A&A 459, 769-775 (2006)
DOI: 10.1051/0004-6361:20053899
L. Vanzi1 - A. Scatarzi2 - R. Maiolino2 - M. Sterzik1
1 - European Southern Observatory, Alonso de Cordova 3107, Vitacura,
Santiago, Chile
2 - Osservatorio
Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
Received 25 July 2005 / Accepted 12 June 2006
Abstract
We present new high-resolution optical spectroscopy of giant HII regions around three young
massive clusters located in the blue dwarf galaxies NGC 5253 and He 2-10.
We used the observations to derive the mass of the clusters under the hypothesis of virialization. The virial masses exceed the optical ones. In one case, however, the virial mass is consistent with that derived from the IR luminosity, pointing to the presence of a hidden stellar population.
The dynamics inferred from the emission lines indicates that the clusters may indeed be
virialized or close to virialization.
We
detect an indication of mass segregation in two clusters. At least in one case, we find that such
mass segregation must have a "primordial'' nature, rather than dynamical,
i.e. associated with the formation mechanism of the cluster. We could resolve two of
the clusters with the spectro-astrometric technique detecting in both cases
evidence of structures on the pc scale.
Key words: galaxies: dwarf - galaxies: individual: NGC 5253 - galaxies: individual: He 2-10 - galaxies: star clusters
Young massive clusters (YMC) have been detected in a large number of
galaxies characterized by high star-formation rates:
interacting (Whitmore et al. 1999); giants (Larsen & Richtler 1999; Larsen 2000); dwarfs (McCrady et al.
2005; Vanzi & Sauvage 2006) and AGNs (Galliano et al. 2005). Some of these
galaxies host few YMCs, others up to a few hundred. It is becoming evident
that YMCs are very important in the star formation process occurring in
galaxies and possibly in the formation of galaxies themselves. They account
for a large fraction of the overall star formation in some cases, and their
winds and ionizing output can be relevant to the properties and
evolution of the hosting system. The luminosity and mass of YMCs are typically
distributed according to a power law (Whitmore et al. 1999; Johnson et al. 2000; Larsen 2002;
Cresci et al. 2005; Vanzi & Sauvage 2006), and evidence has been collected to show that
this power law could evolve toward the distribution observed for globular
clusters, with the less massive clusters being evaporated, while the most
massive and gravitationally bound ones survive over a Hubble time (Fall & Rees 1977;
Fall & Zhang 2001). Typical
masses of YMCs are in the range 104-
.
It is also becoming
evident that in their early phases, YMCs are deeply embedded in dust and
molecular clouds. The embedded phase is estimated to last only a few Myrs.
We obtained high dispersion spectroscopy in the visible band of three classical
YMCs with the purpose of characterizing their properties and those of the
surrounding medium. Two of the observed clusters are located in NGC 5253, a
well-known blue dwarf galaxy characterized by a recent episode of star
formation, a large number of clusters (van den Bergh 1980), and sub-solar
abundances (Kobulnicky et al. 1999). In this paper we have assumed a distance to
NGC 5253 of 3.3 Mpc (Gibson et al. 2000). The clusters observed in NGC 5253 are
numbers 1 and 5 following the nomenclature of Calzetti et al. (1997). Their
ages are 8-12 Myr and 2.5-4.4 Myr, respectively, according to the HST
observations of Calzetti et al. (1997). The estimated stellar masses are
<
and about 2-
,
respectively, again according to the estimates of Calzetti et al. (1997).
Vanzi & Sauvage (2004) instead derive a total mass
of 0.82-
and an age younger than 2 Myr for cluster 5
from a careful modeling of the IR emission of the cluster. The discrepancy
by almost one order of magnitude between the optical and IR estimate of the
mass is not surprising since it has been found that NGC 5253-5 is embedded
in dust and extincted by about 8 magnitudes in the optical, so that a large
fraction of its stellar content could easily escape optical observations.
The third cluster is located in He 2-10, which is also a blue dwarf galaxy at a
distance of 9 Mpc (Johansson 1987). This cluster is identified as "A'' by Johnson
et al. (2000) who derive a mass of 1.6-
.
The age of the
cluster is estimated to be about 3-6 Myr based on the presence of WR stars.
This paper is organized as follow, we briefly describe the observations in Sect. 2, and in Sect. 3 we present the analysis of the data including line profiles, reddening, ionization status of the interstellar medium, and spectro-astrometry. In Sect. 4 we summarize our conclusions.
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Figure 1:
Two dimensional spectra centered on [OIII]5007 and H![]() ![]() |
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The observations were obtained during the nights of March 13 and March 15, 2003, at
the ESO - VLT with the red arm of the cross-dispersed echelle spectrometer
UVES. The spectral range covered is 4800-5680 Å and 5840-6715 Å with a
gap produced by the fact that the spectrum is imaged on two CCDs separated by 1 mm. We used a slit of
,
oriented as the parallactic angle, obtaining a spectral resolution of about 80 000. The spatial scale
along the slit is 0.182
per pixel. The integration time was 60 min divided in three 20-min exposures for NGC 5253-1 and NGC 5253-5 and 15 min divided in 15 1-min exposures for the cluster in He 2-10.
The seeing during the observations was about 1
.
The spectra were reduced using the UVES pipeline provided by ESO. The wavelength
calibration was obtained using a Th-Ar comparison spectrum.
Two dimensional spectra, 12
wide, centered on [OIII]5007 and H
are shown in Fig. 1.
One dimensional spectra were extracted from the 2D spectra with a width of 3 pixels, corresponding to apertures of about
.
A spectrum of the star LTT 3218 observed with a slit
was used for the calibration in flux, the data reduction was done in MIDAS. The 2D spectrum of the star was smoothed to a resolution of 50 Å and compared to the template of Hamuy et al. (1992) to derive a 2D instrument response curve.
The new observations were used to derive the properties of the giants HII regions and the ionizing clusters.
In Fig. 2 we show the profiles of
for the three clusters, centered on the center
of the cluster and
extracted with an aperture of 3 pixels so that the final aperture is about
.
We used ALICE in MIDAS to fit the line
profiles. It was not possible to obtain satisfactory fits with a single
Gaussian and, in all cases, at least two Gaussian components were necessary. The
parameters of the fits are given in Table 1 for
,
[OIII] 5007,
and [NII] 6583. We estimated the global error on the line fluxes to be on the order of 1%, which includes the statistical error and the error introduced by the calibrations.
The error on the velocities can be estimated by examining the values in Table 1 for different spectral lines, and it is typically less than 2 km s-1.
In the case of He 2-10 and NGC 5253-5, the two-component fit
gives a relatively narrow component superimposed
on a significantly broader one (a factor of about 3-4 broader than the narrow
component), in agreement with the multiple "narrow'' components superimposed on a
single broader one found by Melnick et al. (1999) in the HII region surrounding
30-Doradus. The case of NGC 5253-1 is different, because the line profile observed is affected by the presence of a region located at about 2
to the
north and characterized by very strong emission lines that extend toward the
cluster and blend with it, see Fig. 1. This region is very close to the cluster NGC 5253-4,
though it cannot be identified with it. The small Gaussian component observed
in this case is almost certainly the tail of the emission lines from the neighboring
region. The fact that, unlike Melnick et al. (1999) we detect, in all cases,
a single narrow component can be attributed to the fact that our aperture
averages over a much larger physical region due to the greater distance of the
targets. The
of the broad components agree with the
observation of Melnick et al. (1999), who attribute this component to a
tenuous highly turbulent gas.
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Figure 2:
Fit with two Gaussians of the
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The broadening of the emission lines in HII regions is produced by a few
fundamental effects, as described by Melnick et al. (1999): mainly thermal
broadening, virial broadening, and local dynamical effects as expanding shells,
filaments, and outflows. Terlevich & Melnick (1981) and Rozas et al. (1998)
report observing an empirical correlation between the total
or
luminosity of giant HII regions and their observed
.
This
correlation can be conceptually understood if the
is mostly determined by
gravity. The HII regions must be virialized or almost virialized.
In this case, an estimate of the mass can be derived. To compare our
observations with this empirical relation, we extracted 1D spectra with an
aperture that includes most of the
emission along the slit and
repeated the double-component fit, the results are given in Table 2. The errors on the quantities related to the lines are the same as in Table 1.
To derive the total
luminosity, we calculated a slit-loss correction
factor assuming the HII regions to be regular and symmetrical. The correction factor
turned out to be about 5 in the case of He 2-10 and NGC 5253-5, which were integrated
over an aperture equal to the FWHM of the spatial profile, but was almost 20 for
NGC 5253-1 where, due to the blending with the neighboring region located to the
north, we were forced to truncate the spatial integration in that direction.
After correcting for the extinction, we obtained 3.83, 8.41, and
erg/s for the total
luminosity of NGC 5253-1, NGC 5253-5 and
He 2-10, respectively. The slit-loss correction introduces a major source of uncertainty on the fluxes.
To evaluate it, we measured the
luminosity of cluster 5 on the HST image of
Calzetti et al. (1997). We obtained a value of
erg/s after correcting for the extinction. This value is about 35% lower than the slit-loss-corrected luminosity obtained from the spectrum, and it gives an estimate of
the approximation introduced. The uncertainty is probably larger for NGC 5253-1 due to the larger correction.
In Fig. 3 we compare our data points with the relation of
Terlevich & Melnick (1981), converted to
assuming a
ratio of 2.86, and Rozas et al. (1998). Both
the total
luminosity and the luminosity of the narrow component of
the fit are shown, along with
the total
luminosity before correction for the slit-loss. As
we used the value of the narrow component deconvolved for the
instrumental broadening (2.2 km s-1) and for a thermal contribution of 9.1 km s-1(typical of the HII regions, Melnick et al. 1999). If the
broad component of the emission lines is associated to highly turbulent gas (Melnick et al. 1999),
it should not be used for dynamical studies.
The errorbar indicates the uncertainty
introduced by the slit-loss correction estimated as described in the previous paragraph.
Given the uncertainties, our data points are consistent with the empirical
relation. Under the assumption that the clusters are virialized, or almost virialized, we
can therefore derive their virial masses as
from the parameters of Table 2. As radius of the HII region, we used half FWHM of the spatial
profile along the slit.
The values obtained in this way must be multiplied by a factor
that takes into account the relation between
and the line of sight velocity dispersion and between the gravitational radius and the half light radius
.
A typical value is
(e.g. Fleck et al. 2006). For a Gaussian profile, though,
FWHM/2. In the last column of Table 2 we list
.
Table 1:
Parameters of the double Gaussian fit executed on the
,
[OIII] 5007, and [NII] 6583 emission lines. The flux of the
components F is given in 10-16 erg/s/cm2, position with respect to the nominal
center wavelength v and
are given in km s-1.
Table 2:
Parameters of the double Gaussian fit executed on
over the bulk of the HII regions. The flux of each component F is given
in 10-15 erg/s/cm2. The position with respect to the nominal center wavelength v and the
in km s-1. The dimension along the slit, on
which the lines were integrated, d is in
,
this is basically the FWHM of
the spatial profile. The virial masses Mare in units of
.
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Figure 3:
Comparison of the
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Figure 4: Extinction as function of space along the slit from south (negative) to north (positive) with 0 indicating the position of the cluster. |
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For NGC 5253-5, Vanzi & Sauvage (2004) derive an upper mass of
,
from the integrated IR luminosity. Calzetti et al. (1997) give a mass of 2-
,
based on optical observations.
There are no IR estimates of the mass of the other two clusters; however, for the most luminous ultra
dense HII region of He 2-10, Vacca et al. (2002) derive a mass of about
from mid-IR observations. Johnson et al. (2000) instead obtain a mass of 1.2-
for region A.
Cabanac et al. (2005), however, show that this region can only partially be associated to the
optical cluster observed by us.
The mass of NGC 5253-5, estimated through the optical luminosity (corrected
for extinction by means of the Balmer decrement), falls short by almost one
order of magnitude when compared to the IR mass. This discrepancy is most likely due to the fact that a large fraction of the stars in the cluster are obscured.
Observations
of YMCs in the IR showed in fact that in their early phases they can be characterized by very high
extinction, largely underestimated by the balmer decrement (Hunt et al. 2001). This is certainly the case for NGC 5253-5 (Vanzi & Sauvage 2004) and He 2-10 (Cabanac et al. 2005).
The situation is more complex when considering the virial masses since they are affected by uncertainties that are difficult to quantify. In this case the discrepancies could be for different reasons. It could be that the HII regions
are not virialized. A number of objections could indeed be raised to the virialization hypothesis, as the clusters are very young, and HII regions are typically irregular and short-living objects. In addition even under virialized conditions, the virial masses include both the
stellar and the gas components. The gas-to-star ratio is unknown, but it can easily be on the order of 1 or more. The HII regions considered are larger than the stellar cluster; however, the total mass of the diffuse ISM is expected to be small. In the case of NGC 5253-5, for instance, half
FWHM of the HII region spatial profile corresponds to 12.7 pc, while Calzetti el al. (1997) give a half light radius of the cluster of about 3 pc. Assuming a standard density of 100 cm-3, the total mass of the diffuse gas in the volume observed would not exceed
.
The factor
is also uncertain as seen before. If we assume
and a gas-to-star ratio of 1, we derive 8.6, 4.0, and
,
respectively, for the HII regions around clusters NGC 5253-1, -5, and He 2-10. The main uncertainty at this point is related to the assumption on the gas-to-star ratio.
The range of masses
calculated by Vanzi & Sauvage (2004) for NGC 5253-5, assuming a Salpeter IMF,
goes from 0.8 to
for a lower mass cutoff between 1 and 0.1
,
respectively. In this sense our virial estimate is consistent with a
standard IMF. The most realistic value calculated for the mass of the cluster is
,
obtained using a Scalo IMF, which is flatter in
the low mass regime and which extends from 0.1 to 100
.
We find that the
comparison of the
fluxes and their broadening can complement the
analysis of the IR light to derive the mass of YMCs.
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Figure 5:
Ratios of [OIII]/
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Table 3:
Line ratios observed (Cols. 2 and 3) and modeled with CLOUDY
(Cols. 4 and 5) for the three
clusters, on the cluster's center (on) and at about 3
off center (off). The effective temperatures
of the ionizing stars and the ionization parameter derived are given in the last two columns.
We derived the extinction in the spatial direction along the slit
by using the observed
ratio and assuming an intrinsic ratio of 2.86
(corresponding to
K and
cm-3, Hummer & Storey 1987).
The results are shown in Fig. 4 and are consistent with
previous determinations. Allen et al. (1976) obtained
in He 2-10 but
with a larger aperture. Gonzalez-Riestra et al. (1987) obtained
in NGC
5253 with an aperture similar to ours. We note that, in the case of NGC 5253-5,
the extinction shows a marked maximum in the proximity of the cluster's center,
identified with position "0'' in the plots, but shifted respect to it by a
fraction of arcsec. The extinction in the other two cases shows a
spatial gradient. Both facts can be understood if the central cluster breaks
the molecular cloud in one preferential direction, as it is very often observed
in the galactic HII regions. At about 2
to the north of NGC 5253-1,
the extinction increases at about the same location of the emission line region
mentioned in Sect. 3.1.
We examined the spatial distribution of the
ratios [OIII]/
and [NII]/
as diagnostics
of the ionization status of the ISM (Fig. 5). The
line ratios show different spatial behaviors in the three clusters. In He 2-10
they share a trend with a maximum close to the position of the cluster. In
NGC 5253 the two ratios are anti-correlated: in NGC 5253-1 [NII]/
is
maximum at the position of the cluster, while in NGC 5253-5 it is
[OIII]/
that peaks on the cluster center. We also note that a situation
similar to the one observed in NGC 5253-5 is also present in the region to the
north of NGC 5253-1.
These clusters
have diameters slightly smaller than 1
in the HST images, therefore
they are marginally resolved by our observations. As a consequence, we are
confident that the line-ratio trend observed in Fig. 5 are real even within
the cluster region. The errors on the line ratios are mostly determined by the spectro-photometry
and are on the order of 2%.
We used CLOUDY (Ferland et al. 1998) to model the ratios observed and, in
particular, to derive the effective temperature of the ionizing sources. The
ionizing stellar spectra were approximated with black bodies. We used densities in
the range 10-100 cm-3 (although the density does not affect significantly these
ratios), and adopted the abundances known for the two galaxies: solar
for He 2-10 and 1/5
for NGC 5253.
We varied the effective temperature of the ionizing stars T* and the
ionization parameter U to match the observed ratios. The use of both [OIII]/H
and [NII]/H
allows us to remove the degeneracy between T* and U.
In Table 3 we compare
the observed line ratios with the best fit obtained with CLOUDY, along with
the inferred effective stellar temperatures T* and the ionizing parameters U.
These values are reported both for the position corresponding to the
cluster's center (indicated as ON) and for the
average of two locations (indicated as OFF), which lie at about 3
from the center.
We find the two quantities well constrained by the
observations; in particular, the uncertainties on T* are about 2000 K.
The observed ratios are corrected for the extinction.
The very
high effective temperatures derived in the case of NGC 5253-5 and He 2-10 are
not surprising as both are well-known WR galaxies (Schaerer et al. 1999).
In both cases the presence of WR
stars has been inferred by the HeII broad feature at 4686 Å . The WR stars are
the evolved phase of the most massive stars, typically above 35 (Schaerer & Vacca 1998). They are very short-lived, less than 1 Myr
(Maeder & Meynet 1994) and have effective temperatures above 30 000 K and up to 105 K (Maeder & Conti 1994). The cluster NGC 5253-1 instead shows lower
T*, consistent with its more advanced stage of evolution
where the number of WR and O stars must be much reduced compared to the
other two cases.
Even more interesting is the clear gradient in temperature
observed in all cases across the clusters. Both He 2-10 and NGC 5253-5 have a higher
temperature on the cluster's center than in the outer parts, while NGC 5253-1
shows the opposite trend with higher temperature in the outer regions.
NGC 5253-1 has an age of about 10 Myr, so
the most massive stars of the cluster must have evolved already and produced
the first supernovae; it is then plausible that the outer gas is ionized by other sources in
the neighborhood of the cluster. In contrast the clusters observed in He 2-10 and NGC 5253-5 are
both very young with ages of a few Myr, and indeed in these cases the hottest stars are
observed in the central beam. Summarizing, in two cases we find indications
of a higher
density of massive stars toward the clusters' centers. It is interesting to
notice that the trend in the values of the line ratios is already observed in the apertures next to the central one, at 0.5
from the center, a distance that, at least in the case of NGC 5253-5, is
comparable to the cluster's size; see next section.
Mass segregation has been previously observed in YMCs in the LMC (de Grijs et al. 2002a) and in M 82 (McCrady et al. 2005). In these cases the most massive stars appeared to be located toward the center of the cluster. An important question within this context is whether mass segregation in clusters is produced by dynamical effects or if it has instead a "primordial'' origin, associated with the formation of the cluster itself. A way to answer this question is to compare the relaxation time with the age of the cluster (de Grijs et al. 2002b).
According to Vanzi & Sauvage (2004), NGC 5253-5 is possibly the youngest YMC observed,
with an age <2 Myr. The relaxation time for this cluster can be derived using expression 6 of Meylan (1987).
We measured the cluster radius on the optical HST images and obtained a FWHM of
0.45
in the F814W filter, or about 7 pc. We used a mass of
.
With these parameters, the relaxation time for a 100
star would be about 50 Myr,
much longer than the age of the cluster. The relaxation time is even longer for less massive stars.
The indication for mass segregation in NGC 5253-5 would therefore suggest a "primordial'' rather
than dynamical effect.
The case for a "primordial'' segregation in NGC5253-5 is even
stronger than in the cluster R136, in 30-Dor, where
mass segregation is also observed but the age of 3-4 Myr seems to be long
enough, compared to the relaxation time, for dynamical effects to be relevant
(Brandl et al. 1996).
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Figure 6:
Profiles of the
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The quality of our data allows us to employ the spectro-astrometry technique to resolve structures in the sources observed, beyond the limits imposed by the seeing. The technique is not new and has been described in detail by Bailey (1998a,b, and references therein). It consists of fitting the spatial profile of the source along the slit and determining its "centroid'' as a function of the wavelength, with a sub-pixel accuracy at the location of spectral features. If the source is extended or contains structures characterized by different spectral features, this will produce a displacement of the centroid of the composite spectrum along the slit. Due to the complexity of the regions observed, which typically include a bright compact source - the cluster - embedded into an extended and diffuse HII region, we have used two Gaussians to fit the spatial profile along the slit. Particular care was taken in the definition of the position of the continuum. Since the continuum signal is very faint in all cases, we summed up all the emission-line-free regions of the spectra in the direction of the dispersion and fit the spatial profile obtained in this way with a single Gaussian.
The
results are shown in Fig. 6 for NGC 5253-5 and He 2-10. The
quality of the data of NGC 5253-1 was not high enough to allow this kind of
analysis. Both
and [OIII] 5007 lines have been used, the normalized line
profiles are plotted in the upper panel of Fig. 6. It can be
seen how the [OIII] line is in both cases slightly narrower than
.
The displacements of the centroids as function of
wavelength are shown in the middle and lower panels. The dashed lines indicate
the position of the continuum
corresponding to the 0 of the spatial scale. We observe that the emission
lines tend to be shifted with respect to the continuum. The shift is about
+0.20
for He 2-10 and -0.10
for the
in NGC 5253-5,
equivalent to 8.7 and 1.6 pc, respectively. In other words, the stellar continuum
and the gas emission do not come from the same region or, at least, their
relative contribution is not uniform over the region observed. In NGC 5253-5, unlike
,
the [OIII] profile is centered on the continuum.
The origin of this difference is not clear; it could be physical (different
[OIII] emitting regions relative to H
or differential extinction between
line and continuum emitting regions), but it could also be due to
uncertainties in the determination of the continuum center, which is
not well-constrained due to low signal-to-noise.
We notice that an offset between the emission lines and the
continuum has been observed, though on a larger scale, by Izotov et al. (1997)
and Vanzi et al. (2000) in SBS 0335-052, so that it must be relatively common to giant HII regions.
One possible explanation is that the ISM surrounding the cluster be
inhomogeneous and the molecular cloud be preferentially evaporated in one direction.
In addition, the emission line centroid
shows a structure at the position of the nominal center of the line, indicated
with 0 km s-1. In He 2-10 this structure is only detected in
,
possibly
because the [OIII] line does not have high enough signal-to-noise, while in NGC
5253-5 the same structure is detected in both lines. In both sources the size
of this structure is about 0.1
in size, corresponding to 1.6 and 4.3 pc, and it
has a broadness 70 km s-1 and 100 km s-1 in velocity in He 2-10 and NGC 5253-5, respectively.
These structures are difficult to interpret. Since they are relatively compact and are
characterized by high velocities, they seem produced by local episodes of winds or by turbolence
more than by the global structure of the regions observed. In fact, if we interpret these features
as due to rotation, we can derive the mass from the size and velocity measured, simply as M=v2r/G. We obtained
and
for He 2-10 and NGC 5253-5, respectively.
We obtained high resolution spectra of three giant HII regions around YMCs hosted by blue dwarf galaxies. The main conclusions from the analysis of the data are the following.
Acknowledgements
This research made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. AS acknowledges the support of ESO under two studentships funded by the Director General Discretionary Funds. We thank the anonymous referee, who contributed very valuable comments that improved this paper, and Giovanni Cresci and Jorge Melnick for reading the manuscript and for useful discussions.