB morphology of the GC Terzan 1 (Harris 1996) plus the best-fitting isochrone (Figure 2) provide the basic properties of Camargo 1105. Such parameters are displayed in Table 1. Camargo 1105 is the most reddened cluster in this sample. The bright stars in the CMD of Camargo 1105 present PMs consistent with cluster members, but the bluer stellar sequence appears to be formed by field stars that survived the decontamination procedure. Figure 2 displays the field-star decontaminated CMD of Camargo 1106 fitting the RGB of M107 (Harris 1996) and a PARSEC isochrone of 12.5 Gyr. The faint stars in the CMD of Camargo 1106 present magnitudes and colors consistent with HB stars. 4. Concluding Remarks This Letter marks the discovery of five new GCs in the Galactic bulge: Camargo 1102, Camargo 1103, Camargo 1104, Camargo 1105, and Camargo 1106. These low-Galactic- latitude GCs present low contrast and are very reddened (Table 1), which is consistent with their location in the very high-extinction and extremely crowded central region of the Milky Way. The new findings are old and metal-poor GCs with ages in the range of 12.5–13.5 Gyr and [Fe/H] ranging from −1.5 to −1.8 dex. Camargo 1102 is located over the Galactic bar on the far side of the Galactic center at a distance from the Sun of 8.2±1.2 kpc, at 0.85±0.28 kpc from the center of the Milky Figure 1. WISE multicolor images (7′×4′) centered on the coordinates of the new GCs. Top panels: Camargo 1106 (right) and Camargo 1105 (left). Middle panels: Camargo 1104 (right) and Camargo 1103 (left). Bottom panels: WISE (right) and 2MASS (left) images of Camargo 1102. 2 The Astrophysical Journal Letters, 860:L27 (5pp), 2018 June 20 Camargo
  3. 3. Table 1 Coordinates and Basic Parameters for the GCs Discovered in This Study Cluster ℓ b [Fe/H] AV MV Age de RGC xGC yGC zGC (°) (°) (dex) (mag) (mag) (Gyr) (kpc) (kpc) (kpc) (kpc) (kpc) Camargo 1102 359.145 5.734 −1.7±0.2 3.6±1 −6.3±0.6 13.3±1 8.2±1.2 0.85±0.28 0.19±1.15 −0.12±0.02 0.822±0.12 Camargo 1103 5.604 −2.121 −1.8±0.3 6.6±1 −6.9±1.0 13.5±1 5.0±0.8 3.03±0.77 −2.99±0.78 −0.07±0.01 −0.503±0.08 Camargo 1104 5.621 −1.778 −1.8±0.3 7.5±1 −5.7±1.7 13.5±0.5 5.4±1.0 2.68±0.98 −2.62±1.0 0.53±0.10 −0.167±0.03 Camargo 1105 359.479 2.017 −1.5±0.2 8.2±1 −6.3±1.0 13.0±1 5.8±0.9 2.18±0.90 −2.17±0.90 −0.05±0.01 0.205±0.03 Camargo 1106 357.351 1.683 −1.5±0.3 5.7±1 −5.7±1.6 12.5±1 4.5±0.4 3.54±0.41 −3.53±0.41 −0.21±0.02 0.131±0.01 3 TheAstrophysicalJournalLetters,860:L27(5pp),2018June20Camargo
  4. 4. Way, and have a vertical distance of 0.822±0.12 kpc. The other GCs discussed in this Letter are located in the near side of our Galaxy, with distances from the Sun in the range of 4.5–5.8 kpc and within 2.1–3.6 kpc from the Milky Way center, but closer to the Galactic plane. My thanks to an anonymous referee for valuable comments, which improved this work. This publication makes use of data products from WISE, 2MASS, and Gaia-DR2. ORCID iDs Denilso Camargo https://orcid.org/0000-0001-7344-4663 References Anguiano, B., De Silva, G. M., Freeman, K., et al. 2016, MNRAS, 457, 2078 Bica, E., Bonatto, C., Barbuy, B., & Ortolani, S. 2006, A&A, 450, 105 Bica, E., Ortolani, S., & Barbuy, B. 2016, PASA, 33, 28B Bonatto, C., & Bica, E. 2007, MNRAS, 377, 1301 Bressan, A., Marigo, P., Girardi, L., et al. 2012, MNRAS, 427, 127 Camargo, D., Bica, E., & Bonatto, C. 2015a, NewA, 34, 84 Camargo, D., Bica, E., & Bonatto, C. 2016a, MNRAS, 455, 3126 Camargo, D., Bica, E., & Bonatto, C. 2016b, A&A, 593, A95 Camargo, D., Bica, E., Bonatto, C., & Salerno, G. 2015b, MNRAS, 448, 1930 Camargo, D., Bonatto, C., & Bica, E. 2015c, MNRAS, 450, 4150 Cohen, R. E., Moni Bidin, C., Mauro, F., et al. 2017, MNRAS, 464, 1874 Di Matteo, P., Gómez, A., Haywood, M., et al. 2015, A&A, 577, 1D Dutra, C. M., Santiago, B. X., & Bica, E. 2002, A&A, 381, 219 Fadely, R., Willman, B., Geha, M., et al. 2011, AJ, 142, 88 Figure 2. Top panels: 2MASS field-star decontaminated CMDs of the newly discovered GCs and their respective reference clusters. The best-fitting PARSEC isochrones are shown as solid lines. Middle panels: Gaia-DR2 PM distribution. The black circles are the stars in the central region of each cluster, while the brown circles represent the stars in the surrounding field. Bottom panels: 2MASS RDPs built after applying color–magnitude filters that select the RGB stars fitted by the isochrones. 4 The Astrophysical Journal Letters, 860:L27 (5pp), 2018 June 20 Camargo
  5. 5. Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, arXiv:1804.09365 Harris, W. E. 1996, AJ, 112, 1487, 2010 edition, arXiv:1012.3224 Kim, D., Jerjen, H., Mackey, D., et al. 2016, ApJ, 820, 119 Kim, D., Jerjen, H., Milone, A. P., et al. 2015, ApJ, 803, 63 Minniti, D., Alonso-Garca, J., Braga, R., et al. 2017a, RNAAS, 1, 16 Minniti, D., Alonso-Garca, J., & Pullen, J. 2017b, RNAAS, 1, 54 Minniti, D., Geisler, D., Alonso-Garcia, J., et al. 2017c, ApJL, 849, L24 Minniti, D., Hempel, M., Toledo, I., et al. 2011, A&A, 527, 81 Minniti, D., Lucas, P. W., Emerson, J. P., et al. 2010, NewA, 15, 433 Moni-Bidin, C., Mauro, F., Geisler, D., et al. 2011, A&A, 535, A33 Piatti, A. E. 2018, MNRAS, 477, 2164 Valenti, E., Ferraro, F. R., & Origlia, L. 2007, AJ, 133, 1287 5 The Astrophysical Journal Letters, 860:L27 (5pp), 2018 June 20 Camargo

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