The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge.

Abstract

Globular star clusters are compact and massive stellar systems old enough to have witnessed the entire history of our Galaxy, the Milky Way. Although recent results suggest that their formation may have been more complex than previously thought, they still are the best approximation to a stellar population formed over a relatively short timescale (less than 1 Gyr) and with virtually no dispersion in the iron content. Indeed, only one cluster-like system (omega Centauri) in the Galactic halo is known to have multiple stellar populations with a significant spread in iron abundance and age. Similar findings in the Galactic bulge have been hampered by the obscuration arising from thick and varying layers of interstellar dust. Here we report that Terzan 5, a globular-cluster-like system in the Galactic bulge, has two stellar populations with different iron contents and ages. Terzan 5 could be the surviving remnant of one of the primordial building blocks that are thought to merge and form galaxy bulges.

Full text

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The cluster Terzan 5 as a remnant of a primordial
building block of the Galactic bulge
F.R. Ferraro1, E. Dalessandro1, A. Mucciarelli1, G.Beccari2, M. R. Rich3, L. Origlia4, B.
Lanzoni1, R. T. Rood5, E. Valenti6,7, M. Bellazzini4, S. M. Ransom8, G. Cocozza4
1Department of Astronomy, University of Bologna, Via Ranzani, 1, 40127 Bologna,
Italy
2ESA, Space Science Department, Keplerlaan 1, 2200 AG Noordwijk, Netherlands
3Department of Physics and Astronomy, Math-Sciences 8979, UCLA, Los Angeles, CA
90095-1562,USA
4INAF- Osservatorio Astronomico di Bologna, Via Ranzani, 1, 40127 Bologna, Italy
5Astronomy Department, University of Virginia, P.O. Box 400325, Charlottesville, VA,
22904,USA
6European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
7Pontificia Universidad Catolica de Chile, Departamento de Astronomia, Avda Vicuña
Mackenna 4860, 782-0436 Macul, Santiago, Chile
8National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
Globular star clusters are compact and massive stellar systems old enough to have
witnessed the entire history of our Galaxy, the Milky Way. Although recent
results1,2,3 suggest that their formation may have been more complex than
previously thought, they still are the best approximation to a stellar population
formed over a relatively short time scale (less than 1 Gyr) and with virtually no
dispersion in the iron content. Indeed, only one cluster-like system (  Centauri) in

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the Galactic halo is known to have multiple stellar populations with a significant
spread in iron abundance and age4,5. Similar findings in the Galactic bulge have
been hampered by the obscuration arising from thick and varying layers of
interstellar dust. Here we report that Terzan 5, a globular-cluster-like system in
the Galactic bulge, has two stellar populations with different iron content and ages.
Terzan 5 could be the surviving remnant of one of the primordial building blocks
that are thought to merge and form galaxy bulges.

We have recently obtained a set of high-resolution images of Terzan 5 in the K and J
bands by using MAD6, a Multi-Conjugate Adaptive Optics demonstrator instrument
installed at the Very Large Telescope (VLT) of the European Southern Observatory
(ESO). MAD operates at near-infrared wavelengths, thus revealing the only component
of stellar radiation that can efficiently cross the thick clouds of dust obscuring the
Galactic bulge. It is able to perform exceptionally good and uniform adaptive optics
correction over its entire field of view (1'x1'), thus compensating for the degradation
effects to the astronomical images induced by the Earth’s atmosphere. In particular, we
have obtained a set of K-band (2.2m) images of Terzan 5 close to the diffraction limit
(Fig. 1). The sharpness and uniformity of the images yields very high quality
photometry, resulting in accurate (K, JK) colour-magnitude diagram (CMD) even for
the very central region of the cluster, and leading to a surprising discovery. We have
detected two well-defined red horizontal branch clumps, separated in luminosity: a
bright horizontal branch (BHB) at K = 12.85 and a faint horizontal branch (FHB) at K =
13.15, the latter having a bluer (JK) colour (Fig. 2).
We have carefully considered whether the double horizontal branch could be
spurious. It is neither due to instrumental effects (Fig. 2), nor to differential reddening7,8

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(as the two horizontal branch clumps in the CMD are separated in a direction which is
essentially orthogonal to the reddening vector), nor to field contamination (while field
stars are expected to be almost uniformly distributed over the MAD field of view, the
radial distributions of the stars belonging to the two horizontal branch clumps are
significantly concentrated toward the cluster centre and are inconsistent with a uniform
distribution at more than 5 level; see Fig. 3a and Supplementary Information). We
have also found that the radial distributions of the two horizontal branch populations are
different (Fig. 3a): according to a Kolmogorov-Smirnov test, the BHB population is
significantly (at > 3.5level) more centrally concentrated than that of the FHB. The
stars belonging to the BHB are also substantially more numerous than those of the FHB
near the cluster centre (that is, at distances r < 20''), becoming progressively more rare
at larger radii (Fig. 3b).
Once alerted to the existence of the double horizontal branch, we have also
identified the feature in optical observations obtained with the Advanced Camera for
Surveys (ACS) on board the Hubble Space Telescope (HST; see Supplementary Fig.
1a). Although the strong differential reddening broadens the colour extension of the
horizontal branch clumps by ~ 1 mag, the optical (I, VI) CMD still shows a clear
bimodal distribution of horizontal branch stars in the direction orthogonal to the
reddening vector (Supplementary Fig. 1b). A hint of a double horizontal branch clump
was already visible in a previously published CMD obtained with HST-NICMOS9-11,
although the shorter colour baseline provided by the J- and H- band observations did
not clearly separate the two clumps.

Hence, we conclude that the existence of the two horizontal branch clumps is a
real feature, and the differing radial distributions may indicate different physical origins

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of the two populations. In particular, a combination of different metallicity and age,
with the population in the BHB clump being more metal-rich and younger than that in
the FHB clump, could in principle reproduce the observed features (Supplementary Fig.
2). The only direct information previously available on the metal content of individual
stars in Terzan 5 was from four bright giants near the Tip of the red giant branch
(RGB), giving an average iron-to-hydrogen abundance ratio [Fe/H] = 0.2 with a
negligible dispersion12. Hence, we quickly secured medium-resolution near-infrared
spectra of 6 horizontal branch stars (3 in each clump) at the Keck Telescope13. The
derived radial velocities for the two groups of stars (85 km s-1 in both cases) are fully
consistent with the previous measures12 and the systemic velocity of Terzan 5 quoted in
the currently adopted globular cluster catalogue14. This confirms that all of the observed
stars are cluster members and suggests that there is no significant kinematical difference
between the two populations (this is also confirmed by proper motion studies; see
Supplementary Information). Furthermore, we have found that the iron content of the
stars in the two clumps differs by a factor of 3 (~ 0.5 dex): the FHB stars have [Fe/H] =
 0.2, while the BHB stars have [Fe/H] = +0.3 (Fig. 4a).
To date, apart from a significant spread in the abundance patterns of a few light
elements (such as Na and O)1, the chemical composition of all globular clusters in the
Galaxy is known to be extremely uniform in terms of iron content, with the only
exception being  Centauri4,5 in the Galactic halo. Hence, Terzan 5 is the first stellar
aggregate discovered in the Galactic bulge that has globular-cluster-like properties but
also with the signatures of a much more complex star formation history.
To further investigate this issue, we have performed a differential reddening
correction15 on the optical ACS catalogue and combined it with the near-infrared data,
thus obtaining the (K, VK) CMD shown in Fig. 4b. The presence of two distinct
populations with a double horizontal branch and (possibly) two separate RGBs can be

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seen in this CMD. The RGB of the most metal-rich population appears to be more bent
(as expected, because of the line blanketing due to a higher metal content). The
observed features can be reproduced with two populations characterized by the
observed metallicities and two different ages: t = 12 Gyr for the FHB and a significantly
younger age (t = 6 Gyr) for the BHB.
Using the number of horizontal branch stars found in the combined MAD and
ACS samples (see Supplementary Information for details), we estimate that the cluster
harbours about 800 FHB stars and 500 BHB stars in total. This is even larger than the
global horizontal branch population of 47 Tucanae16, thus suggesting that Terzan 5 is
more massive than previously thought (Supplementary Information).
The evidence for two distinct stellar populations and for a very large total mass
suggests that Terzan 5 has experienced a quite troubled formation history. It might be
the merger-product of two independent stellar aggregates17. Although such a possibility
seems to be unlikely for globular clusters belonging to the Galactic halo, the chance of
capturing a completely independent stellar system should be significantly larger if the
orbits are confined within the Galactic bulge. In this scenario, however, it is not easy to
explain why the metal-rich population is more centrally concentrated than the metal-
poor one. Moreover, globular clusters younger than 10 Gyr are very rare in our
Galaxy18. Rather, Terzan 5 could be a complex  Centauri-like system4,5 or the nuclear
remnant of a disrupted galaxy, similar to the M 54-Sagittarius system19,20 or the Carina
dwarf spheroidal21 in the metal-rich regime. The remnant of a disrupted dwarf galaxy
would naturally present a larger central concentration of the metal-rich (and younger)
population22, as commonly observed in the satellites of the Milky Way and M31. On the
other hand, the strict similarity in iron abundance between Terzan 5 and the Galactic

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bulge population is fully compatible with the hypothesis that the (partial) disruption of
its progenitor has contributed to the formation of the Galactic bulge23.
Possible relics of the hierarchical assembly of the Galactic halo have been
recently identified at high Galactic latitudes24. Terzan 5 may be the first example of the
sub-structures that contributed to form the Galactic bulge. Indeed, our discovery could
be the observational confirmation that galactic spheroids originate from the merging of
pre-formed, internally evolved stellar systems, and that other similar objects might be
hidden in the heavily obscured central region of the Galaxy.
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Supplementary Information accompanies the paper on www.nature.com/nature.
Acknowledgements: We thank the MAD team at ESO and in particular P. D’Amico for performing the
observations with MAD. This research was supported by “Progetti Strategici di Ateneo 2006 ” (
University of Bologna), “Progetti di Ricerca di Interesse Nazionale 2007 and 2008” (Istituto Nazionale di
Astrofisica), Agenzia Spaziale Italiana and the Ministero dell'Istruzione, dell'Universitá e della Ricerca.
We also acknowledge support from the ESTEC Faculty Visiting Scientist Programme. R.M.R. is
supported by the NSF and STScI; R.T.R. is partially supported by STScI. This research has made use of
the ESO/ST-ECF Science Archive facility, which is a joint collaboration of the ESO and the Space
Telescope - European Coordinating Facility. Part of the data presented here were obtained at the W.M.
Keck Observatory, which is operated as a scientific partnership among the California Institute of
Technology, the University of California and the NASA. The Observatory was made possible by the
financial support of the W.M. Keck Foundation.
Author contributions: F.R.F. designed the study and coordinated the activity. E.D., A.M. G.B., E.V. and
G.C. analysed the photometric dataset. R.M.R. and L.O. secured and analyzed the Keck spectra. A.M.
designed the reddening correction routine. M.B. performed radial distributions tests. E.D. and M.B
performed the proper motion analysis. F.R.F., B.L. and L.O. wrote the paper. S.R., R.M.R., R.T.R. and
M.B. critically contributed to the paper presentation. All the authors contributed to discussion of the
results and commented on the manuscript.
Correspondence to: F.R.Ferraro1 Correspondence and requests for materials
should be addressed to F.R.F. francesco.ferraro3@unibo.it

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Fig. 1. MAD image of Terzan 5 in the K band. Observations were performed
at the ESO-VLT (Paranal, Chile) on August 2008, through J and K filters.
Exposure times were about two minutes in each filter. Shown is the best image
obtained in the K band (the image size is 1'x1', north is up, east is left). The
measured full-width at half-maximum (FWHM) of stars is 0.1'', the Strehl ratio
ranges between 15% and 24% over the entire field of view. The quality of the J
image is slightly worse (FWHM  0.24'' and Strehl ratio below 10%), but still
much better than normally obtained with ground-based observations. A small
(16'' x 16'') portion of the K image sampling the very central region of Terzan 5
is shown magnified. The cluster centre of gravity (marked with the white cross)
has been determined by averaging the positions of the resolved stars and
following the same procedure adopted in previous studies25. It is located at right
ascension  = 17 h 48 m 4.85 s, declination = 24° 46' 44.6'', which is ~ 3''
south-east from the centre listed in the most commonly adopted globular cluster
catalogue14, but in good agreement (within the errors     0.5'') with the
determination obtained from HST-NICMOS observations9. The barycenters of

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the two horizontal branch populations are coincident with the gravity centre
within the errors.

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Fig. 2. The two horizontal branch clumps of Terzan 5. Main panel, MAD
(K, JK) CMD of the central region of Terzan 5. Inset, magnified view of the
horizontal branch region, with the two horizontal branch clumps marked with red
arrows. Terzan 5 is heavily obscured by thick clouds of dust (this effect is
commonly called “reddening”) intervening between the system and the
observer, in a way that strongly depends on the direction of the line of sight
(“differential reddening”)7,8. The effect of reddening on the K magnitude and the
J-K colour is indicated by the reddening vector plotted in the main panel.
Several tests have been performed on the images and the catalogue to exclude
any possible spurious effect from the instrument or the reduction procedure.
Stars in the two clumps do not show any peculiar spatial distribution on the
detector. Moreover, the two clumps are not spuriously produced by the variation
in size and shape of the Point Spread Function, or the local level of the
background. Error bars (1 s.e.m ) are plotted at different magnitude levels.
The contamination from Galactic bulge field stars in this CMD is negligible. In
the 1 arcmin2 field of view of MAD, we estimate (Supplementary Information)
that 11 and 8 field stars should contaminate the faint and bright horizontal
branch selection boxes (while we count 299 FHB stars and 310 BHB stars in
the entire MAD sample).

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Fig. 3. Radial distribution of the two horizontal branch populations in
Terzan 5.
a, Cumulative radial distribution of the observed BHB stars (red line) and the
FHB population (blue line), compared to that of field stars (solid black line), as a
function of the projected distance from the cluster centre of gravity. The field
distribution has been obtained from a synthetic sample of 100,000 points
uniformly distributed in X and Y over the MAD field of view.
b, Ratio between the number of observed BHB and FHB stars computed over
areas of increasing radius, ra. Points with ra < 30'' refer to the MAD sample,
those corresponding to larger radii have been computed by also using the ACS
data. The grey area around the black line represents the 1 uncertainty region.
BHB stars are substantially more numerous than FHB stars in the cluster centre
and they rapidly vanish at ra > 50''.

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Fig. 4. Iron abundance and ages of the two populations.
a, Combined J-band spectra near the 1.1973 m iron line for three FHB (left)
and three BHB (right) stars, as obtained with NIRSPEC at Keck II on 2 July
2009 (coloured lines). The measured equivalent widths of the lines and suitable
spectral synthesis12 yield iron abundances [Fe/H] ≈ 0.2  0.1 and [Fe/H] ≈ +0.3
 0.1, respectively. The black solid lines correspond to the best-fit synthetic
spectra obtained for temperatures and gravities derived from evolutionary
models reproducing the observed colours of the horizontal branch stars: Teff =
5000 K and log g = 2.5 for the FHB stars, Teff = 4500 K and log g = 2.0 for the
BHB stars. For sake of comparison, we also plot (as black dashed lines) the
synthetic spectra obtained by adopting the same atmospheric parameters, but
[Fe/H] = 0.3 for the FHB and [Fe/H] = 0.2 for the BHB.
From the measured spectra we also derived the stellar radial velocities and
found an average value of 85 km s-1 ( = 9 km s-1) and 85 km s-1 ( = 10 km s-1)
for the FHB and BHB stars, respectively (the typical uncertainty on the
individual measure is of the order of 3 km s-1). These values are fully consistent

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with the previously measured radial velocities of four giants (Vr = 932 km s-1)12
and the value (Vr = 9415 km s-1) listed for Terzan 5 in the currently adopted
globular cluster catalogue14. This observational fact confirms that the horizontal
branch stars for which we have secured spectra are cluster members, and
suggests that there is no significant kinematical difference between the two
populations.
b, (K, VK) CMD of Terzan 5 obtained by combining VLT-MAD and HST-ACS
data corrected for differential reddening. Two isochrones26 with [Fe/H] = 0.2
(heavy element mass fraction Z=0.01, and helium mass fraction Y=0.26) and t =
12 Gyr (blue line), and with [Fe/H] = +0.3 (Z=0.03, Y=0.29) and t = 6 Gyr (red
line) are overplotted to the data by adopting a colour excess8 E(BV) =
2.380.05 and a distance8 d = 5.90.5 kpc. Note that the CMD cannot be
reproduced by two isochrones with the measured metallicities and the same
age. Owing to the large scatter at the turn-off level, we estimate that the
uncertainty on the younger component age is about 2 Gyr.