We present a photometric study of NGC3258 and NGC3268 globular cluster systems (GCSs) with a wider spatial coverage than previous works. This allowed us to determine the extension of both GCSs, and obtain new values for their populations. In both galaxies, we found the presence of radial colour gradients in the peak of the blue globular clusters. The characteristics of both GCSs point to a large evolutionary history with a substantial accretion of satellite galaxies.Instituto de Astrofísica de La Plat

Bassino, Lilia Patricia

Caso, Juan Pablo

Galaxies

English

El Servicio de Difusión de la Creación Intelectual

galaxies
Conference Report
Revisiting the Globular Cluster Systems of NGC 3258
and NGC 3268
Juan Pablo Caso 1,2,* and Lilia Bassino 1,2
1 Instituto de Astrofísica de La Plata (CCT La Plata—CONICET, UNLP), Facultad de Ciencias Astronómicas y
Geofísicas de la Universidad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA La Plata, Argentina;
lbassino@fcaglp.unlp.edu.ar
2 Consejo Nacional de Investigaciones Científicas y Técnicas, Rivadavia 1917,
C1033AAJ Ciudad Autónoma de Buenos Aires, Argentina
* Correspondence: jpcaso@fcaglp.unlp.edu.ar
Academic Editors: Ericson D. Lopez and Duncan A. Forbes
Received: 27 June 2017; Accepted: 22 August 2017; Published: 31 August 2017
Abstract: We present a photometric study of NGC 3258 and NGC 3268 globular cluster systems (GCSs)
with a wider spatial coverage than previous works. This allowed us to determine the extension of
both GCSs, and obtain new values for their populations. In both galaxies, we found the presence
of radial colour gradients in the peak of the blue globular clusters. The characteristics of both GCSs
point to a large evolutionary history with a substantial accretion of satellite galaxies.
Keywords: galaxies: elliptical and lenticular, cD; galaxies: evolution; galaxies: star clusters:
individual: NGC 3258 & NGC 3268
1. Introduction
The majority of the members in globular cluster (GC) populations are usually old stellar systems
(e.g., [1,2]). They were formed under extreme environmental conditions, which are probably reachable
only in massive star formation episodes during major mergers [3]. This implies a direct connection
between the episodes that built up the globular cluster systems (GCSs) and the stellar population
of the host galaxy. Hence, the study of a GCS is important to obtain a comprehensive picture of
the evolutionary history of galaxies.
Our target galaxies, NGC 3258 and NGC 3268, are thought to make-up the bulk of the Antlia
galaxy cluster, located in the Southern sky at a low Galactic latitude (≈19 deg). The central part
seems to consist of two groups, each one dominated by one of these giant ellipticals (gEs) with similar
luminosity. These two subgroups might be in a merging process, but surface brightness fluctuations
distances [4–6] and radial velocities analysis [7,8] are not conclusive.
There are several studies about these GCSs [9–11] and the connection between their bright-end
and ultra-compact dwarfs [12,13]. The aim of the present study is to complement previous ones
by taking advantage of wider and deeper datasets. This results in a more accurate contamination
estimation, and the possibility of calculating the GCSs’ spatial extensions and total populations.
2. Materials and Methods
The dataset consists of two wide fields (36× 36 arcmin2) obtained at the Cerro Tololo 4-m telescope
with the MOSAIC II camera in filters (C, T1). One field contains both galaxies, while the other
is located to the east (Figure 1). These double the areal coverage from Dirsch et al. [9], the more
extended GCSs study of these galaxies. We also used four fields from VLT obtained with the FORS1
camera (6.8× 6.8 arcmin2) in filters (V, I) (see Figure 1). Table 1 contains the basic information of
the observations. For MOSAIC data, we selected T1,0 = 23.85 as the magnitude limit; for fainter
Galaxies 2017, 5, 48; doi:10.3390/galaxies5030048 www.mdpi.com/journal/galaxies
Galaxies 2017, 5, 48 2 of 7
magnitudes, the completeness falls below 60%. In the case of FORS1 data, we selected V0 = 25.5 as the
magnitude limit, which implies a completeness limit of 60% for GCs at less than 1 arcmin from the
galaxies centre and 70% for larger galactocentric distances.
Figure 1. The MOSAIC (white regions) and FORS1 (black regions) fields are overlaid on
a 70× 70 arcmin2 DSS image of the Antlia cluster. North is up and east is to the left.
Table 1. Basic data from observations. FWHM: full width at half maximum.
Name Obs. Date Exp. Time Typical FWHM
MOSAIC data
Central Field 4/5 April 2002 4× 600 s in C− 4× 600 s in T1 1′′
East Field 24/25 May 2004 7× 900 s in C− 5× 600 s in T1 1.1′′
FORS1 data
27/28 March 2003 5× 300 s in V − 5× 700 s in I 0.6′′
GC candidates were selected in both datasets from point sources with colours in the usual range
of GCs, i.e., 0.9 < (C− T1)0 < 2.3 and 0.4 < (V − I)0 < 1.6, and fainter than MV ≈ −10.5 to avoid
ultra-compact dwarfs (e.g., [14,15]), which implies T1,0 > 21.6 and V0 > 22.2 at the assumed distance.
We refer to Caso et al. [16] for further details on the data reduction and photometry.
3. Results
3.1. GCSs Spatial Extension
In order to complement the available datasets, taking advantage of their different properties,
we derived the radial distributions of the GC candidates following a two-step procedure. First, we fitted
power-laws to blue and red subpopulations, masking regions close to neighbour galaxies and bright
stars. Considering that previous studies point to an overlap of both GCSs (e.g., [10]), we fitted the
radial distributions iteratively, avoiding regions where the other GCS might be contributing to the
observed surface density. We assumed that the outer limit of a GCS is achieved when its GC surface
density is equal to 30 per cent of the background level. This criterion has been applied to several GCS
studies, including those with a large field of view (FOV) (e.g., [17,18]). The background region selected
to correct for contamination in the MOSAIC photometry is located in the eastern portion of our field,
at more than 25 arcmin from both galaxies. It spans 489 arcmin2, and the mean projected densities
Galaxies 2017, 5, 48 3 of 7
of point sources with similar colours and magnitudes than GC candidates are ≈0.45 arcmin−2 and
≈0.2 arcmin−2, respectively, for blue and red GC candidates.
In the case of NGC 3258, the blue GCS reaches ≈17 arcmin (i.e., ≈170 kpc), while the red
subpopulation appears more concentrated towards the galaxy, with an extension of ≈6 arcmin
(i.e., ≈60 kpc). For NGC 3268, the extension of both subpopulations are similar, ≈14 arcmin and
≈12 arcmin for blue and red GCs, respectively.
For the innermost regions of the GCSs, we obtained the radial profiles from the FORS1 data,
applying background and completeness corrections to the GC surface densities. In these cases, we fitted
a modified Hubble distribution [9,19]:
n(r) = a
(
1 +
(
r
r0
)2)−β
(1)
to consider the usual flattening in the inner GC radial profiles [20–22]. Figure 2 shows the background
and completeness corrected radial profiles from FORS1 data for blue (filled squares) and red (filled
circles) subpopulations. Open symbols represent the radial profiles obtained from MOSAIC data,
properly scaled to match the deeper FORS1 data. Solid curves indicate the fitted Hubble distributions,
while dashed ones correspond to the scaled power-laws fitted to MOSAIC data. Table 2 shows the
parameters associated with the Hubble profiles.
0.0 0.5 1.0−
0.
5
0.
5
1.
5
2.
5
log (r [arcmin])
lo
g 
(σ 
[N
 ar
cm
in
−
2  
])
NGC 3258
0.0 0.5 1.0−
0.
5
0.
5
1.
5
log (r [arcmin])
lo
g 
(σ 
[N
 ar
cm
in
−
2  
]) NGC 3268
Figure 2. Background and completeness corrected radial profiles from FORS1 data for blue
(filled squares) and red (filled circles) globular clusters (GCs). The open symbols shows the MOSAIC
data, properly scaled. Solid curves represent the Hubble modified profile fitted to FORS1 data, and
dashed ones the power-laws fitted to MOSAIC data.
The wider FOV allows us a better determination of the radial profile and the background
level, which implies a more accurate estimation of the radial extension in both systems. The joint
analysis of MOSAIC and FORS1 data results in a more general fit of the GCs radial distributions than
previous studies.
Galaxies 2017, 5, 48 4 of 7
Table 2. Parameters of the modified Hubble distribution fitted to the FORS1 radial profiles.
NGC 3258 NGC 3268
Blue GCs Red GCs Blue GCs Red GCs
a 2.23± 0.05 2.05± 0.06 1.85± 0.03 2.09± 0.06
r0 1.11± 0.10 1.05± 0.08 1.40± 0.10 0.77± 0.08
beta −3.5± 0.2 −5.8± 0.3 −3.4± 0.26 3.6± 0.3
3.2. Total Population of the GCSs
In order to calculate the population of the GCSs we obtained the background and completeness GC
luminosity functions (GCLFs) from FORS1 data for both galaxies (Figure 3). It is largely documented
in literature that GCLF in elliptical galaxies can be approximated by Gaussian profiles with a turn-over
magnitude (TOM) MV = −7.4 (e.g., [1,23]). In both galaxies, the expected TOM was close to our
magnitude limit. Hence, we used the distance moduli calculated by Tully et al. [6] to determine
the TOM, mM = 32.56± 0.14 for NGC 3258 and mM = 32.74± 0.14 for NGC 3268. The rest of the
Gaussian parameters were fitted from the data. From this procedure, we established that GCs brighter
than V0 = 25.5 represent ≈62% for NGC 3258 and ≈56% in the case of NGC 3268.
21 22 23 24 25 26
0
50
10
0
15
0
20
0
V0
N
NGC 3258
21 22 23 24 25 26
0
50
10
0
15
0
20
0
V0
N
NGC 3268
Figure 3. In each panel the smoothed GC luminosity function from FORS1 data is represented with a
solid line, while the dashed one represents the Gaussian fitted to the data. The shaded regions indicate
the regions with fainter magnitudes than our limit due to the completeness drop.
Then, we numerically integrated the GC radial profiles derived in the previous section along
the entire radial extension of the GCSs. Finally, we corrected these values by the fraction of missing GCs
due to our completeness limits. The results indicate a population of ≈6600 blue GCs and ≈1400 red
ones for NGC 3258, which implies a fraction of red-to-total GCs of fred ≈ 0.18. In the case of NGC 3268,
we obtained ≈5200 blue and ≈3000 red GCs, with fred ≈ 0.38.
The fraction of red GCs calculated in this work for both galaxies is similar to the values indicated
by Bassino et al. [10]. Our results point to more populated GCSs, mainly due to the larger radial
extension derived in this contribution. For instance, in case we integrate our radial distribution
for NGC 3258 up to 10 arcmin, the number of GCs is in agreement with Bassino et al. [10] values.
The same analysis for NGC 3268 results in a larger number of GCs.
Galaxies 2017, 5, 48 5 of 7
3.3. Radial Gradients in the Colour Distribution
The left panels of Figure 4 show the smoothed and background corrected colour distribution
for GC candidates around NGC 3258 (upper panel) and NGC 3268 (lower panel) from the MOSAIC
data. The sample was split in three radial regimes. We statistically subtracted the contamination
and applied the algorithm Gaussian mixture modeling (GMM) [ 24] to the clean samples to calculate
the colour for the blue GC peak in the three ranges. After repeating the procedure 25 times in order
to reduce statistical noise, the mean colours in the (C − T1) filters resulted 1.27± 0.01, 1.21± 0.01,
and 1.12± 0.02, respectively, in the case of NGC 3258, and 1.37± 0.02, 1.28± 0.02, and 1.22± 0.03,
respectively, for NGC 3268. These mean colours are indicated in the panels with vertical lines.
The right panels of Figure 4 are analogues for the FORS1 data. The smaller FOV and the lower
sensitivity of (V − I) colours with respect to (C− T1) make results noisy, but blue peaks seem to get
bluer for GC candidates at galactocentric distances larger than 150”.
1.0 1.5 2.0
0
10
20
30
40
50
(C−T1)0
N
 1' < r <  3'
 3' < r <  6.5'
 6.5' < r <  10'
NGC 3258
1.0 1.5 2.0
0
5
10
15
20
25
30
35
(C−T1)0
N
 1' < r <  3'
 3' < r <  6'
 6' < r <  10'
NGC 3268
0.6 0.8 1.0 1.2 1.4 1.6
0
10
20
30
40
50
60
70
(V−I)0
N
 20'' < r <  90''
 90'' < r <  150''
 r >  150''
NGC 3258
0.6 0.8 1.0 1.2 1.4 1.6
0
10
20
30
40
50
60
70
(V−I)0
N
 20'' < r <  90''
 90'' < r <  150''
 r >  150''
NGC 3268
Figure 4. Left panels: smoothed colour distributions for GC candidates from MOSAIC data, split in
three radial distance regimes. Vertical lines indicate mean colours for the blue subpopulation, obtained
with Gaussian mixture modeling (GMM) [24]. Right panels: analogue figures for GC candidates from
FORS1 data.
4. Summary
The rich and spatially extended GCSs in both galaxies point to rich evolutionary histories.
The radial gradient in the colour peak of blue GCs found in NGC 3258 and NGC 3268 could be
explained by the correlation between the colour and metallicity for blue GCs and the galaxy masses
(e.g., [25,26]), and points to the relevant role of the accretion of satellite galaxies in the build-up of the
outer regions of GCSs. Similar radial gradients have been found in other giant ellipticals, pointing to
two phases in galaxy formation (e.g., [27]).
Acknowledgments: This research was funded with grants from Consejo Nacional de Investigaciones Científicas
y Técnicas de la República Argentina (PIP 112-201101-00393), Agencia Nacional de Promoción Científica y
Tecnológica (PICT-2013-0317), and Universidad Nacional de La Plata (UNLP 11-G124), Argentina.
Author Contributions: This paper was totally carried out by the authors.
Conflicts of Interest: The authors declare no conflict of interest.
Galaxies 2017, 5, 48 6 of 7
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Revisiting the globular cluster systems of NGC3258 and NGC3268

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Revisiting the globular cluster systems of NGC3258 and NGC3268

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