Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A82 | |
Number of page(s) | 10 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201014864 | |
Published online | 06 October 2010 |
Integral-field near-infrared spectroscopy of two blue dwarf galaxies: NGC 5253 and He 2-10![[*]](/icons/foot_motif.png)
G. Cresci1,2 - L. Vanzi3 - M. Sauvage4 - G. Santangelo5,6,7,1 - P. van der Werf8
1 - Osservatorio astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
2 -
Max-Planck-Institut fuer Extraterrestrische Physik, Postfach 1312, Garching 85741, Germany
3 -
Department of Electrical Engineering, Pontificia Universidad Catolica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
4 -
Laboratoire AIM, CEA/Irfu, CNRS/INSU - Université Paris Diderot, CEA-Saclay, 91191 Gif sur-Yvette-Cedex, France
5 -
European Southern Observatory, Karl Schwarzschild str.2, 85748 Garching bei Muenchen, Germany
6 -
Universitá di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
7 -
INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
8 -
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
Received 26 April 2010 / Accepted 8 July 2010
Abstract
We present integral field spectroscopy in the near infrared (NIR)
of He 2-10 and NGC 5253, two well known nearby dwarf irregular
galaxies showing high star-formation rates. Our data provide an
unprecedented detailed view of the interstellar medium and star
formation in these galaxies, allowing us to obtain spatially resolved
information from the NIR emission and absorption line tracers. We study
the spatial distribution and kinematics of different components of the
interstellar medium (ISM) mostly through the Bracket series lines, the
molecular hydrogen spectrum, [FeII] emission, and CO absorptions.
Although the ISM is mostly photo-excited, as derived by the [FeII]/Br
and
line ratios, some regions corresponding to non-thermal radio sources show a [FeII]/Br
excess due to a significant contribution of SN driven shocks. In
He 2-10 we find that the molecular gas clouds, as traced by
CO(2-1) and
infrared line, show consistent morphologies and velocities when studied
with the two different tracers. Moreover, there is a clear association
with the youngest super star clusters as traced by the ionized gas. In
the same galaxy we observe a cavity depleted of gas, which is
surrounded by some of the most active regions of star formation, that
we interpret as a signature of feedback-induced star formation from
older episodes of star formation. Finally, we measured high turbulence
in the ISM of both galaxies,
km s-1, driven by the high star-formation activity.
Key words: galaxies: dwarf - galaxies: individual: He 2-10 - galaxies: individual: NGC 5253 - galaxies: ISM - galaxies: starburst
1 Introduction
Detailed observations of intense star formation and mass assembly
are a key ingredient in probing the role of various mechanisms in
shaping the galaxies and moderating their evolution. Starburst galaxies
are complex systems, where gas is converted into stars on a relatively
short timescale with great efficiency: the theoretical progress in this
area is still partly dependent on highly simplified recipes for the
physical mechanisms that drive the formation of new stars and their
interplay with the interstellar medium (ISM). Building a complete
theoretical picture is important to fully understand the mechanism
responsible for the formation and evolution of star-forming galaxies at
high redshift (), where the bulk of star formation in the Universe took place (e.g. Rudnick et al. 2006).
Important progress has been made in recent years by direct observations
of distant galaxies, but at those distances even the largest sources
are just a few arcsec in size (e.g. Cresci et al. 2006), preventing us from distinguishing the different mechanisms at work. Although perfect analogs of high-z
galaxies are not available locally owing to the different conditions in
the ISM, gas content, accretion rate, and chemical properties, studying
star-forming galaxies at a much higher physical resolution is critical
to investigate some key properties that are virtually inaccessible at
high redshift. Nearby starburst systems have already helped in the past
decades to understand some of the processes that regulate and trigger
star formation on galactic scales, e.g. the interaction with close
companions or large molecular clouds, and internal mechanisms such as
feedback from previous generations of massive stars. Blue compact dwarf
galaxies are particularly suited for this task, as they show typically
sub-solar abundances, thus providing the opportunity to observe star
formation in a relatively pristine environment and with less
contamination by the underlying evolved stellar population than in
larger galaxies.
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Figure 1:
K-band continuum images of He 2-10 (left panel) and NGC 5253 (right panel), extracted from the SINFONI datacube. The Br |
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The population of young super massive stellar clusters (SSCs) that
accounts for a large fraction of the star formation of the entire
galaxy was identified in starburst systems both locally and at high-z (e.g. de Grijs et al. 2003; Elmegreen et al. 2005).
These clusters could be the progenitors of the old globular clusters
observed in most galaxies today (de Grijs & Parmentier 2007), thus providing further evidence for the link between the starburst phenomenon and the formation of galaxies.
Moreover, star-forming regions with properties consistent with the more massive HII regions detected locally are observed up to
(Swinbank et al. 2009),
and local dwarf galaxies have been identified as scaled down analogs of
clumpy star-forming galaxies at high redshift (Elmegreen
et al. 2009). Dwarf
galaxies indeed show high gas fraction, high turbulence, a large Jeans
length compared to their size and a clumpy appearance, similar to what
is observed in high-z forming disks (see e.g. Genzel et al. 2006, 2008; Cresci et al. 2009; Forster-Schreiber et al. 2009).
In this picture, the clumpy morphology comes from gravitational
instabilities in gas with high turbulent speed compared to the
rotational one, and high gas mass fraction compared to stars. Although
the mechanism providing the gas in the local and high-z galaxies
might be different (mergers and interactions are required locally,
while cosmological accretion through cold flows might be dominant at
high redshift, see e.g. Dekel et al. 2009), the physical process regulating the star formation in clusters are probably the same in the two environments.
In the effort of understanding the physical processes at play in galactic-scale star formation, we thus undertook a survey campaign of nearby dwarf starburst galaxies using integral field spectroscopy in the near infrared (NIR). This technique provides an unprecedented detailed view of the interstellar medium and star formation in these galaxies, allowing us to obtain spatially resolved information from the emission and absorption line tracers in the NIR. The results derived from the study of the nearby starburst galaxy II Zw 40 were presented in Vanzi et al. (2008), while in this paper we discuss the observations obtained on He 2-10 and NGC 5253, two very well known examples of nearby starbursts. Both galaxies are dwarf-irregular, with estimated distances of 9 Mpc for He 2-10 (Vacca & Conti 1992) and 3.3 Mpc for NGC 5253 (Gibson et al. 2000): due to their relatively close distances and remarkable properties they have been studied in detail in the past.
1.1 He 2-10
He 2-10 has nearly solar metalicity (
,
Kobulnicky et al. 1999a)
and hosts an intense episode of star formation as revealed by classical
indicators such as the strong optical and infrared emission lines
(Vacca & Conti 1992; Vanzi & Rieke 1997), the Wolf-Rayet features (Vacca & Conti 1992), intense mid-IR (Sauvage et al. 1997) and far-IR continuum (Johansson 1987). Some compact radio sources were observed by Johnson & Kobulnicky (2003), who characterized their spectra as thermal, measured their sizes as 2-4 pc and their masses in excess of
.
A number of young super massive clusters in the star-forming region of the galaxy, with typical masses up to
and ages of few Myr have been found in optical and UV studies (e.g. Johnson et al. 2000; Vacca & Conti 1992).
However, these observations reveal clusters that are relatively
unobscured and are probably not the youngest star-forming regions.
Cabanac et al. (2005,
hereafter C05) observed in the IR, to penetrate the obscuring material,
and detected several compact sources that could be associated with the
radio sources previously mentioned, finding that most of them are
ultra-compact HII regions and two are possible SN remnants (see
Fig. 1).
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Figure 2:
Near-infrared H and K spectrum extracted with a circular aperture of
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Kobulnicky et al. (1995) derived a total molecular gas mass of
and an atomic gas mass of
through single-dish observations of CO(1-0) and CO(2-1) and VLA HI
21 cm. Thanks to the interferometric resolution, they detected a
CO plume extended about
to the southeast, and a similar but more extended feature in HI. They
interpreted these features as a sign of a merger with a companion
galaxy. This interpretation is supported by the detection of an
extended component of CO(3-2) molecular gas to the northeast of the
main body of the galaxy by Vanzi et al. (2009), interpreted as additional evidence of a detached cloud expelled in the merger that He 2-10 is experiencing.
1.2 NGC 5253
NGC 5253 is an irregular dwarf galaxy in the Centaurus Group and
another interesting target to study young starbursts. It is indeed one
of the closest starburst galaxies, with a heliocentric distance of only
3.3 Mpc (Gibson et al. 2000), so that
corresponds to 16 parsec, and one of the youngest starburst galaxies known (Rieke et al. 1988), as implied by the detection of spectral features arising from Wolf-Rayet stars (Schaerer et al. 1997)
and from its almost entirely thermal radio spectrum with very little
synchrotron emission from supernova remnants (Beck et al. 1996). Its metalicity is sub-solar and about
(Kobulnicky et al. 1999).
At centimetric wavelengths, Turner et al. (1998) and Turner et al. (2000) have observed several nebulae, which they derive to be ionized by
200-1000 O stars, suggesting that very large clusters are the preferred
mode of star formation in the central regions of the galaxy. Calzetti
et al. (1997)
found that they span a range of masses and ages, and identified six
main clusters, one of which powers a giant HII region that is the most
prominent object in the galaxy (Vanzi & Sauvage 2004). Cresci et al. (2005)
detected more than 300 clusters combining high-resolution NIR and
optical imaging, with derived ages spanning a range between 2 and
50 Myr and masses of up to
.
The paper is organized as follows: Sect. 2 provides the details of the observations, in Sect. 3 we describe in detail the results, in Sect. 4 we discuss a possible origin of the cavity detected in the ISM of He 2-10, while in Sect. 5 we summarize our results.
2 Observations and data reduction
The observations were obtained with the integral field spectrograph SINFONI at the ESO-VLT (Eisenhauer et al 2003, Bonnet et al. 2004) as part of the ESO program 075.B-0648 ( He2-10 and NGC5253) and 074.A-9011 ( He2-10).
We used the
pixel scale, which provides a field of view of
.
Due to the large apparent sizes of these two galaxies, we were able to
cover only part of the optical extent of the targets with SINFONI. For
He 2-10 our pointing is centered on the bright central nucleus,
generally referred to as region A in the literature, which is an arc of
UV-bright super star clusters that dominate the emission of the galaxy
(Vacca & Conti 1992).
In NGC 5253 our observations were centered at the brightest HII
region, i.e. cluster 5 according to the notation of Calzetti
et al. (1997).
For each galaxy we obtained two data cubes using the H and K gratings, providing one spectral resolution R= 3000 and 4000 respectively. He 2-10 was observed on 2005 March 24 and 2005 April 3 for a total integration time on-source of 20' in both H and K band. NGC 5253 was observed on 2005 April 3, with an integration time of 10' in both bands.
The data were reduced with the SINFONI custom reduction package SPRED (Abuter et al. 2006), which includes all of the typical reduction steps applied to near-IR spectra with the additional routines necessary to reconstruct the data cube. After background subtraction, which was performed with off sky frames obtained interspersed with the on-source exposures, the data were flat-fielded and corrected for dead/hot pixels. Telluric correction and flux calibration were carried out using A- and B-type stars, after removing the stellar features. Residuals from the OH line emission were minimized with the methods outlined in Davies (2007).
A crude estimate of the seeing limited spatial resolution achieved was
obtained with the standard star observations. These show a FWHM
for both galaxies, corresponding to a linear resolution of 26.2 and 9.6
pc at the distances assumed for He 2-10 and NGC 5253
respectively.
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Figure 3:
Br |
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Figure 4:
Br |
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3 Analysis
In Figs. 3 and 4 we show the line maps, extracted as explained in Sect. 2, in Br
2.167
m, [FeII] 1.643
m,
2.121
m rest wavelength for both galaxies. The contours of the K-band
continuum from the SINFONI data-cubes are drawn for comparison. Below
we will mainly use these tracers to study the properties and dynamics
of the different phases of the ISM in the two galaxies: ionized gas
through recombination lines of H and other species like Fe, warm
molecular gas through
H 2, and dust extinction using ratios of well-modeled lines.
3.1 Ionized hydrogen
The ionized gas distribution, which we can best trace in our data through the Br
emission, allows us to identify the main regions of star formation in
the two galaxies. We refer to the first panel of Figs. 3 and 4 for He 2-10 and NGC 5253 respectively.
In He 2-10 three main regions are prominent in Br.
The central region, where the emission peaked, is also bright in H
,
which coincides with the region called L4 by C05. This area is bright
and extended in the radio continuum and is identified as knot 4 by
Kobulnicky & Johnson (1999, hereafter KJ99).
A second bright Br
region is seen to the southeast of the previous one. It is not bright in H
,
but it is detected in the I, K, and N band continuum (C05), probably due to higher extinction (see Sect. 3.4). It coincides with source L5 of C05 and with the radio knot 5 of KJ99.
Finally, a region to the northwest of the central one is also detected in H
as a diffuse nebula, bright in L and 10
m (knot L1 in C05) and coincident with the radio sources knot 1 and 2 of KJ99 (see Fig. 1).
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Figure 5:
Map of the [FeII]/Br |
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The K continuum detected in SINFONI and shown in Figs. 1 and 3 in contours is identical to the K image obtained by C05. The comparison of the Br
morphology and the continuum shows a clear difference, with the maximum of the K continuum located
to the NW of the Br
one, and a plume of three clusters bright in continuum but undetected in Br
.
These bright K sources at the center of the galaxy (L4a, L3a,b and L6
according to C05) are actually older clusters of more evolved stars
(see Sect. 3.5).
NGC 5253 offers a significantly different view. It was already noted by Vanzi & Sauvage (2004) and Cresci et al. (2005)
that the radiation emitted by this galaxy is dominated at all
wavelengths by one single compact source, the cluster identified with
number 5 by Calzetti et al. (1997) at the center of our pointing. This source is a super massive cluster with an age of just a few Myr and a mass of
,
which also dominates the emission in our Br
map (see Figs. 1 and 4). The Br
source is coincident with the K continuum pick. We also detect the source identified as 5a by Vanzi & Sauvage (2004) to the NE of source 5 and cluster 4 of Calzetti et al. (1997).
All the other main clusters fall outside our field of view. Yet a
number of fainter sources are detected, mostly in our continuum image
without a counterpart in the ionized gas map, and an extended diffuse
component in the emission line.
3.2 Ionized iron
The forbidden lines of [FeII] are emitted in partially ionized
regions in thin shells at the edge of HII regions, or in more extended
regions produced by shocks (Mouri et al. 2000).
Therefore these lines have been used as tracers of young supernova
remnants (SNR) in starburst galaxies and AGN (Moorwood & Oliva 1988; Oliva et al. 1990; Colina 1993; Vanzi & Rieke 1997; Alonso-Herrero et al. 2003).
However, the observation of these systems with high enough angular
resolution, as well as the observation of star-forming regions in the
Galaxy, allowed to establish that it is not rare to detect purely
photo-excited lines in objects still dominated by young massive stars
(e.g. Vanzi et al. 2008). The ratio [FeII](
)/Br
can be used to distinguish thermal excitation in SN shocks from
photoionization: in the first case the ratio is typically higher than a
few tens, while in the second one it is significantly lower than 1,
with typical values of 0.1 or less (Alonso-Herrero et al. 1997).
In He 2-10 the bulk of the [FeII] emission is almost coincident with the Br
one, mainly corresponding to the radio knots 4 and 1.2 of KJ99. However, the [FeII] peak is located
to the southwest of the Br
one, and the emission significantly extends toward the west, reaching
the radio source knot 3 of KJ99. A fainter source of [FeII] is also
visible beyond the north edge of the radio source 4. At the position of
knots 1-2 the iron is faint, while at the location of knot 5 only a
diffuse component can be seen. The region with much fainter Br
emission around knots 1, 3, and 4 (see Sect. 3.1) is evident also in [FeII].
In NGC 5253 the [FeII] line is brightest at the location of cluster 5. It is interesting that we detect a second bright source
to the northwest of cluster 5, and another one
to the east, which have no obvious counterpart in the continuum or in Br
.
To better quantify the nature of these [FeII] sources, we used the ratio [FeII]/Br
as a diagnostic of the excitation mechanism. To minimize the effect of
extinction and band shift we used the observed ratio
[FeII](1.644)/Br12(1.641), and rescaled the Br12 flux to the
corresponding Br
assuming an intrinsic ratio Br12/Br
(Hummer & Storey 1987, for a case B recombination with N=104 cm-3 and
K). The maps of the obtained ratio are shown in Fig. 5.
The highest values are in the range between 1 and 7, therefore
consistent with a significant contribution by SNe in the regions with
bright [FeII] emission. The data also support the suggestion of Johnson
& Kobulnicky (2003)
that knot 3 in He 2-10 is a non-thermal radio source. Indeed, this
region (shown in the radio 3.6 cm continuum contours from Johnson
& Kobulnicky 2003, in Fig. 5) is coincident with one of the regions with high [FeII]/Br
ratio in our data. Moreover, C05 claim that the more extended radio
knot 4 is a complex mix of normal HII regions and SNR, because it shows
a non-thermal signature with a slightly negative spectral index
.
Indeed we find in the northern region of this radio knot the other
region with a significantly enhanced ratio, which confirms their
analysis.
At the location of the radio knot 4 and 1.2 in He 2-10 and cluster
5 in NGC 5253 the [FeII]/Br
ratio is lower, with values in the range
0.1-0.3, indicating regions dominated by photoionization by young massive stars.
That in both galaxies the ratio FeII/Br
does not reach the maximum values observed in pure SN remnants seems to
indicate that in all cases we do not observe pure shock-excited [FeII]
lines, but possibly an additional contribution from the underlying
stellar population. Despite this caveat, assuming that all the iron
emission with [FeII]/Br
is dominated by the emission by SNR, we can estimate a SN rate for the two galaxies with the following relation:
where
![$L_{[{\rm FeII}]}({\rm tot})$](/articles/aa/full_html/2010/12/aa14864-10/img38.png)

![$L_{\rm [FeII]}(SNR)$](/articles/aa/full_html/2010/12/aa14864-10/img39.png)



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(2) | ||
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(3) |
We compared these values with the expected number of SNe as derived from the far-infrared (FIR) luminosity observed in these galaxies, according to the relation between FIR and SN rate calibrated by Mannucci et al. (2003). With this method, we obtain a SN rate of


3.3 Molecular hydrogen
The H2 line at 2.212 m rest wavelength and additional fainter lines in the H and K
bands allow us to trace the location and physical status of the warm
molecular gas in the two galaxies. The molecular hydrogen lines in the
NIR are actually mainly excited through fluorescence, i.e. absorption
of UV photons, or thermally through collisions. The two different
mechanisms mostly excite different ro-vibrational levels, so that it is
possible to distinguish between the two processes by comparing the
ratios of some of the lines with models (e.g. Black &
van Dishoeck 1987). The
2-1/1-0 S(1) ratio is however disgnostic in the low-density limit, as
it always thermalizes at high density (see Sternberg & Dalgarno 1989).
Table 1:
Comparison of the observed ratios of H2 lines to H2(1-0) S1 at 2.12
in four different regions of both galaxies (see text) with fluorescent
and thermal excitation models by Black & van Dishoeck (1987).
As shown in Fig. 3, in He 2-10 we detect three main regions of H2 emission at 2.212 m.
All of them are extended, and appear to be diffuse with filamentary
components. They are generally not clearly associated to any feature
observed in the continuum or in other lines, except for the cloud
corresponding to the radio knots 1 and 2, where Br
and continuum emission is also detected at corresponding location. In particular, the K band nucleus does not show a clear counterpart in H2, and the H2 filaments extends beyond the bright Br
regions, similar to what was observed by Vanzi et al. (2008) in II Zw 40.
However, if we compare our H2 observations with the study of the molecular gas in this galaxy as traced by the CO(J=2-1) transition observed by Santangelo et al. (2009) in the millimeter at the Submillimeter Array (SMA) telescope, we find a strong spatial correlation between the CO and H2 clouds. Although their data cover a larger field of view with lower angular resolution (
), the correlation between the positions of the clouds in the different species is clearly visible in Fig. 6, with all the detected CO clouds showing a counterpart in the H2.
The correspondence in spatial distribution is not completely
systematic, because the E-W structure to the SE of L1 and the peak just
east of L4 do not have an obvious counterpart in the CO map. However, a
good agreement is found also in velocity, as shown in Sect. 3.6 and Fig. 10 (see also Henry et al. 2007), which provides further support for the interpretation that the CO and H2 we detect come from the same molecular structures. The particular shape of the H2 emission is probably because we preferentially see H2 on the surface of the clouds, where it can be heated by UV photons or hit by shock waves.
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Figure 6:
The H2(2.212
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The situation is different in NGC 5253 (see Fig. 4), where the brightest peaks of the H2
emission can be associated to clusters 5, 4, and 5a. But some
differences in the spatial extend of the emission in the ionized and
molecular hydrogen are present, because the region to the north-west of
cluster 5 which emits in Br
has no counterpart of bright H2 emission, and cluster 5 itself is elongated toward the southwest in H2 in a region with little Br
and higher dust extinction (see Sect. 3.4).
To better understand the different excitation mechanisms responsible for the H2 emission, we extracted 1D K-band spectra from different regions in the two galaxies, and compared the ratios of the detected H2 lines to the 1-0 S(1) line at 2.12
.
For He 2-10 we extracted one spectrum at the location of both the eastern and western
H2 clouds (with an aperture of
), as well as one spectrum at the emission peak at the center (
)
and one in the gas bridge corresponding to the radio knot 3 (
). For NGC 5253 we extracted one spectrum at the peak in cluster 5 (
), one in the cluster 5a to the NE (
), and two more in the extended H2 emission to the SW (
)
and W (
)
from cluster 5. The center of all regions is marked with a cross in Figs. 3 and 4. In Table 1 we compare the measured ratios with the predictions of the Black & van Dishoeck (1987) model for different excitation mechanisms.
The 2-1S lines in the K band spectrum are particularly useful to distinguish between fluorescence and thermal excitation, because they are expected to be very weak in the latter case. Most of the lines were detected in different regions of both galaxies, which strongly suggests a dominant fluorescence excitation process. The measured ratios are lower than what is expected for pure fluorescence, allowing for some thermal contribution and suggesting that dense photodissociation regions are the dominant source of H2 emission (see also Hanson et al. 2002). Although the S/N ratio for these faint lines is not optimal for a full 2D spatial analysis, we obtained maps of the most significant line ratios and verified that there is no relevant deviation from the behavior sampled by the ratios reported in Table 1. In particular, we do not detect any region where shocks alone may account for the excitation of H2, suggesting that dense photodissociation regions are the dominant source of H2 emission.
It is an interesting point that even in the regions where [FeII] is strong, the
H2 lines ratios does not support a dominant shock origin. This can possibly be understood by inspecting Fig. 5, where we see that a large fraction of the [FeII]/Br
map shows values that are not strongly characteristic of shock-excited
[FeII] emission (i.e. ratios in the range 1-2). Thus it appears that
even when [FeII] emission is strong, shock-excitation mechanisms are
never absolutely dominant, although contributing to the collected
emission. The same situation is revealed by the
H 2 lines ratios.
Considering the actual nature of the regions where we observe the
emission, i.e. galaxy centers populated by massive young clusters and
dense HII regions, it is not surprising that this mixed situation is
prevalent: there are copious amounts of UV light available as well as
strong winds from stars and supernovae sweeping the ISM. We should
expect to see both of these components contributing to the excitation
of emitting sources.
3.4 Dust
We obtained an extinction map for the two galaxies using the decrement of the observed ratio Br12/Br
with respect to the theoretical value of 0.189 calculated by Hummer & Storey (1987) for case B recombination, T=104 K and n=104 cm-3, assuming the Rieke & Lebowsky (1985) extinction law. The resulting extinction maps are shown in Fig. 7 for the two galaxies, where the contours of the K continuum are overplotted for reference.
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Figure 7:
Extinction map in He 2-10 and NGC 5253 from Br12/Br |
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In He 2-10 the extinction is ranging between
,
with higher values at the edges of the knots, corresponding to the
position of cluster L4a, and at the location of cluster L5 and L1. C05
use the data of Vanzi & Rieke 1997 to derive
from Br
/Br10, consistent with the values found here. This value is also consistent with the
of Henry et al. (2007), corresponding to
.
In NGC 5253 the extinction is higher at the location of the clusters, reaching its maximum value up to AV=12 in cluster 5. This agrees with the results of Vanzi & Sauvage (2004),
who find that the brightest optical clusters of this galaxy are all
characterized by a near-infrared excess that is explained by the
combined effect of extinction and emission by dust. They estimate
for cluster 5, but Calzetti et al. (1997) concluded that the cluster has to be embedded in a thick gas cloud with
using the H
/H
ratio, in agreement with our estimate. Moreover, the dust reddening is
less homogeneous than in He 2-10, and one of the dust lanes
crossing the galaxy (see Cresci et al. 2005) is also evident in our extinction map, south of cluster 5 in the NW-SE direction between cluster 5 and cluster 4.
3.5 Stellar features
The strongest stellar absorption features in the available spectra are the CO bands at
,
whose depth increases with decreasing stellar temperature and
increasing stellar luminosity, which is therefore most evident in
regions with red giant and supergiant stars. Following Leitherer
et al. (1999), we use the spectroscopic CO index
(Doyon et al. 1994; Puxley et al. 1997)
and the equivalent width (EW) of the band to measure the age of the
stellar population for the clusters old enough to show absorption
features in their spectra.
In He 2-10 the most prominent sources in absorption are the
cluster L4 and the arc of old clusters detected only in continuum L3/6
(see Fig. 3). We measure a
and
for the cluster and the arc, extracting the spectra in a circular aperture of 2 pixels and
pixels respectively. We also measure an EW in the rest frame wavelength range
for the two regions of 16 Å and 12 Å. Comparing these values
with the stellar population synthesis model Starburst99 (Leitherer
et al. 1999), and using the Br
detection to remove the degeneracy between young and old ages for a given
or EW, we derive an age of
8.3 Myr for the cluster L1 and an age between 14.5-40 Myr for the clusters in the arc.
These older clusters are lying at the center of the cavity between knot 1, 3 and 4 detected in all the species discussed before (see Fig. 3 and Sects. 3.1-3.3). We will discuss the properties of the cavity and its possible origin in greter detail in Sect. 4.
In NGC 5253 we detected CO bands at low S/N only for a small cluster north of cluster 5, pointing toward a very young stellar population in this galaxy.
3.6 Gas kinematics
We derived radial velocities and velocity dispersions by fitting a
function to the continuum-subtracted spectral profile of the emission
lines at each spatial position in the datacube for both galaxies. The
function fitted was a convolution of a Gaussian with one spectrally
unresolved emission line profile of a suitable sky line, i.e. a high
signal and non-blended line in the appropriate band. A minimization was
performed in which the parameters of the Gaussian were adjusted until
the convolved profile best matched the data. During the minimization,
pixels in the data that consistently deviated more than
from the average were rejected, and were not used in the analysis. The obtained Br
velocity and velocity dispersion maps, corrected for instrumental broadening, are shown in Figs. 8 and 9
for the two galaxies. The kinematics derived with the same method, but
using the other emission lines discussed, are consistent with Br
,
but with lower S/N. The errors on the radial velocities range from 5 to 20 km s-1, and areas with larger errors are not shown in the maps.
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Figure 8:
Gas kinematics from Br |
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Figure 9:
Gas kinematics from Br |
Open with DEXTER |
![]() |
Figure 10:
Comparison between the CO(J=2-1) and H2 velocity field. Right panel: the CO(J=2-1) velocity map obtained by Santangelo et al. (2009) is shown, with the H2 contours superimposed for reference. Left panel: the H2(2.212
|
Open with DEXTER |
The gas kinematics is complex in both galaxies, with measured dispersions as high as 30-80 km s-1.
The high measured line widths are probably produced by the combination
of star formation driven turbulence and outflows in the medium. A
coherent velocity pattern is also observed in the gas kinematics of the
galaxies. In particular, a receding area is detected in the
northwestern part of He 2-10, with a velocity difference of 50 km s-1 with respect to the continuum peak. This is fully consistent with the CO data of Santangelo et al. (2009, see Sect. 3.3). In Fig. 10 we compare the CO(J=2-1) velocity map (left) and the H2
one (right) on the same LSR velocity scale: the two gas components also
have a similar velocity, with the western cloud showing a difference of
35 km s-1 with respect to the rest of the galaxy.
The molecular gas kinematics both for CO and H2 is fully consistent with the one traced by the ionized gas. This supports a picture in which the HII regions move at the velocities of the molecular clouds from which they were born. A different result was obtained with similar observations by Vanzi et al. (2008) in the galaxy II Zw 40, where at least one giant molecular cloud is detected with different morphology and dynamics with respect to the HII regions.
We can derive a value for the
ratio for this galaxy using the observed velocity gradient and integrated dispersion, finding
.
This value is lower than that observed in local spiral galaxies (
), but comparable with the low
ratios observed in high-z turbulent disks (see e.g. Cresci et al. 2009; Epinat et al. 2009; Law et al. 2009). Because the Jeans length
in a gas rich system is proportional to
(Genzel et al. 2008),
this supports the idea that the clumpy morphology of both high redshift
disks and blue dwarf galaxies comes from gravitational instabilities in
gas with high turbulent speed compared to the rotation speed, and a
high mass fraction compared to the stars (see Elmegreen
et al. 2009).
In NGC 5253 a higher velocity dispersion area (
km s-1) is centered around cluster 5 (see also Vanzi et al. 2006),
probably produced by local episodes of winds or by turbulence induced
by the stronger star-formation activity in the young central cluster.
There are hints of an approaching area to the east of cluster 5, but
the S/N is too low in that area to place firm
constraints. No clear evidence of global rotation is detected in either
galaxy in the area covered by our field of view.
4 Feedback-induced star formation in He 2-10?
We have already shown in Sects. 3.1-3.3
that a cavity depleted of gas is present in He 2-10 in all the
species discussed, surrounded by some of the most actively star-forming
regions of the galaxy, as knots 1, 3, and 4 (see Fig. 3).
Interestingly, an arc of clusters with older stellar population is
laying roughly at the center of this structure, as shown in Sect. 3.5 and in Fig. 3. The [FeII]/Br
ratio, that is a tracer of shock excited gas, is measured to be higher at the edges of the cavity (see Sect. 3.2).
All these findings suggest a link between the formation of the cavity
in the gas distribution and the triggering of the star formation
activity in the surrounding regions, which is probably connected with
the shocks responsible for the [FeII] emission.
We can test this hypothesis by comparing the age of the clusters in the
surrounding area with the expected travel time of a shockwave that may
be responsible for the formation of the cavity in the gas. Given the
large uncertainties on both the ages of the stellar populations and on
the velocity of the shock, a more accurate comparison is not possible
with the available data.
Assuming that the center of the cavity is the oldest cluster in the
area L3b (age 12.8 Myr from H
Equivalent Width measured by C05), the main clusters in the region surrounding the cavity are L4a (distance
,
age
6.4 Myr), L4b (
,
5.8 Myr), and L1 (
,
5.2 Myr). It can be already seen how there is a tendency for
clusters closer to the center to be older, as expected if an expanding
shell is the trigger for their formation. If we assume a shell velocity
of
50 km s-1 (e.g. Martin 1998; Fujita et al. 2003), and a minimum time for SNe explosions in the central cluster of
6.3 Myr (Leitherer et al. 1999)
in order to create the shell through feedback, we estimate that the
shock shell reached the location of the three clusters 5.9 Myr,
5.7 Myr and 5.5 Myr ago, respectively. These estimates agree
with the measured ages of the clusters, given the large uncertainties,
showing that the same event might be responsible for the formation of
the cavity sweeping away the gas and triggering the star formation at
its edge compressing the denser material.
5 Conclusions
We obtained integral field spectroscopy in the near-infrared of two nearby starburst galaxies: He 2-10 and NGC 5253. The study of the spectral features observed provided a detailed view of the environment where the star formation is occurring. We found that
- 1.
- The ionized gas traced by Br
is dominated by a single young super massive cluster in NGC 5253, while in He 2-10 the emission is due to several regions of active star-formation.
- 2.
- Although the ISM is mostly photo-excited, as derived by the [FeII]/Br
and H2 line ratios, some regions show a [FeII]/Br
excess that is probably due to a significant contribution by SN driven shocks. The location of the [FeII] bright regions corresponds in He 2-10 to the positions of radio sources classified as non-thermal by Johnson & Kobulnicky (2003) and Cabanac et al. (2005). We estimate the SN rates from the detected [FeII] emission, finding results that are compatible with the star-formation rates as derived by Br
emission and supporting this scenario.
- 3.
- The dust extinction in NGC 5253 is higher at the location of the main young cluster 5, reaching values as high as
, shich shows that these young clusters are embedded in thick dust clouds.
- 4.
- The molecular gas clouds as traced by CO(2-1) and H2 infrared lines in He 2-10 show consistent velocity and morphology, and an association with the formation of the youngest super star cluster as traced by the ionized gas.
- 5.
- We measured high turbulence in the ISM of both galaxies,
km s-1, driven by the high star-formation activity. The clumpy morphology is related to the high gas fraction and high gas turbulence compared to the rotation speed, as in high-s disks.
- 6.
- An arc of old clusters (age
15-40 Myr) is observed at the center of a cavity depleted of gas in He 2-10, surrounded by some of the most actively and young star-forming regions of the galaxy. This suggests a link between the feedback of the clusters in the arc, the formation of the gas cavity, and the triggering of the star formation at its edges. The estimates of the shock shell velocity agree with the measured ages of the young clusters (
5-12 Myr).
P.v.d.W. would like to thank Liviu Stirbat and Pascal Baars for discussions on these galaxies during the early stages of the project. This research has 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. We received support from Basal Center for Astrophisics and Associated Technologies PFB-06. L.V. was supported by CONICYT through project Fondecyt N. 1095187. G.C. acknowledges PUC for the support received during his stay in Chile.
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Footnotes
All Tables
Table 1:
Comparison of the observed ratios of H2 lines to H2(1-0) S1 at 2.12
in four different regions of both galaxies (see text) with fluorescent
and thermal excitation models by Black & van Dishoeck (1987).
All Figures
![]() |
Figure 1:
K-band continuum images of He 2-10 (left panel) and NGC 5253 (right panel), extracted from the SINFONI datacube. The Br |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Near-infrared H and K spectrum extracted with a circular aperture of
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Br |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Br |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Map of the [FeII]/Br |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
The H2(2.212
|
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Extinction map in He 2-10 and NGC 5253 from Br12/Br |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Gas kinematics from Br |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Gas kinematics from Br |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Comparison between the CO(J=2-1) and H2 velocity field. Right panel: the CO(J=2-1) velocity map obtained by Santangelo et al. (2009) is shown, with the H2 contours superimposed for reference. Left panel: the H2(2.212
|
Open with DEXTER | |
In the text |
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