Kinematic Decoupling of Globular Clusters with Extended
  Horizontal-Branch
arXiv:0704.0486v1  [astro-ph]  4 Apr 2007
Accepted for Publication in ApJ Letters, April 2007
Preprint typeset using LATEX style emulateapj v. 08/13/06
KINEMATIC DECOUPLING OF GLOBULAR CLUSTERS
WITH EXTENDED HORIZONTAL-BRANCH
Young-Wook Lee
, Hansung B. Gim
, & Dana I. Casetti-Dinescu
Accepted for Publication in ApJ Letters, April 2007
ABSTRACT
About 25% of the Milky Way globular clusters (GCs) exhibit unusually extended color distribution
of stars in the core helium-burning horizontal-branch (HB) phase. This phenomenon is now best
understood as due to the presence of helium enhanced second generation subpopulations, which has
raised a possibility that these peculiar GCs might have a unique origin. Here we show that these
GCs with extended HB are clearly distinct from other normal GCs in kinematics and mass. The
GCs with extended HB are more massive than normal GCs, and are dominated by random motion
with no correlation between kinematics and metallicity. Surprisingly, however, when they are excluded,
most normal GCs in the inner halo show clear signs of dissipational collapse that apparently led to the
formation of the disk. Normal GCs in the outer halo share their kinematic properties with the extended
HB GCs, which is consistent with the accretion origin. Our result further suggests heterogeneous
origins of GCs, and we anticipate this to be a starting point for more detailed investigations of Milky
Way formation, including early mergers, collapse, and later accretion.
Subject headings: Galaxy: formation – globular clusters: general – stars: horizontal-branch
1. INTRODUCTION
The discovery of multiple stellar populations in the
most massive GC ωCen (Lee et al. 1999), together
with the fact that the second most massive GC M54
is a core of the disrupting Sagittarius dwarf galaxy
(Layden & Sarajedini 2000), have strengthen the view
that some of the massive GCs might be remaining cores
of disrupted nucleated dwarf galaxies (Freeman 1993).
Among their several peculiar characteristics, both ωCen
and M54 have extended horizontal-branch (EHB), with
extremely hot horizontal-branch (HB) stars well sep-
arated from redder HB (Lee et al. 1999; Rosenberg,
Recio-Blanco, & Garcia-Marin 2004). High pre-
cision Hubble Space T elescope (HST ) photometry
(Bedin et al. 2004) has discovered that ωCen also has
a curious double main-sequence (MS). Recent studies
have shown that both these peculiar colour-magnitude
diagram (CMD) characteristics are best understood as
due to the presence of helium enhanced second gen-
eration subpopulations (Norris 2004; Lee et al. 2005;
Piotto et al. 2005; D’Antona et al. 2005). Further-
more, the prediction of the models (Lee et al. 2005;
D’Antona et al. 2005) that most of the GCs with EHB
would have double or broadened MSs are now confirmed
by HST/ACS (Advanced Camera for Survey) pho-
tometry (Piotto et al. 2007). This ensures that EHBs
are strong signature of the presence of multiple popula-
tions in GCs. A significant fraction (∼30%) of the helium
enriched subpopulation observed in these peculiar GCs
is also best explained if the second generation stars were
formed from enriched gas trapped in the deep gravita-
tional potential well while these GCs were cores of the
ancient dwarf galaxies (Bekki & Norris 2006). Despite
the lack of apparently wide spread in iron-peak elements
1 Center for Space Astrophysics & Department of Astronomy,
Yonsei University, Seoul 120-749, Korea (ywlee@csa.yonsei.ac.kr)
2 Department of Astronomy, Yale University, New Haven, CT
06520, USA
in most of these GCs, all of these recent developments
suggest that GCs with EHB are probably not genuine
GCs, but might have a unique origin in the formation
history of the Galaxy.
In order to test this working hypothesis further, we
have carefully surveyed 114 GCs with reasonably good
CMDs, and found that 28 (25%) of them have EHB
(Lee et al., in preparation). Their NGC numbers are:
2419, 2808, 5139, 5986, 6093, 6205, 6266, 6273, 6388,
6441, 6656, 6715, 6752, 7078, and 7089 for the GCs with
strongly extended HB; and 1851, 1904, 4833, 5824, 5904,
6229, 6402, 6522, 6626, 6681, 6712, 6723, and 6864 for
the GCs with moderately extended HB, including those
with bimodal HB distributions. We will collectively call
all of them as “EHB GCs”. Our selection of EHB GCs
were based on the reddening independent criteria on
CMD in B&V passbands (∆VHB > 3.5 for strongly ex-
tended HB; either 3.0 < ∆VHB < 3.5 or ∆(B − V )HB
> 0.78 with clear bimodal colour distribution for mod-
erately extended HB). But, since their appearances on
CMDs are distinct enough from GCs with normal HB
(Piotto et al. 2002), our selection agrees well with the re-
sult based on smaller sample and other measures of HB
temperature extension (e.g., Recio-Blanco et al. 2006).
We have then investigated their properties compared to
other normal GCs.
2. LUMINOSITY FUNCTION AND KINEMATICS
First of all, from the luminosity function (Fig. 1), we
found that EHB GCs are among the brightest GCs of
the Milky Way, including 11 out of 12 brightest GCs
(see also Recio-Blanco et al. 2006). It is surprising to
see that not a single EHB GC is fainter than MV = -
7. Careful inspection of all CMDs confirms that this is
not due to the smaller number of HB stars in fainter
GCs. Because of significant fraction (18 - 51%) of the
helium enriched bluer subpopulation observed in EHB
GCs, its presence on the HB would be reliably detected
(> 5 - 10 stars) even in a cluster of MV = -6 or -5 if it
http://arxiv.org/abs/0704.0486v1
2 Lee, Gim, & Casetti-Dinescu
-2 -4 -6 -8 -10 -12
40 EHB
Normal
Fig. 1.— The histogram of MV for 114 Milky Way GCs (data
from Harris 1996). Blue and red are GCs with strongly and mod-
erately extended HBs, respectively. EHB GCs are clearly brighter
(more massive) than normal GCs.
existed. This result is perhaps already suggesting that
EHB GCs might have a peculiar origin, as their inferred
current stellar mass, which might represent only a small
fraction of their original mass, is comparable with that
of low-luminosity dwarf galaxies in the Local Group.
Motivated by this, we have investigated the kinemat-
ics of EHB GCs, in order to see whether their kinematic
properties are also distinct from other normal GCs. Fol-
lowing previous investigation (Zinn 1993), we have first
divided GCs into three subgroups (Fig. 2) based on the
HB morphology and metallicity diagram (Lee, Demar-
que, & Zinn 1994). Metal-poor ([Fe/H] < -0.8) GCs
in the “Old halo (OH)” group have bluer HB morphol-
ogy at a given metallicity, and those in the “Younger
halo (YH)” group have redder HB morphology at fixed
metallicity. The OH group, in the mean, is probably
older than the YH group by ∼1 Gyr (Rey et al. 2001;
Salaris & Weiss 2002). The metal-rich ([Fe/H] > -0.8)
GCs are further classified as “disk/bulge (D/B)” group.
EHB GCs belong in all three subgroups, although the
majority of them are in the OH group. Note also that
most (94%) GCs in YH group are in the outer halo
(galactocentric distance, Rgc > 8 kpc), while the ma-
jority (80%) of GCs in OH and D/B groups are in the
inner halo (Rgc < 8 kpc).
The result of the kinematic analysis based on
the constant-rotational-velocity solutions (Zinn 1993;
Frenk & White 1980) and the updated database of Har-
ris (Harris 1996) is presented in Table 1. When all the
GCs are considered, we are basically confirming the con-
clusion of the previous work (Zinn 1993). YH group is
dominated by random motion with no sign of significant
rotation (Vrot), while OH group shows some prograde
rotation and a smaller line-of-sight velocity dispersion
(σlos). D/B group is mostly supported by rotation with
a relatively small σlos. We find, however, EHB GCs, both
belonging to YH and OH groups, are dominated by ran-
dom motion and show no signs of rotation. Consequently,
when they are excluded from the sample, normal GCs in
OH group show increased rotation (from 1.5 to 1.8σ from
zero Vrot) and higher value of Vrot/σlos. The same trend
is also observed in the normal GCs in D/B group, but
with much larger uncertainty. When only comparably
bright (MV < -6) GCs are considered, the differences
become significantly larger (2.5σ from zero Vrot). The
TABLE 1
KINEMATICS OF GLOBULAR CLUSTERS BASED ON
RADIAL VELOCITY ALONE*
Group N Vrot σlos Vrot/σlos
All GCs
All Halo 71 25±27 124±10 0.20±0.22
YH 25 -18±66 153±22 -0.12±0.43
OH 46 40±27 104±11 0.38±0.26
D/B 14 168±28 65±12 2.57±0.65
EHB GCs
All EHB 24 10±32 93±13 0.11±0.34
OH 18 4±35 91±15 0.05±0.38
Normal
All Halo 48 32±39 137±14 0.24±0.29
YH 20 -42±80 162±26 -0.26±0.49
OH 28 70±39 111±15 0.63±0.36
D/B 13 188±22 48±9 3.94±0.89
Normal (MV < -6)
All Halo 39 37±44 139±16 0.26±0.32
YH 17 -69±81 160±27 -0.44±0.51
OH 22 105±42 103±15 1.02±0.43
D/B 11 195±27 52±11 3.76±0.95
*For Rgc < 40 kpc and excluding GCs with (cosψ) > 0.2
above analysis, based only on the radial velocity data,
provides good reason to suspect that EHB GCs are kine-
matically decoupled from other normal GCs, especially
in OH group. Below, we investigate this in more detail
using the measurements of full spatial motions and or-
bital parameters now available for 49 GCs in our sample
(Dinescu et al. 2003).
In Figure 3, we have plotted kinematic parameters ob-
tained from full spatial motions as a function of metallic-
-1.0 -0.5 0.0 0.5 1.0
EHB OH+D/B
EHB YH
Normal OH+D/B
Normal YH
Red Blue
HB Type
Fig. 2.— The subdivision of GCs in the HB morphology ver-
sus metallicity diagram. The filled circles are GCs either in the
“old halo (OH)” or metal-rich ([Fe/H] > -0.8) “disk/bulge (D/B)”
groups, while open circles are those in the “younger halo (YH)”
group. The EHB GCs belong in all three subgroups, but most of
them are in OH group. The updated database (Lee et al. 1994)
consisting 95 GCs in Rgc < 40 kpc zone are compared with model
HB isochrones (Rey et al. 2001) in solid lines, the upper being
older by 1.1 Gyr. Short dashed line has the same age as the upper
solid line, but is for EHB GCs with 15 % of helium enhanced (Y
= 0.33) subpopulation (Lee et al. 2005).
Kinematic Decoupling of EHB GCs 3
      
-2000
-1500
-1000
EHB OH
EHB YH
Normal OH+D/B (P)
Normal OH+D/B (R)
Normal YH
47Tuc
      
200 M54
      
-2.5 -2.0 -1.5 -1.0 -0.5  
47Tuc
[Fe/H]
0.0 -2.5 -2.0 -1.5 -1.0 -0.5  
[Fe/H]
0.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
-1000
47Tuc
[Fe/H]
Fig. 3.— The relationship between kinematics derived from full spatial motions and metallicity. From (a) to (f), total orbital energy
(Etot), maximum distance perpendicular to the Galactic plane (Zmax), velocity component perpendicular to the plane (W ), rotational
velocity (Θ), orbital eccentricity, and the angular momentum component associated with Θ (LZ) are plotted as a function of metallicity
(Zinn 1993), respectively. Red filled circles are normal GCs with prograde rotation in OH and D/B groups. All of them are in the inner
halo. Red open circles are normal GCs with retrograde rotation in OH and D/B groups. Only 3 of them are in the inner halo.
ity, which shows more directly the systematic differences
between EHB and normal GCs. In all panels of Fig-
ure 3, EHB GCs have diversity in kinematics, and show
no correlations with metallicity (correlation coefficient,
r, of -0.02 to -0.31 with high p-values of 0.25 to 0.94).
Normal YH GCs, mostly in the outer halo, show kine-
matically hot signatures (high Etot, Zmax, & eccentricity,
and large velocity dispersion) (Mackey & Gilmore 2004).
To our surprise, however, when EHB GCs are excluded,
most normal GCs with prograde rotation in OH and D/B
groups (red filled circles) show clear signs of dissipational
collapse. Etot, Zmax, W velocity, and perhaps orbital ec-
centricity are all decreasing with increasing metallicity,
among which the ‘chevron’ shape of W velocity distri-
bution is most impressive. Rotational velocity, however,
is increasing with metallicity, and LZ appears to be con-
served. The orbital properties of NGC 6528 are known to
be highly affected by the potential of the bar because of
its proximity (Dinescu et al. 2003). Thus, excluding this
one deviant point in panels (d) and (e), we obtain strong
correlations for Etot, Zmax, |W |, Θ, and eccentricity (r
= -0.58, -0.71, -0.95, 0.89, and -0.77, respectively) with
small p-values (0.05, 0.01, 3.1× 10−6, 0.0002, and 0.005,
respectively). In other words, the correlations are highly
significant at the level of 95%, 99%, 99.999%, 99.98%,
and 99.5%, respectively. As expected, however, corre-
lation is low (r = 0.17) for LZ with a high p-value of
0.59.
All of these trends observed for normal GCs with pro-
grade rotation in OH and D/B groups are fully consistent
with the model first envisioned by Eggen, Lynden-Bell,
& Sandage (1962), where metal enrichment went on as
dissipational collapse continued. Although these results
are based on relatively modest numbers of GCs with full
spatial motion information, their coherent behaviours in
all panels of Figure 3, together with statistically signifi-
cant correlations, confirm that we are detecting real sig-
natures. Also, these results are consistent with the kine-
matics solution obtained from radial velocity alone (Ta-
ble 1), which is based on a larger sample of GCs. We
argue, therefore, (1) EHB GCs in our sample are indeed
kinematically decoupled from most of the normal GCs in
OH group, and (2) when EHB GCs are excluded, we are
detecting clearer signatures of dissipational collapse in
the inner halo, which apparently led to the formation of
the Galactic disk (Zinn 1993; Mackey & Gilmore 2004).
The kinematics of EHB GCs, which are not following
the dissipational collapse, are more consistent with what
one would expect among the relicts of primeval star-
forming subsystems that first formed the nucleus (EHB
GCs with low Etot and Zmax) and halo (EHB GCs with
high Etot and Zmax) of the Galaxy through both dissipa-
tional and dissipationless mergers, as has been predicted
by recent ΛCDM simulations for “high-σ peaks” (e.g.,
Diemand, Madau, & Moore 2005; Moore et al. 2006).
As described above, a significant fraction of the helium
enriched subpopulation also favours building block origin
of EHB GCs. Normal YH GCs in the outer halo share
4 Lee, Gim, & Casetti-Dinescu
their kinematic properties with the outlying EHB GCs,
which is consistent with the view (Searle & Zinn 1978)
that they were originally formed in the outskirts of iso-
lated building blocks and later accreted to the outer halo
of the Galaxy when their parent dwarf galaxies, like the
Sagittarius, were merging with the Milky Way. The GCs
with EHB also tend to show more extended Na-O and
Mg-Al anticorrelations (Gratton 2007). Therefore, the
suggested connection between some of these GCs with
strong chemical inhomogenity and orbital parameters
(Carretta 2006) might be due to the diversity of kine-
matics among EHB GCs.
According to the present picture, most of the normal
GCs with retrograde rotation in OH and D/B groups
could have also originated from the subsystems with ret-
rograde rotation. Interestingly, their relatively confined
distributions both in the angular momentum phase space
(Helmi et al. 1999) and velocity space are not inconsis-
tent with the possibility that some or most of them were
former members of parent dwarf galaxies hosting two
EHB GCs, ωCen and/or NGC 6723 (Lee et al., in prepa-
ration). Their distribution in velocity space is also well
consistent with the model prediction of the tidal debris
from ωCen’s parent dwarf system (Mizutani, Chiba, &
Sakamoto 2003), which was presumably formed in the
outer halo and accreted to the inner halo. Note that a
similar minor merging of subsystem with the thin disk
(Quinn, Hernquist, & Fullagar 1993) could have also
changed some of the original kinematic properties of two
disk GCs in Figure 3 (47Tuc and M71).
3. DISCUSSION
The clear differences in kinematics and mass be-
tween GCs with and without EHB are strong evidence
that they have different origins. Our results suggest
present-day Galactic GCs are most likely an ensemble
of heterogeneous objects originated from three distinct
phases of the Milky Way formation: (1) remaining cores
or central star clusters of building blocks that first assem-
bled to form the nucleus and halo of the proto-Galaxy
(Bromm & Clarke 2002; Santos 2003; Bekki 2005;
Kravtsov & Gnedin 2005; Moore et al. 2006), (2) gen-
uine GCs formed in the dissipational collapse of a
transient gas-rich inner halo system that eventually
formed the Galactic disk (Eggen et al. 1962), and (3)
genuine GCs formed in the outskirts of outlying building
blocks that later accreted to the outer halo of the Milky
Way (Searle & Zinn 1978). In this picture, relicts of
first building blocks that formed the flattened nucleus
(Kravtsov & Gnedin 2005; Moore et al. 2006) are now
observed as relatively metal-poor EHB GCs (e.g., NGC
6266, 6522, and 6626) having low Etot and Zmax near the
centre. Formation of the slowly-rotating gas-rich inner
halo system that later collapsed in phase (2) is still most
unclear, but it is attractive to speculate that leftover gas
from “rare peaks” (building blocks hosting EHB GCs)
in the inner halo and gas from continuously falling “less
rare peaks” (Moore et al. 2006) led to the formation of
this structure, perhaps with the aids of some heating
feedbacks (e.g., Schawinski et al. 2006) soon followed by
cooling. Several lines of further study will certainly help
to shed more light into the picture briefly sketched here.
For example, search for the tidal streams that might be
associated with EHB GCs, dark matter search in the
outlying EHB GCs where preferential disruption of dark
matter halo (Saitoh et al. 2006) might be less severe,
kinematics analyses of extragalactic GC systems along
with the ultraviolet survey for EHB GC candidates,
together with more detailed high resolution ΛCDM
simulations.
We thank R. Zinn, R. Larson, and P. Demarque for
helpful discussions, C. Chung for his assistance in HB
isochrone construction, and H.-Y. Lee for her assistance
in CMD compilation. Support for this work was provided
by the Creative Research Initiatives Program of the Ko-
rean Ministry of Science & Technology and KOSEF, for
which we are grateful.
REFERENCES
Bedin, L. R., Piotto, G., Anderson, J., Cassisi, S., King, I. R.,
Momany, Y., & Carraro, G. 2004, ApJ, 605, L125
Bekki, K. 2005, ApJ, 626, L93
Bekki, K. & Norris, J. E. 2006, ApJ, 637, L109
Bromm, V. & Clarke, C. J. 2002, ApJ, 566, L1
Carretta, E. 2006, AJ, 131, 1766
D’Antona, F., Bellazzini, M., Caloi, V., Pecci, F. F., Galleti, S., &
Rood, R. T. 2005, ApJ, 631, 868
Diemand, J., Madau, P., & Moore, B. 2005, MNRAS, 364, 367
Dinescu, D. I., Girard, T. M., van Altena, W. F., & Lopez, C. E.
2003, AJ, 125, 1373
Eggen, O. J., Lynden-Bell, D., & Sandage, A. R. 1962, ApJ, 136,
Freeman, K. C. 1993, in ASP Conf. Ser. 48, The Globular Clusters-
Galaxy Connection, ed. G. H. Smith & J. P. Brodie (San
Francisco: ASP), 608
Frenk, C. S. & White, S. D. M. 1980, MNRAS, 193, 295
Gratton, R. 2007, in ASP Conf. Ser., From the Stars to Galaxies,
ed. A. Vallenari & R. Tantalo, (Venice: ASP), in press
Harris, W. E. 1996, AJ, 112, 1487
Helmi, A., White, S. D. M., de Zeeuw, P. T., & Zhao, H. 1999,
Nature, 402, 53
Kravtsov, A. V. & Gnedin, O. Y. 2005, ApJ, 623, 650
Layden, A. C. & Sarajedini, A. 2000, AJ, 119, 1760
Lee, Y.-W., Demarque, P., & Zinn, R. 1994, ApJ, 423, 248
Lee, Y.-W., Joo, J.-M., Sohn, Y.-J., Rey, S.-C., Lee, H.-C., &
Walker, A. R. 1999, Nature, 402, 55
Lee, Y.-W. et al. 2005, ApJ, 621, L57
Mackey, A. D. & Gilmore, G. F. 2004, MNRAS, 355, 504
Mizutani, A., Chiba, M., & Sakamoto, T. 2003, ApJ, 589, L89
Moore, B., Diemand, J., Madau, P., Zemp, M., Stadel, J. 2006,
MNRAS, 368, 563
Norris, J. E. 2004, ApJ, 612, L25
Piotto, G. et al. 2002, A&A, 391, 945
Piotto, G. et al. 2005, ApJ, 621, 777
Piotto, G. et al. 2007, preprint (astro-ph/0703767)
Quinn, P., Hernquist, L, & Fullagar, D. 1993, ApJ, 430, 74
Recio-Blanco, A., Aparico, A., Piotto, G., de Angeli, F., &
Djorgovski, S. J. 2006, A&A, 452, 875
Rey, S.-C., Yoon, S. J., Lee, Y.-W., Chaboyer, B., & Sarajedini, A.
2001, AJ, 122, 3219
Rosenberg, A., Recio-Blanco, A., & Garcia-Marin, M. 2004, ApJ,
603, 135
Saitoh, T. R., Koda, J., Okamoto, T., Wada, K., & Habe, A. 2006,
ApJ, 640, 22
Salaris, M. & Weiss, A. 2002, A&A, 388, 492
Santos, M. R. 2003, in ESO Astrophysics Symposia, Extragalactic
Globular Cluster Systems, ed. M. Kissler-Patig (Garching:
Springer-Verlag), 348
Schawinski, K. et al., 2006, Nature, 442, 888
Searle, L & Zinn, R. 1978, ApJ, 225, 357
Zinn, R. 1993, in ASP Conf. Ser. 48, The Globular Clusters-Galaxy
Connection, ed. G. H. Smith & J. P. Brodie (San Francisco:
ASP), 38
