In this letter, we present preliminary results of the analysis of Flamingos-2 and GMOS-S photometry of the globular cluster (GC) system of the elliptical galaxy NGC1395. This is the first step of a long-term Brazilian-Argentinian collaboration for the study of GC systems in early-type galaxies. In the context of this collaboration, we obtained deep NIR photometric data in two different bands (J and Ks), which were later combined with high quality optical Gemini + GMOS photometry previously obtained by the Argentinian team. This allowed us to obtain different color indices, less sensitive to the effect of horizontal branch (HB) stars for several hundreds of GC candidates, and to make an initial assessment of the presence or absence of multiple GC populations in colors in NGC1395.Instituto de Astrofísica de La PlataMuseo de Astronomía y Geofísic

Chies Santos, Ana

Cortesi, Arianna

Escudero, Carlos Gabriel

Faifer, Favio Raúl

Forte, Juan Carlos

Oliveira, Claudia M.

Sesto, Leandro Alberto

Smith Castelli, Analía Viviana

Galaxies

English

SEDICI - Repositorio de la UNLP

galaxies
Letter
The Brazil–Argentina Gemini Group for the Study of
Globular Cluster Systems (BAGGs GCs):
FLAMINGOS-2 and GMOS Data for NGC 1395
Favio Faifer 1,2,*,†,‡, Carlos Escudero 1,2,‡, Analía Smith Castelli 2,‡, Juan Forte 3,4,‡,
Leandro Sesto 1,2,‡, Ana Chies Santos 5,‡, Arianna Cortesi 6,‡ and Claudia Mendes de Oliveira 6,‡
1 Facultad de Ciencias Astronómicas y Geofísicas, National University of La Plata, La Plata B1900FWA,
Argentina; cgescudero@fcaglp.unlp.edu.ar (C.E.); sesto@fcaglp.unlp.edu.ar (L.S.)
2 Instituto de Astrofísica de La Plata (CCT La Plata, CONICET-UNLP), La Plata B1904CMC, Argentina;
asmith@fcaglp.unlp.edu.ar
3 The National Scientific and Technical Research Council (CONICET), CABA C1425FQB, Argentina;
planeta.jcf@gmail.com
4 Planetario de la Ciudad de Buenos Aires, CABA C1425FGC, Argentina
5 Departamento de Astronomia, Instituto de Física, Universidade Federal do Rio Grande do Sul,
Porto Alegre 90040-060, Brazil; ana.chies@ufrgs.br
6 Instituto de Astronomia, Geofísica e Ciências Atmosféricas da U. de São Paulo, Cidade Universitária,
05508-900 São Paulo, Brazil; aricorte@googlemail.com (A.C.); claudia.oliveira@iag.usp.br (M.d.O.)
* Correspondence: favio@fcaglp.unlp.edu.ar; Tel.: +54-221-423-6591
† Current address: Paseo del Bosque s/n, La Plata CP B1900FWA, Argentina.
‡ These authors contributed equally to this work.
Academic Editors: Duncan A. Forbes and Ericson D. Lopez
Received: 1 July 2017; Accepted: 7 August 2017; Published: 14 August 2017
Abstract: In this letter, we present preliminary results of the analysis of Flamingos-2 and GMOS-S
photometry of the globular cluster (GC) system of the elliptical galaxy NGC 1395. This is the first
step of a long-term Brazilian–Argentinian collaboration for the study of GC systems in early-type
galaxies. In the context of this collaboration, we obtained deep NIR photometric data in two different
bands (J and Ks), which were later combined with high quality optical Gemini + GMOS photometry
previously obtained by the Argentinian team. This allowed us to obtain different color indices,
less sensitive to the effect of horizontal branch (HB) stars for several hundreds of GC candidates,
and to make an initial assessment of the presence or absence of multiple GC populations in colors in
NGC 1395.
Keywords: early-type galaxies; globular cluster; galaxy halos
1. Introduction
Globular clusters (GCs) are powerful probes to study the evolutionary histories of galaxies, as they
are good tracers of galactic star forming episodes. They are found around all major galaxies and can be
easily observed far beyond the Local Group [ 1]. As GCs are intrinsically old objects, their integrated
properties could give us information about the physical conditions in the interstellar medium at the
moment of formation of their host galaxies.
GC systems in massive galaxies are known to present a bimodal optical color distribution ([2,3]).
This phenomenon has been identified in different galaxies and it has been shown that this effect is
more clearly detectable when metallicity-sensitive color indices (such as (g′ − z′) or (C − T1)) are
used ([4,5]). Therefore, color bimodality is usually interpreted as evidence for the existence of two
GC sub-populations: the “blue” subpopulation (metal-poor clusters associated with the halo) and the
Galaxies 2017, 5, 38; doi:10.3390/galaxies5030038 www.mdpi.com/journal/galaxies
Galaxies 2017, 5, 38 2 of 6
“red” one (metal-rich clusters linked to bulge/disc). These sub-populations would have been formed
in at least two stages (e.g., [6,7]): blue GCs would have been a primordial population formed at high
redshifts in protogalactic fragments (e.g., [8,9]), while the red GCs would have formed later, during
a gas-rich merging of these fragments (e.g., [10]).
However, it was suggested that the bimodality may be an artifact arising from the non-linearity
in the color–metallicity (C–M) relation of GCs ([11,12]). In the [12] scenario, the morphology of the
horizontal branch (HB) produces a “wavy” pattern in the C–M relation in such a way that it is possible
to obtain a bimodal color distribution from a unimodal metallicity distribution. Despite several
empirical C–M calibrations have been published in the last decade, there is still a lot of controversy
about the existence of the alleged “wavy” pattern, i.e., how strong it is, and how much it influences
our interpretation of multiple GC populations based on broad-band colors ([13–16]).
Although the combination of optical and NIR colors is expected to help to mitigate the
“age–metallicity degeneracy” ([17]), it is still not clear how to interpret the lack of multi-modality
in some systems observed with optical/NIR filters ([18]). The study of NGC 3115 by [15] (multi-band
photometry and spectra centered on the CaT ), suggests that if the underlying metallicity bimodality
is real, it should be detected in all colors as well as in metal-sensitive indices. This means that, if the
bimodality appears only in some colors, it would be due to the “wavy” pattern in the C–M relation.
In the framework of our BAGGs GCs collaboration, we are involved in obtaining deep optical and
NIR multi-band photometry, as well as deep MOS spectroscopy, of GC systems belonging to massive
early-type galaxies. We expect that this data, combined with different single stellar population models
(SSP), helps to mitigate the age–metallicity degeneracy and allows us to measure several color-indices
less sensitive to the effect of HB stars. By combining kinematics and color–metallicity measurements
of GC systems in a self-consistent manner, we hope to be able to recover the evolutionary histories of
the galaxies and shed light on the assembly history of the halos of the galaxies.
As a first step of our collaboration’s observing campaign, in semester 2015B, we obtained data
of the GC system of the giant elliptical galaxy NGC 1395 (D ∼ 21.4 Mpc, MB = −21.02 mag), one of
the dominant galaxies of the Eridanus group. This galaxy harbors thousands of GCs and shows
a clear bimodal optical color distribution (Escudero+, in prep.). Here, we present preliminary results
of an analysis of deep NIR photometric images taken in two different photometric bands (J and Ks),
obtained with Gemini+Flamingos-2. The photometry obtained from these images was combined with
high quality optical photometry from previous Gemini+GMOS runs.
2. Data
Within the BAGGs GCs collaboration, we obtained NIR images of two fields of the elliptical
galaxy NGC 1395 using Gemini + Flamingos-2 in the J and Ks bands (Program GS-2015B-Q-38).
The total on-source exposure times were 30 × 50 s in both fields for the J band, and, in the Ks band,
302 × 12 s on-source for the field containing the galaxy and 277 × 12 s on-source for the other field.
The results presented here are based on a preliminary reduction performed with THELI ([19]). We used
DAOPHOT ([20]) to obtain psf photometry of all the detected sources in the fields. The NIR photometry
was calibrated using 2MASS objects present in our fields.
We also worked on a mosaic of four image fields of NGC 1395, previously obtained with
Gemini+GMOS by the Argentinian team (Figure 1). These optical images were observed in the
g′ (4 × 180.5 s), r′ (4 × 120.5 s), i′ (4 × 150.5 s) and z′ (4 × 150.5 s) bands, using 2× 2 binning which
gives a scale of 0.146”/pix (Programs GS-2012B-Q-44 and GS-2014B-Q-28). In this work, we present
the results obtained from the two fields that overlap with the NIR fields. The reduction of this data set
was performed through the Gemini/GMOS IRAF package in the usual way (see [21] for more details).
The detection and classification of the sources was made using a combination of SExtractor ([22])
and different IRAF tasks as is explained in [23]. Finally, we obtained DAOPHOT photometry in each
band and we calibrated our final photometric catalogue using Sloan standard stars observed on the
Galaxies 2017, 5, 38 3 of 6
same night as our target. The complete optical photometric analysis will be presented in Escudero+
(in prep.).
We used RA and DEC coordinates to match the optical candidates and to build a master catalogue
which includes all the unresolved sources detected and measured in the g′, r′, i′, z′, J and Ks bands.
In order to obtain a clean sample of GC candidates as possible and to have reliable colors in all the
bands, we cut the photometric sample in the range 18 < g′ < 24 mag with color cuts similar to those
in Escudero+ (2015a). The final catalogue includes 650 candidates detected in all the bands (red circles
in Figure 1).
Figure 1. DSS red image of NGC 1395 showing the optical globular cluster (GC) candidates
(blue squares). The GC candidates detected in the NIR images (∼650) are shown in red. The size of this
image is 12× 17 arcmin. North is up and east is to the left.
3. Results
Our first goal is to try to identify the presence of different color GC subpopulations. To this aim,
we built several color histograms and color–color diagrams. As an example, in Figure 2, we present
the different color histograms obtained by combining g′ magnitudes with the rest of our photometric
bands. At this stage of our work, we did not apply any statistical test but rather made a preliminary
identification of the modal peaks of the different colors. As in other giant early-type galaxies,
the (g′ − z′) color distribution of NGC 1395 looks bimodal ([24]). Two main peaks are clearly identified
in modal colors of (g′ − z′) = 0.93 and (g′ − z′) = 1.33 mag. However, the appearance of the color
distribution strongly depends on the combined bands. All the optical, optical-NIR and most of the
purely NIR color distributions show signs of multiple subpopulations. Some of them, such as (g′ − i′),
(g′ − z′) and (g′ − Ks), look strongly bimodal with possible substructures. As previously noticed by
other authors (e.g., [25]), in the particular case of colors involving only NIR bands, the situation is not
so clear.
Galaxies 2017, 5, 38 4 of 6
Figure 2. Histograms and density color distributions for different band combinations and for objects
with g′ < 24 mag. The considered bins are in the range 0.04–0.1 mag. The density distributions were
built using a Gaussian kernel with 0.02 < σ < 0.05 mag. To avoid superposition, we shifted some
histograms by a constant value, as indicated in the figures.
The color–color diagrams show signs of different degrees of non-linearity. As an example,
Figure 3 shows a (g′ − Ks) vs. (g′ − z′) diagram with evidence of a slight non-linearity which is in
good agreement with the results of previous studies ([26–28]). Interestingly, most of the smoothed
diagrams obtained from our photometry suggest that, at least, two main subpopulations in color
are present.
Figure 3. (Left): Color–color (g′−Ks) vs. (g′− z′) diagram built from our optical and NIR photometric
sample (objects with g′ < 24 mag); (Right): Smoothed version of the same diagram as that on the left.
The elliptical symbol depicts the kernel used to smooth the original diagram.
4. Discussion
As it has already been noticed by other authors, in some galaxies, the usual description
of color histograms on unimodal or bimodal bases by fitting Gaussian distribution seems to be
Galaxies 2017, 5, 38 5 of 6
simplistic. For example, [29] has shown that the (g′ − z′) color distributions of the GCs of several
galaxies in Virgo and Fornax seem to display a common and synchronized multi-population pattern.
Furthermore, [16] proposed that in some massive galaxies, the metallicity distribution might be
thought of as the result of a rapid sequence of individual GC formation events leading to an apparent
“continuous" metallicity distribution. In the particular case of NGC 1395, though preliminary, our results
seem to fit in these pictures. That is, they seem to present a bimodal color distribution in some
photometric band combinations, but show evidence of substructures in these color patterns.
To clarify all these issues, high-quality multi-band photometry and deep spectroscopy for a large
sample of GCs belonging to different systems, are clearly needed. In the framework of our BAGGs
GCs collaboration, we expect to obtain deep spectroscopy for a sub-sample of GC candidates in order
to determine spectroscopic ages and metallicities to test the photometric results.
Acknowledgments: This research was funded with grants from Consejo Nacional de Investigaciones Científicas
y Técnicas de la República Argentina, Agencia Nacional de Promoción Científica y Tecnológica, and Universidad
Nacional de La Plata, Argentina.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Brodie, J.; Strader, J. Extragalactic Globular Clusters and Galaxy Formation. Ann. Rev. Astron. Astrophys.
2006, 44, 193.
2. Ashman, K.; Zeph, S. Merging and Interacting Galaxies: Sites of Globular Cluster Formation; Astronomical
Society of the Pacific: San Francisco, CA, USA, 1993; Volume 48, p. 776.
3. Elson, R.; Santiago, B. The globular clusters in M87: A bimodal colour distribution. Mon. Not. R. Astron. Soc.
1996, 278, 617.
4. Barmby, P.; Huchra, J.P.; Brodie, J.P.; Forbes, D.A.; Schroder, L.L.; Grillmair, C.J. M31 Globular Clusters:
Colors and Metallicities. Astron. J. 2000, 119, 727.
5. Sinnott, B.; Hou, A.; Anderson, R.; Harris, W.E.; Woodley, K.A. New g’r’i’z’ Photometry of the NGC 5128
Globular Cluster System. Astron. J. 2010, 140, 2101.
6. Forbes, D.A.; Brodie, J.P.; Grillmair, C.J. On the Origin of Globular Clusters in Elliptical and cD Galaxies.
Astron. J. 1997, 113, 1652.
7. Forte, J.; Vega, E.; Faifer, F.; Smith Castelli, A.; Escudero, C.; González, N.; Sesto, L. Globular clusters: DNA of
early-type galaxies? Mon. Not. R. Astron. Soc. 2014, 441, 1391.
8. Beasley, M.; Baugh, C.; Forbes, D.; Sharples, R.; Frenk, C. On the formation of globular cluster systems in
a hierarchical Universe. Mon. Not. R. Astron. Soc. 2002, 333, 383.
9. Muratov, A.; Gnedin, O. Modeling the Metallicity Distribution of Globular Clusters. Astrophys. J. 2010,
718, 1266.
10. Li, H.; Gnedin, O.Y. Modeling the Formation of Globular Cluster Systems in the Virgo Cluster. Astrophys. J.
2014, 796, 10.
11. Richtler, T. Some remarks on extragalactic globular clusters. Bull. Astron. Soc. India 2006, 34, 83.
12. Yoon, S.; Yi, S.; Lee, Y. Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies.
Science 2006, 311, 1129.
13. Moyano Loyola, G.; Faifer, F.; Forte, J. Globular Clusters: Chemical Abundance—Integrated Colour
calibration. Bol. Asoc. Argent. Astron. 2010, 53, 133.
14. Usher, C.; Forbes, D.; Brodie, J.; Foster, C.; Spitler, L.; Arnold, J.; Romanowsky, A.; Strader, J.; Pota, V.
The SLUGGS survey: Calcium triplet-based spectroscopic metallicities for over 900 globular clusters.
Mon. Not. R. Astron. Soc. 2012, 426, 1475.
15. Cantiello, M.; Blakeslee, J.; Raimondo, G.; Chies-Santos, A.; Jennings, Z.; Norris, M.; Kuntschner, H. Globular
clusters of NGC 3115 in the near-infrared. Demonstrating the correctness of two opposing scenarios.
Astron. Astrophys. 2014, 564, L3.
16. Harris, W.; Ciccone, S.; Eadie, G.; Gnedin, O.; Geisler, D.; Rothberg, B.; Bailin, J. Globular Cluster Systems in
Brightest Cluster Galaxies. III: Beyond Bimodality. Astrophys. J. 2017, 835, 101.
Galaxies 2017, 5, 38 6 of 6
17. Worthey, G. Comprehensive stellar population models and the disentanglement of age and metallicity effects.
Astrophys. J. Suppl. 1994, 95, 107.
18. Chies-Santos, A.; Larsen, S.; Cantiello, M.; Strader, J.; Kuntschner, H.; Wehner, E.M.; Brodie, J.P. An optical/NIR
survey of globular clusters in early-type galaxies. III. On the colour bimodality of globular cluster systems.
Astron. Astrophys. 2012, 539, 54.
19. Schirmer, M. THELI: Convenient Reduction of Optical, Near-infrared, and Mid-infrared Imaging Data.
Astrophys. J. Suppl. 2013, 209, 21.
20. Stetson, P.B. DAOPHOT—A computer program for crowded-field stellar photometry. Publ. Astron. Soc. Pac.
1987, 99, 191
21. Escudero, C.; Faifer, F.; Bassino, L.; Calderón, J.; Caso, J. The extremely populated globular cluster system of
the lenticular galaxy NGC 6861. Mon. Not. R. Astron. Soc. 2015, 449, 612.
22. Bertin, E.; Arnouts, S. SExtractor: Software for source extraction. Astron. Astrophys. Suppl. 1996, 117, 393.
23. Faifer, F.; Forte, J.; Norris, M.; Bridges, T.; Forbes, D.; Zepf, S.; Beasley, M.; Gebhardt, K.; Hanes, D.; Sharples, R.
Gemini/GMOS imaging of globular cluster systems in five early-type galaxies. Mon. Not. R. Astron. Soc.
2011, 416, 155.
24. Escudero, C.; Sesto, L.; González, N.; Faifer, F.; Smith Castelli, A.; Forte, J. Comparación fotométrica en
imágenes Gemini/GMOS. Bol. Asoc. Argent. Astron. 2015, 57, 19.
25. Cho, H.; Blakeslee, J.; Chies-Santos, A.; Jee, M.; Jensen, J.; Peng, E.; Lee, Y. The Globular Cluster System of
the Coma cD Galaxy NGC 4874 from Hubble Space Telescope ACS and WFC3/IR Imaging. Astrophys. J.
2016, 822, 95.
26. Blakeslee, J.; Cho, H.; Peng, E.; Ferrarese, L.; Jordán, A.; Martel, A. Optical and Infrared Photometry of
Globular Clusters in NGC 1399: Evidence for Color-Metallicity Nonlinearity. Astrophys. J. 2012, 746, 88.
27. Forte, J.; Faifer, F.; Vega, E.; Bassino, L.; Smith Castelli, A.; Cellone, S.; Geisler, D. Multicolour-metallicity
relations from globular clusters in NGC 4486 (M87). Mon. Not. R. Astron. Soc. 2013, 431, 1405.
28. Powalka, M.; Lançon, A.; Puzia, T.H.; Peng, E.W.; Liu, C.; Muñoz, R.P.; Blakeslee, J.P.; Côté, P.; Ferrarese, L.;
Roediger, J.; et al. The Next Generation Virgo Cluster Survey (NGVS). XXV. Fiducial Panchromatic Colors of
Virgo Core Globular Clusters and Their Comparison to Model Predictions.Astrophys. J. Suppl. 2016, 227, 12.
29. Forte, J. Supra-galactic colour patterns in globular cluster systems.Mon. Not. R. Astron. Soc. 2017, 468, 3917.
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The Brazil-Argentina gemini group for the study of globular cluster systems (BAGGs GCs): Flamingos-2 and GMOS data for NGC 1395

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The Brazil-Argentina gemini group for the study of globular cluster systems (BAGGs GCs): Flamingos-2 and GMOS data for NGC 1395

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