Issue |
A&A
Volume 517, July 2010
|
|
---|---|---|
Article Number | A90 | |
Number of page(s) | 13 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/201014390 | |
Published online | 13 August 2010 |
Tol 2240-384 - a new low-metallicity AGN
candidate![[*]](/icons/foot_motif.png)
Y. I. Izotov1,2,4 - N. G. Guseva1,2 - K. J. Fricke1,3 - G. Stasinska4 - C. Henkel1 - P. Papaderos5,6
1 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
2 - Main Astronomical Observatory, Ukrainian National Academy of
Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
3 - Institut für Astrophysik, Göttingen Universität,
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
4 - LUTH, Observatoire de Paris, CNRS, Universite Paris Diderot, Place
Jules Janssen, 92190 Meudon, France
5 - Centro de Astrofísica da Universidade do Porto, Rua das Estrelas,
4150-762 Porto, Portugal
6 - Department of Astronomy, Oskar Klein Centre, Stockholm University,
106 91 Stockholm, Sweden
Received 9 March 2010 / Accepted 23 April 2010
Abstract
Context. Active galactic nuclei (AGNs) have
typically been discovered in massive galaxies of high metallicity.
Aims. We attempt to increase the number of AGN
candidates in low metallicity galaxies. We present VLT/UVES and
archival VLT/FORS1 spectroscopic and NTT/SUSI2 photometric observations
of the low-metallicity emission-line galaxy Tol 2240-384 and
perform a detailed study of its morphology, chemical composition, and
emission-line profiles.
Methods. The profiles of emission lines in the UVES
and FORS1 spectra are decomposed into several components with different
kinematical properties by performing multicomponent fitting with
Gaussians. We determine abundances of nitrogen, oxygen, neon, sulfur,
chlorine, argon, and iron by analyzing the fluxes of narrow components
of the emission lines using empirical methods. We verify with a
photoionisation model that the physics of the narrow-line component gas
is similar to that in common metal-poor galaxies.
Results. Image deconvolution reveals two
high-surface brightness regions in Tol 2240-384 separated by
2.4 kpc. The brightest southwestern region is surrounded by
intense ionised gas emission that strongly affects the observed B-R
colour on a spatial scale of 5 kpc. The profiles
of the strong emission lines in the UVES spectrum are asymmetric and
all these lines apart from H
and H
can be fitted by two Gaussians of
-92 km s-1
separated by
80 km s-1
implying that there are two regions of ionised gas emitting narrow
lines. The oxygen abundances in both regions are equal within the
errors and in the range
O/H = 7.83-7.89. The shapes of the H
and H
lines are more complex. In particular, the H
emission line consists of two broad components of
km s-1
and 2300 km s-1, in addition
to narrow components of two regions revealed from profiles of other
lines. This broad emission in H
and H
associated with the high-excitation, brighter southwestern H II
region of the galaxy is also present in the archival low- and
medium-resolution VLT/FORS1 spectra. The extraordinarily high
luminosity of the broad H
line of
erg s-1
cannot be accounted for by massive stars at different stages of their
evolution. The broad H
emission persists over a period of 7 years, which excludes
supernovae as a powering mechanism of this emission. This emission most
likely arises from an accretion disc around a black hole of mass
10
.
Key words: galaxies: fundamental parameters - galaxies: active - galaxies: starburst - galaxies: ISM - galaxies: abundances
1 Introduction
Active galactic nuclei (AGNs) are understood to be powered by massive
black holes at the centers of
galaxies, accreting gas from their surroundings. They are usually
found in massive, bulge-dominated galaxies and their gas metallicities
are generally high (Storchi-Bergmann
et al. 1998; Ho 2009; Hamann et al. 2002). However,
it remains unclear whether AGNs in low-metallicity low-mass galaxies do
exist. Groves et al. (2006)
and Barth et al. (2008)
searched the Sloan Digital Sky Survey (SDSS) spectroscopic galaxy
samples for low-mass Seyfert 2 galaxies. In particular, Groves et al. (2006) used a
sample of
23 000 Seyfert 2 galaxies selected by Kauffmann et al. (2003) and
found only 40
Seyfert 2 galaxies among them with masses lower than 10
.
They demonstrated, however, that the metallicities of these AGNs are
around solar or slightly subsolar. The same high metallicity range is
found in the SDSS sample of 174 low-mass broad-line AGNs of Greene & Ho (2007). On the
other hand, Izotov et al. (2007)
and Izotov & Thuan (2008)
demonstrated that broad-line AGNs with much lower metallicities
probably exist, although they occupy a region
in the diagnostic diagram differing from that of more metal-rich AGNs
and
are extremely rare. They identified
four of these galaxies, which were found to have oxygen abundances
O/H in the range
7.36-7.99 on the basis of a systematic search for extremely
metal-deficient emission-line dwarf galaxies in
the SDSS Data Release 5 (DR5) database of
675 000 spectra. The absolute magnitudes of those
four low-metallicity AGNs are typical of dwarf galaxies, their host
galaxies have a compact structure,
and their spectra resemble those of low-metallicity high-excitation
H II regions. Izotov
et al. (2007) found that there is however a striking
difference: the strong permitted emission lines, mainly the H
6563 line,
show very prominent broad components
characterised by properties unusual for dwarf galaxies:
1) their H
full widths at zero intensity (FWZI) vary from 102 to
158 Å,
corresponding to expansion velocities between 2200 and
3500 km s-1; 2) the
broad H
luminosities
are
extraordinarily large, between
and
erg s-1.
This is higher than the range 1037-1040 erg s-1
found by Izotov et al. (2007)
for the other
emission-line galaxies (ELGs) with broad-line emission. The ratio of H
flux in the broad component to that in the narrow component varies
from 0.4 to 3.4, compared to 0.01-0.20 for the other galaxies;
3) the Balmer lines exhibit a very steep decrement, which is
indicative of collisional excitation and the broad emission originating
in very dense gas
(
cm-3).
To account for the broad-line
emission in these four objects, Izotov
et al. (2007) considered various physical
mechanisms such as Wolf-Rayet (WR) stars, stellar winds from Ofp or
luminous blue variable stars, single or multiple supernova (SN)
remnants propagating in the interstellar medium, and SN bubbles. While
these mechanisms may be able to produce
to 1040 erg s-1,
they cannot generate yet higher
luminosities, which are more likely associated with SN shocks or AGNs. Izotov et al. (2007)
considered type IIn SNe because their H
luminosities are higher (
1038-1041 erg s-1)
than those of the other SN types and they decrease less rapidly. Izotov & Thuan (2008) found
no significant temporal evolution of broad H
in all four galaxies over a period of 3-7 years. Therefore,
the IIn SNe mechanism may be excluded, leaving
only the AGN mechanism capable of accounting for the high luminosity of
the
broad H
emission. However, we also have difficulty with this mechanism. In
particular, all four galaxies are present in neither the ROSAT
catalogue of the X-ray sources nor the NVSS catalogue of radio sources.
High-ionisation emission lines such as He II
4686 or [Ne V]
3426 are
weak or not detected in optical spectra. Based on the observational
evidence, Izotov & Thuan (2008)
concluded that all four
studied galaxies most likely belong to the very rare type of
low-metallicity AGNs in which non-thermal ionising radiation is
strongly diluted by the radiation of a young massive stellar
population.
The fifth galaxy of this type, Tol 2240-384, was
first spectroscopically studied by Terlevich
et al. (1991) and Masegosa
et al. (1994). However, in those low-resolution
spectra, some important emission lines, such as [O II]
3727,
[Ne III]
3868, and H
6563, are
missing. This, in particular, precludes abundance determination and the
detection of broad hydrogen emission. Kehrig et al. (2006,2004) studied
Tol 2240-384 spectroscopically, and Kehrig
et al. (2006) derived the oxygen abundance of this
galaxy,
O/H =
.
We note that no broad emission was
reported by Terlevich et al.
(1991), Masegosa et al.
(1994), and Kehrig
et al. (2006).
In this paper, we present 8.2 m Very Large
Telescope (VLT) spectroscopic observations
and 3.5 ESO New Technology Telescope (NTT) photometric
observations
of this emission-line galaxy. Its optical spectrum shows the very broad
components of hydrogen emission lines and is similar to those
found previously by Izotov et al.
(2007) and Izotov &
Thuan (2008) for the four other galaxies. We describe
observations in Sect. 2. The morphology of the galaxy is
discussed
in Sect. 3 and its location in the emission-line diagnostic
diagram is
discussed in Sect. 4. Element abundances are derived in
Sect. 5.
The kinematics of the ionised gas from narrow emission lines is
discussed
in Sect. 6. We discuss in Sect. 7 the properties of
the broad emission and derive the mass of the central black hole
assuming an AGN mechanism for the
origin of the broad line emission. Our conclusions are summarized in
Sect. 8.
![]() |
Figure 1:
Flux-calibrated VLT/UVES spectrum of Tol 2240-384, obtained on
23 August 2009, corrected for the redshift of z
= 0.07595 [ESO program 383.B-0271(A)] (upper spectrum).
The lower spectrum is the upper spectrum downscaled by a factor of 100.
The scale of the ordinate is that for the upper spectrum.
Note the broad emission in the hydrogen line H |
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2 Observations
2.1 Spectroscopy
A new optical spectrum of Tol 2240-384 was obtained using the
8.2 m Very Large Telescope (VLT) on 2009 August 23
[ESO program 383.B-0271(A)].
The observations were performed using the UVES echelle spectrograph
mounted at the UT2. We used the gratings CD#1 with the central
wavelength 3460 Å, CD#2 with the central wavelength
4370 Å, CD#3 with the central wavelength 5800 Å, and
CD#4 with the central wavelength 8600 Å.
The slits were used with lengths of 8
and 12
for the blue (CD#1 and CD#2) and red (CD#3 and CD#4) parts of the
spectra,
respectively, and with a width of 3
.
The angular scale along the slit was 0
246
and 0
182
for the blue
and red arms, respectively. The above instrumental set-up resulted in a
spectral range
3000-10 200 Å
over 131 orders and a resolving power
/
of
80 000.
The total exposure time was 2960 s for gratings CD#1 and CD#3,
and 2970 s for gratings CD#2 and CD#4, divided into 2 equal
subexposures.
Observations were performed at airmass
1.2 with gratings CD#1 and
CD#3 and
1.5
with gratings CD#2 and CD#4. The seeing was
2
.
The Kitt Peak IRS spectroscopic standard star Feige 110 was
observed for flux calibration. Spectra of thorium (Th) comparison arcs
were obtained for wavelength calibration.
We supplemented the UVES observations with ESO archival data of Tol 2240-384 (ESO program 69.C-0203(A)). These observations were obtained on 12 September, 2002 with the FORS1 spectrograph mounted at the UT3 of the 8.2 m ESO VLT. The observing conditions were photometric throughout the night.
Two sets of spectra were obtained. Low-resolution spectra were
obtained with a grism 300 V (3850-7500)
and a blocking filter GG 375.
The grisms 600B (
3560-5970)
and 600R (
5330-7480)
for the blue and red wavelength ranges were used in the
medium-resolution observations. To avoid second-order contamination,
the red part of the spectrum was obtained with the blocking filter
GG 435.
A long (418
)
slit with a width of 0
51
was used.
The spatial scale along the slit was 0
2
pixel-1and the resolving power
/
in the low-resolution mode and
/
and 1160
in a medium-resolution mode for the 600B and 600R grisms, respectively.
The spectra were obtained at airmass
1.2-1.4.
The seeing was
1
2
during the low-resolution observations,
1
during the medium-resolution observations in the blue range, and 1
5
during the medium-resolution observations in the red range.
The total integration time for the low-resolution observations was
360 s
(
s). The longer
exposures were taken for the medium-resolution observations and
consisted of 2160 s (
s) and
1800 s (
s) for the blue and
red parts, respectively.
The two-dimensional spectra were bias subtracted and
flat-field corrected using IRAF.
We then used the IRAF
software routines IDENTIFY, REIDENTIFY, FITCOORD, and TRANSFORM to
perform wavelength
calibration and correct for distortion and tilt for each frame. Night
sky subtraction was performed using the routine BACKGROUND. The level
of
night sky emission was determined from the closest regions to the
galaxy that are free of galaxian stellar and nebular line emission, as
well as of emission from foreground and background sources.
The one-dimensional spectra were then extracted from the
two-dimensional frame using the APALL routine. We adopted extraction
apertures of 3
4
for the UVES spectrum and 0
51
4
for FORS1 spectra. Before extraction, the two distinct two-dimensional
UVES spectra, the three distinct two-dimensional low-resolution FORS1
spectra, and the three distinct two-dimensional medium-resolution FORS1
spectra
were carefully aligned with the routine ROTATE using the spatial
locations of the brightest parts in
each spectrum, so that the spectra were extracted at the same positions
in all
subexposures. We then summed the individual spectra from each
subexposure after manual removal of the cosmic ray hits.
The resulting UVES spectrum of Tol 2240-384 is shown in
Fig. 1.
A strong broad H
emission line is present in the spectrum, very
similar to the one seen in the spectra of the four low-metallicity AGN
candidates discussed by Izotov
et al. (2007) and Izotov
& Thuan (2008). The broad component in the H
emission line is much weaker,
suggesting a steep Balmer decrement and hence that the broad emission
originates in a very dense gas.
The extracted medium-resolution and low-resolution spectra of
Tol 2240-384
are shown in Figs. 2a,b,
respectively. As in
the UVES spectrum, a broad H
emission line is detected. The signal-to-noise ratio of the FORS1
spectra is higher than that of the UVES spectrum because of the lower
spectral resolution of the former. This allows us to detect broad
components of the H
and H
emission lines.
As in the UVES data, the broad H
emission line in the
FORS1 spectra is much weaker than the broad H
emission line.
![]() |
Figure 2:
Flux-calibrated and redshift-corrected archival VLT/FORS1
medium-resolution ( left) and low-resolution (
right) spectra of Tol 2240-384 obtained on 12
September 2002 [ESO program 69.C-0203(A)] (upper spectra). The
lower spectra are the upper spectra downscaled by a factor
of 100. The scale of the ordinate is that for the upper
spectra. Note the strong broad emission in the hydrogen line H |
Open with DEXTER |
![]() |
Figure 3:
Left: deconvolved R image
of Tol 2240-384 revealing two high-surface brightness regions
separated by 1
|
Open with DEXTER |
2.2 Photometry
To gain additional insight into the morphological and photometric
properties of Tol 2240-384, we study archival [ESO program
71.B-0509(A)]
images for this system in the Bessel filters U (
s),
B (
s), and R
(
s). These data were
taken
with the SUSI2 camera (0
161 pixel-1)
attached to the 3.5m ESO NTT in seeing conditions of
1
in U and B, and 0
9
in R. Image reduction and analysis was carried out
using MIDAS and additional routines developed by ourselves. From the
available calibration exposures, it was not possible to establish
photometric zero points with an accuracy better than
0.2 mag.
In the following, we therefore restrict ourselves to discussing the
morphology and the relative colour distribution of
Tol 2240-384. From the approximate apparent B
band magnitude of 16 mag that we obtained for this system, and assuming
a distance of 310 Mpc from the NASA/IPAC Extragalactic
Database (NED) (corrected for Virgocentric infall and based on H0
= 73 km s-1 Mpc-1),
we estimate its absolute magnitude to be
-21 B mag.
This is 2-4 mag brighter than the absolute SDSS g
magnitudes
of four low-metallicity AGNs studied by Izotov
& Thuan (2008).
3 Morphology of Tol 2240-384
The morphology of Tol 2240-384, as inferred from the combined R
band
exposure, is illustrated with the contours in Fig. 3 (left). It can be
seen that the galaxy is unresolved and shows merely a slight NE-SW
elongation on a projected scale of kpc. On the same
panel, we display the R band image after Lucy
deconvolution (Lucy 1974), which
contains two main high-surface brightness regions, separated by 1
6
(2.4 kpc). The southwestern region (labelled A)
coincides with the surface brightness maximum of the galaxy and is
about ten times more luminous than the northeastern region B.
This result was checked and confirmed using
a flux-conserving unsharp masking technique (Papaderos
et al. 1998).
In Fig. 3
(right), we show the uncalibrated B-R
map
of Tol 2240-384 with the overlaid contours depicting the
morphology of the
deconvolved R image. The colour map
reveals a strong colour contrast of nearly 0.8 mag between the
SW and NE half of the galaxy with a relatively sharp transition between
these two regions at the periphery of knot A.
Knots A and B are located respectively within the red (-0.1 mag)
(shown by purple, blue, and green colours in Fig. 3 (right)) and blue (
-0.1 mag)
(shown by white and red colours in Fig. 3 (right))
halves of Tol 2240-384. They do not, however, spatially
coincide with the
locations where the extremal colour indices are observed. More
specifically, the reddest and bluest features on the colour map are
offset by between 0
5
and 0
9
from regions A and B. The extended, almost uniformly
red colour pattern in the SW half of Tol 2240-384
is indicative of intense ionised gas emission surrounding the brightest
region A on spatial scales of
5 kpc. Strong H
emission, with an equivalent width of 1300 Å in the UVES
spectrum, registered within the R band
transmission curve, can readily shift optical colours by more than
0.5 mag. We note that extreme contamination of optical
colours by extended and intense nebular line emission several kpc away
from young stellar clusters has been observed in several low-mass
starburst galaxies (e.g., Papaderos
et al. 1998; Izotov et al. 1997; Papaderos
et al. 2002). As we discuss below, intense H
emission, including a strong broad component, is associated with
region A. In contrast, no appreciable ionised gas emission is present
in
the NE part of the galaxy, indicating that the blue B-Rcolour
in region B and its surroundings is mainly due to stellar
emission.
In Fig. 4,
we show the surface brightness profiles (SBPs) of Tol 2240-384
in U (squares), B (dots) and R
(open circles). The SBPs were
computed with the method iv in Papaderos
et al. (2002) and shifted vertically to an equal
central surface brightness. The point spread function (PSF) in the B,
derived from two well-exposed
nearby stars in the field is included for comparison.
We note that the slight bump in the SBPs at a photometric radius
6
reflects the luminosity contribution of the
fainter knot B. In agreement with the evidence from the deconvolved R band
image (Fig. 3),
the surface photometry does not support the
existence of a bulge component in Tol 2240-384. It can be seen
that all SBPs have an exponential slope in their outer parts.
This component, reflecting the emission from the host galaxy of
Tol 2240-384, contains approximately 50% of the total
luminosity
of the galaxy.
The effective radius of 1.2 kpc determined for Tol 2240-384 is
a factor of between 2 and 3 larger than typical
values for blue compact dwarf (BCD) galaxies (cf. Papaderos
et al. 2006). This is also the case for the
exponential scale length
kpc, derived from a linear fit to the B band SBP
for
1
8.
![]() |
Figure 4:
Surface brightness profiles (SBPs) of Tol 2240-384 in U,
B, and R computed with
method iv in Papaderos et al.
(2002).
The point spread function (PSF) in the B band,
derived from two well-exposed nearby stars in the field of view, is
shown with the gray curve. The straight line shows a linear fit to the
host galaxy of Tol 2240-384 for radii |
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4 Location in the emission-line diagnostic diagram
Figure 5
shows the position of Tol 2240-384 (represented by a star) in
the classical [O III] 5007/H
vs. [N II]
6583/H
diagram (Baldwin et al. 1981,
hereafter BPT), in addition to other objects shown for comparison. The
four objects from Izotov & Thuan
(2008) are represented by filled circles and lie in the same
region. The asterisks represent the broad-line AGNs with low black hole
masses from Greene & Ho (2007).
The two low-luminosity broad-line AGNs NGC 4395 and Pox 52 (Barth et al.
2004; Kraemer
et al. 1999) are represented by an open circle and
an open square, respectively. For reference, the cyan dots represent
all the galaxies from the SDSS DR7 with flux errors smaller than 10%
for each of the four emission lines, H
,
[O III]
5007, H
and
[N II]
6583. The emission line fluxes
were measured using the technique developed by Tremonti
et al. (2004) and were taken from the SDSS website
. These galaxies are
distributed into two wings, the left one interpreted as star-forming
galaxies and the right one containing AGN hosts. The dashed line
represents the empirical divisory line between star-forming galaxies
and AGNs drawn by Kauffmann
et al. (2003), while the continuous line represents
the upper limit for pure star-forming galaxies from Stasinska
et al. (2006). One can see that Tol 2240-384, as
well as the four objects from Izotov
& Thuan (2008) lie in the low metallicity part of
what is usually considered as the region of star-forming galaxies.
However, it has been shown by Stasinska
et al. (2006) that an AGN hosted by a low
metallicity galaxy would be difficult to distinguish in this diagram,
even if the active nucleus contributed significantly to the emission
lines (which is not the case in Tol 2240-384, as discussed
later in this paper). The active galaxies from the Greene
& Ho (2007) sample occupy a different zone in the BPT
diagram, closer to the ``AGN wing'', probably
because their metallicities are higher than that of
Tol 2240-384 and the four galaxies from Izotov
& Thuan (2008), but lower than those of the bulk of
AGN
hosts.
![]() |
Figure 5:
The Baldwin-Phillips-Terlevich (BPT) diagram (Baldwin
et al. 1981) for narrow emission lines. Plotted are
the |
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![]() |
Figure 6:
Decomposition of strong emission line profiles into two Gaussian
components in the UVES spectrum of Tol 2240-384 for:
a) [O II]
|
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5 Element abundances
5.1 Empirical analysis
We derived element abundances from the narrow emission-line fluxes,
using a classical empirical method.
Thanks to the high spectral resolution, the narrow emission lines in
the UVES spectrum are resolved and found to have an
asymmetric shape, implying the presence of two kinematically distinct
emission-line regions in the SW part of the galaxy
(Fig. 6).
On the other hand,
these lines are not resolved in the FORS1 spectra due to insufficient
spectral resolution and have therefore the full widths at half maximum (FWHM)
corresponding to the instrumental ones.
Therefore, in the case of the
UVES spectrum, the narrow emission lines were deblended and element
abundances were derived from the emission line fluxes for every
emitting region.
To improve the accuracy of the abundance determination, we also derived
element abundances for the total emission-line fluxes, including both
blueshifted and redshifted components of the emission lines.
The fluxes in all spectra were measured using Gaussian fitting with the
IRAF SPLOT routine. They were corrected for both extinction, using the
reddening curve
of Whitford (1958), and
underlying
hydrogen stellar absorption, derived simultaneously by an iterative
procedure described by Izotov
et al. (1994) and using the observed decrements of
the narrow hydrogen Balmer lines. The extinction coefficient C(H)
and equivalent width of hydrogen absorption lines EW(abs) are derived
in such a way to obtain the closest agreement between the
extinction-corrected and theoretical recombination hydrogen
emission-line fluxes normalised to the H
flux. It is assumed that EW(abs) is the same
for all hydrogen lines. This assumption is justified by the
evolutionary stellar population synthesis models of González
Delgado et al. (2005).
The extinction-corrected total fluxes 100I(
)/I(H
)
of the narrow lines from the UVES spectrum as well as fluxes of the
blueshifted
and redshifted components, and the extinction coefficient
C(H
),
the equivalent width of the H
emission line EW(H
),
the H
observed flux F(H
), and the equivalent width of
the underlying hydrogen absorption lines EW(abs) are given in
Table 1.
We note that total fluxes of hydrogen emission lines corrected for
extinction and underlying hydrogen absorption (Col. 2 in
Table 1)
are very close to the theoretical recombination values of Hummer & Storey (1987)
suggesting that the extinction
coefficient C(H
)
is reliably derived. We obtained EW(abs)
of
0.2 Å,
which is much smaller than the equivalent widths of hydrogen emission
lines,
implying that the effect of underlying absorption on the emission line
fluxes is very small,
2 percent
for H9
3835
and much lower
for stronger lines.
In Table 1,
we also show the emission-line fluxes and other parameters for the
medium- and low-resolution FORS1 spectra. We note that the extinction
coefficient C(H
)
(Table 1)
derived from the FORS1 spectra is significantly higher than that
derived from the UVES spectrum. This difference is probably caused by
the FORS1 spectra being obtained at the relatively high airmass of
1.2-1.4 with
the narrow 0
51
slit. Therefore,
these spectra are affected by atmospheric refraction. This effect
is seen by comparing the continuum slopes of the UVES and FORS1
spectra (Figs. 1
and 2),
respectively. The continuum in the UVES spectrum
is blue, while it is reddish in the FORS1 spectra. This effect is
somewhat larger for the medium-resolution spectrum.
We conclude that the FORS1 data are somewhat uncertain for the analysis
of physical conditions and the abundance determination.
Table 1: Extinction-corrected narrow emission-line fluxes.
Table 2: Physical conditions and element abundances.
The physical conditions, and the ionic and total heavy element
abundances in the H II regions of
Tol 2240-384 were derived following Izotov
et al. (2006) (Table 2). In particular for
the O2+, Ne2+, and Ar3+,
we adopt
the temperature (O
III) directly derived from the [O III]
4363/(
4959 +
5007)
emission-line ratio. The electron temperatures
(O II)
and
(S III)
were derived from the empirical relations by
Izotov et al. (2006).
We used
(O
II) for the calculation of
O+, N+, S+,
and Fe2+ abundances and
(S III)
for the calculation of S2+, Cl2+,
and Ar2+ abundances.
The electron number densities
(O II)
and
(S
II) were obtained from the [O II]
3726/
3729 and
[S II]
6717/
6731 emission-line ratios,
respectively.
The low-density limit holds for the H II
regions
that exhibit the narrow line components considered here. The element
abundances then do not depend sensitively on
.
The electron temperatures
(O III),
(O
II), and
(S III),
electron number densities
(O II)
and
(S
II), the ionisation correction factors (ICFs),
and
the ionic and total O, N, Ne, S, Cl, Ar, and Fe abundances derived from
the
forbidden emission lines are given in Table 2. It can be seen that
the element abundances derived for the blueshifted and redshifted
components, and from the total fluxes in the UVES spectrum are very
similar.
They are also consistent with the element abundances derived from the
low-resolution FORS1 spectrum. However, the element abundances derived
from
the medium-resolution FORS1 spectrum are somewhat different. This is
apparently due to the larger effect of the atmospheric refraction in
the FORS1 medium-resolution spectrum resulting in a lower electron
temperature
(O
III) and thus higher oxygen abundance.
For the oxygen abundance, we adopt the value
O/H =
.
This
value is consistent within the errors with the value of
obtained by Kehrig et al. (2006).
However, for its absolute B magnitude of
-21 mag, Tol 2240-384 is
(
O/H)
0.7 dex below the
oxygen abundance derived from the metallicity-luminosity relation for
ELGs
by Guseva et al. (2009)
(their Fig. 9).
This deviation is most likely an indication of the extreme current star
formation in Tol 2240-384. This is similar to the lower-metallicity BCD
SBS 0335-052E with extreme star formation, which for its absolute
magnitude of
-17 mag
is also by
(
O/H)
0.7 dex below the value from
the relation by Guseva et al.
(2009). The oxygen abundance in Tol 2240-384
is within the range of the oxygen abundances obtained by Izotov et al. (2007) and
Izotov & Thuan (2008)
for the four low-metallicity AGN candidates.
The abundance ratios N/O, Ne/O, S/O, Cl/O, Ar/O, and Fe/O obtained for
Tol 2240-384 from the UVES spectrum and
for the four galaxies agree well.
5.2 Photoionisation model
We computed a photoionisation model of the narrow-line region to see
whether the derived oxygen abundance is compatible with the observed
temperature for a bona fide H II
region. No information is available about the morphology of the nebular
gas, so the model from this point of view is poorly constrained. For
the ionising source, we adopt the radiation from a starburst model
computed
with STARBURST99 (Leitherer
et al. 1999; Smith et al. 2002) at
appropriate metallicity and adopt an age of
1 Myr (the results would not be fundamentally different for
another age). The luminosity is adjusted to reproduce the observed H
flux. The corresponding total mass of the burst is
,
so the effects of statistical fluctuations to represent the ionising
radiation field are completely negligible. We used the photoionisation
code PHOTO (Stasinska 2005), and
varied the elemental abundances and density distribution as free
parameters. As already obvious from previous work (Stasinska
& Izotov 2003), the He II
4686 line in
many H II galaxies can only be
explained by an additional ionising source. Whether this is a
population of binary stars, hot white dwarfs, or something else is
unclear at the moment.
As in Stasinska & Izotov (2003),
we simply mimicked this additional X-ray component by bremstrahlung at
106 K with the luminosity needed to
explain the luminosity of the He II
4686 line.
This additional component has no detectable effect on the other lines.
A uniform density or constant pressure model did not allow us to fit
all the constraints satisfactorily, and we had to resort to a
two-density model, with an inner zone of density 10 cm-3,
and an outer thick shell of density 200 cm-3.
With this geometry, we were able to find a model, model M1, that
reproduces the observed line ratios satisfactorily. In Table 3, the line ratios (in
units
(H
))
predicted by this model are compared to the observed narrow ones in the
total UVES spectrum (Table 1).
The chemical composition of model M1
is given in Table 4
and compared to both the one derived from the empirical method and the
solar value. Since we were able to reproduce the observed [O III]
4363/
5007 ratio,
the abundances of N, O, and Ne are of course similar in the
two approaches (note that we did not reproduce the [N II]
5755/
6584 ratio,
but it was not used either in the empirical approach as it relies on an
extremely weak line). The abundances of S, Ar, and Fe are less
certain because the ionisation structure of these elements is not very
well known.
We note that we had to adopt a far lower C/O ratio, than in the Sun (Asplund et al. 2009) or in
low-metallicity emission-line galaxies (e.g., Izotov & Thuan 1999; Garnett et al.
1997) to diminish the cooling and match the observed
[O III]
4363/
5007 ratio. This procedure is
often used when no ultraviolet data are available to directly constrain
the carbon abundance, but the carbon abundance obtained in this way may
not be correct. Models with extra heating or more complex morphology
and a different C/O would be equally valid from a photoionisation point
of view. In summary, the photoionisation analysis faces problems that
are similar to those of many bona fide low
metallicity H II regions, but
does not indicate any additional problem. In terms of its abundance
pattern, Tol 2240-384 is well within the trends exhibited
in general by metal-poor galaxies (Izotov
et al. 2006). We note only
that iron has not been depleted much, compared to H II
galaxies of similar metallicities. Photoionisation models for
Tol 2240-384 are discussed further in Sect. 6.
6 Kinematics of the ionised gas from the narrow emission lines
We show in Table 5
the parameters of the blueshifted and redshifted narrow components of
the strongest emission lines in the UVES spectrum.
The two components are separated by 78 km s-1that
varies only slightly from one emission line to another. On the other
hand, the FWHMs of different lines differ somewhat.
We note that the strong forbidden nebular emission lines [O III]
4959,
5007,
[O II]
3726,
3729, and [Ne III]
3868
have FWHMs in the range 73-80 km s-1
and are narrower than the
permitted hydrogen and helium lines. The FWHMs of
the blueshifted and redshifted components of the weaker auroral
[O III]
4363 emission line are more
uncertain. The FWHMs of hydrogen lines are larger,
at
85 km s-1.
The largest FWHM of
92 km s-1
is found for the He I
5876
emission line.
How can we reconcile the FWHM differences
of the forbidden and permitted lines?
Forbidden and permitted lines probe different parts of the
emitting regions (e.g., Filippenko 1985).
It is probable that the detected emission of hydrogen and helium lines
includes a significant fraction from dense parts of
emitting regions with number densities above the critical density
of 105-107 cm-3
for forbidden nebular lines. At these
densities, the Balmer decrement is steeper than the pure recombination
value because of
the contribution of the collisional excitation. The denser regions
appear to be
characterised by a higher velocity dispersion. If this were the case
then
we would expect to measure smaller FWHMs from H
to H
because of
the decreasing fraction of emission caused by collisional excitation.
The inspection of Table 5
shows that this is indeed the case. The widths
of the blueshited and redshifted He I
5876
emission lines are even larger than that of the narrow hydrogen lines
due to the significant contribution of collisional excitation from the
populated high-lying metastable level 23S.
Similar evidence of narrow components in the permitted emission lines
has been
found in some Seyfert 1 galaxies (e.g., Filippenko &
Halpern 1984; Mullaney
& Ward 2008).
Table 3: Comparison of photoionisation models with observations.
Table 4: Comparison of abundances in Tol 2240-384 with the solar values.
Table 5: Narrow components of strong emission lines in the UVES spectrum.
7 Broad emission
Using Gaussian fitting, we reassembled the H
and H
emission lines, including broad components,
in the UVES spectrum, and the H
,
H
,
and H
emission lines
in the medium-resolution FORS1 spectrum. Results of the fitting for the
H
emission line in the UVES spectrum are shown in Fig. 7 and the parameters
of the broad components for the H
,
H
,
and H
emission lines are shown in Table 6. It can be seen from
this Table that
the broad emission of H
and H
lines could be fitted by two Gaussians. We note that the fitting is
more uncertain for the H
emission line because
of its significantly lower flux compared to that of the H
emission
line, especially in the UVES spectrum. The broad emission of the H
emission line in the FORS1 spectrum is yet weaker than that
of the H
emission line and could be fitted using only a single Gaussian.
In addition, this emission is contaminated by the [O III]
4363
emission line, making the flux of the broad H
component more uncertain. We also note that the fluxes of the emission
lines are lower in the FORS1 spectrum presumably due to the smaller
aperture. The FWHMs of the broad H
emission line in the UVES and FORS1 spectra are in fair agreement,
which is indicative of rapid gas motion with velocities of
2000 km s-1.
![]() |
Figure 7:
Decomposition of the H |
Open with DEXTER |
The observed H-to-H
flux ratios of
10
for the broadest
components in both the UVES and FORS1 spectra are significantly higher
than the recombination value of
3 expected for the low-density
ionised gas (Table 6).
This large ratio may in part be caused by dust extinction. However, the
correction for extinction with C(H
) = 0.28
and 0.83, respectively,
derived from the decrement of the narrow Balmer hydrogen lines in the
UVES and FORS1 spectra (Table 1)
implies an H
-to-H
flux ratio of
6.
We were unable to derive the dust
extinction in the region with broad hydrogen emission. However, we
could argue
that this extinction is not significantly higher than that in the
region of
the narrow line emission. Otherwise, the extinction-corrected broad
H
-to-H
flux ratio would imply a relatively low ionised gas
density and thus the presence of a broad component in the strongest
forbidden
emission line, [O III]
5007.
However, this broad [O III]
emission is not seen implying an electron number density
106 cm-3,
comparable to or higher than the critical electron number density for
the
[O III]
4959, 5007 emission lines. The
broad component
is probably present in the [O III]
4363
emission line of the medium-resolution FORS1 spectrum. The critical
density for this auroral
line is
108 cm-3.
The line may therefore originate in the dense regions, while the
nebular [O III]
4959, 5007
emission lines do not. However, the [O III]
4363
emission line is much weaker and is too
close to the stronger H
4340
emission line to draw more
definite conclusions about the presence of its broad component.
In Fig. 8,
we show
the CLOUDY model predictions of the theoretical H-to-H
flux
ratio as a function of number density for three values of the
ionisation
parameter
,
-2, and -3, respectively. The higher ionisation
parameter corresponds to stronger gas heating. For a fixed oxygen
abundance, this corresponds to a higher ionised-gas temperature.
In the CLOUDY modelling, we chose
O/H = 7.6.
We also adopted a power-law distribution
for the ionising radiation with
and an upper cutoff
of
corresponding to the photon energy of 10 Ryd.
The H-to-H
flux ratio at low
in Fig. 8
is constant and corresponds to the recombination value. With increasing
,
the
contribution of the collisional excitation becomes important resulting
in an increase in the H
-to-H
flux ratio, and the effect is stronger for the models with higher
where high flux ratios are achieved at lower
electron number densities because of the higher electron temperatures.
To correct the observed broad emission for extinction, we
adopt for the extinction coefficients C(H)
the respective values obtained from the observed decrement of the
narrow Balmer hydrogen lines (Table 1). Thus, C(H
)
is equal to 0.28 and 0.83 for the UVES and FORS1 spectra, respectively.
We indicate in Fig. 8
by dash-dotted and dashed horizontal lines the extinction-corrected
broad H
-to-H
flux ratios of 6.44 and 5.86 for the UVES and FORS1 spectra,
respectively. These values are significantly higher than the
recombination ones and imply a high density of the region with broad
emission. In particular, we obtain from the modelled H
-to-H
flux ratios with
the range of the electron number densities between the two dotted
vertical lines in Fig. 8
of
-
cm-3
to account for the observed
ratios.
At a distance D = 310 Mpc to
Tol 2240-384, we obtain the extinction-corrected H
luminosity
(H
)
= 3
1041 erg s-1
from
the UVES data. This high luminosity can probably only be explained by
the broad emission originating in a type IIn SN or an AGN, as discussed
by Izotov et al. (2007)
and Izotov & Thuan (2008).
However, broad emission was present over a period of
7 years
as demonstrated by the FORS1 observations in 2002 and the UVES
observations in 2009. This rules out the hypothesis that the broad line
fluxes are caused by type IIn SN because their H
fluxes
should have decreased significantly over this time interval.
![]() |
Figure 8:
The dependence of the theoretical H |
Open with DEXTER |
Table 6: Broad components of strong emission lines.
There remains the AGN scenario. Tol 2240-384 was
detected
in neither the NVSS radio catalogue nor the ROSAT catalogue, suggesting
that it is a faint radio and X-ray source,
similar to the objects discussed by Izotov
& Thuan (2008). What about its optical spectra? Can
accretion discs around
black holes in these low-metallicity dwarf galaxies account for their
spectral properties? The spectrum of Tol 2240-384, which is
similar to those of the four objects discussed by Izotov
& Thuan (2008), does not show
clear evidence of an intense source of hard
nonthermal radiation: the [Ne V] 3426,
[O II]
3727, He II
4686, [O I]
6300,
[N II]
6583, and [S II]
6717,
6731 emission lines, which are usually found in the spectra of AGNs,
are weak or not detected. However, if, as argued above, the density of
the broad line region were
-
cm-3,
the forbidden lines would be very weak or suppressed, except perhaps
for [O III]
4363. The
flux of the broad He II
4686 line
depends on both the spectral energy distribution of the non-thermal
radiation and the ionisation parameter, but it is not expected to be
higher than 20% of the H
line flux as seen in
Stasinska (1984). A broad feature
with such a low flux would be undetected
in our spectra. Some radiation from the central engine may escape to
large distances and give rise to narrow lines. This possibility is
roughly accounted for by the X-ray radiation included in
model M 1 to explain the observed He II
4686 flux.
Model M2 provides another solution, where the radiation field added to
the stellar radiation is more typical of an active nucleus. We chose
the broken power-law distribution of Kraemer
& Crenshaw (2000) and adjusted the luminosity of the
radiation leaking out of the broad line region to reproduce the
observed He II
4686 flux. Under this
condition, the fraction
of the H
emission produced by the power-law ionising radiation is
2% of that
produced by the stellar ionising radiation.
The radiation field being different from that
in model M1, some small adjustments are needed to the density
distribution to reproduce the observed [O III]
5007/[O
II]
3727 flux ratio, as well as to
the abundances. Since this radiation field is more efficient at heating
the gas, it is no longer necessary to reduce the carbon abundance to
reduce the amount of cooling. In this model, the temperature in the low
excitation zone is lower, therefore a slightly higher oxygen abundance
is needed to reproduce the fluxes of the oxygen lines. As can be seen
in Table 4,
the C/O abundance ratio in this model is much closer to both the solar
value
and the value expected for the Tol 2240-384 metallicity
(Izotov &
Thuan 1999; Garnett
et al. 1997), thus is far more satisfactory. This
model has however some drawbacks with respect to model M1, the
major one being that it predicts a [Ne V]
3426 flux of
5% of H
,
which should have been noted in the observed spectrum. To improve on
photoionisation modelling by including a proper treatment of the broad
line zone, more complete observational constraints would be useful.
Assuming that an AGN mechanism is responsible for the broad
hydrogen
emission, we now estimate the mass of the central black hole. Greene & Ho (2007) derived the
following relation between the central black hole mass and broad H
emission line characteristics, using the
Greene & Ho (2005)
relation between the AGN continuum and H
luminosity
and the Bentz et al. (2006)
relation between the AGN radius and continuum luminosity
where







8 Conclusions
We have presented 8.2 m Very Large Telescope (VLT) observations with the UVES and the FORS1 spectrographs, and 3.5m ESO New Technology Telescope (NTT) U,B,R imaging of the low-metallicity emission-line galaxy Tol 2240-384. We have studied the morphology of Tol 2240-384, the kinematics of the ionised gas, the element abundances, and the broad hydrogen emission in this galaxy. We have arrived at the following conclusions:
- 1.
- Image deconvolution reveals two high-surface brightness
regions in Tol 2240-384 separated by 2.4 kpc and
differing in their luminosity by a factor of
10. The brightest southwestern region is surrounded by intense ionised gas emission, which strongly affects the observed B-R colour on a spatial scale of
5 kpc. This high-excitation H II region is associated with broad H
and H
emission. Surface photometry does not indicate, in agreement with the results of image deconvolution and unsharp masking, the presence of a bulge in Tol 2240-384.
- 2.
- We derived the oxygen abundance
O/H = 7.85
0.01 in Tol 2240-384, which is consistent within the errors with the value of 7.77
0.08 derived earlier by Kehrig et al. (2006).
- 3.
- The emission line profiles in the high resolution UVES
spectrum reveal
the presence of two narrow components with a radial velocity difference
of
78 km s-1. Furthermore, the full widths at half maximum (FWHMs) of the narrow lines differ. Strong forbidden nebular lines [O III]
4959, 5007, [O II]
3726, 3729, and [Ne III]
3868 have FWHMs of 73-80 km s-1. The FWHMs of hydrogen lines are larger,
85 km s-1 and decrease from H
to H
emission lines. The largest FWHM of
92 km s-1 is found for the He I
5876 emission line. This data suggest that narrow permitted hydrogen and helium lines probe the denser inner parts of the emitting regions compared to the forbidden lines.
- 4.
- Both UVES and FORS1 spectra reveal the presence of very
broad hydrogen
lines with FWHMs greater than
2000 km s-1. The steep Balmer
decrement of the broad hydrogen lines and the very high luminosity of
the broad H
line
erg s-1suggest that the broad emission arises from very dense and high luminosity regions such as those associated with supernovae of type IIn or with accretion discs around black holes. However, the presence of the broad H
emission over a period of 7 years rules out the SN mechanism. Thus, the emission of broad hydrogen lines in Tol 2240-384 is most likely associated with an accretion disc around a black hole.
- 5.
- There is no obvious spectroscopic evidence of a source of
non-thermal hard ionising radiation in Tol 2240-384.
However, none is expected if, as we argue, the density of the broad
line region is
-
cm-3.
- 6.
- Assuming that the broad emission in Tol 2240-384 is powered
by an AGN, we have estimated a mass for the central black hole of
10
.
Y.I.I., N.G.G. and K.J.F. are grateful to the staff of the Max Planck Institute for Radioastronomy for their warm hospitality and acknowledge support through DFG grant No. FR 325/58-1. P.P. has been supported by a Ciencia 2008 contract, funded by FCT/MCTES (Portugal) and POPH/FSE (EC), and by the Wenner-Gren Foundation. 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. The Sloan Digital Sky Survey (SDSS) is a joint project of The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, Princeton University, the United States Naval Observatory, and the University of Washington. Funding for the project has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.
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Footnotes
- ... candidate
- Based on observations collected at the European Southern Observatory, Chile, ESO program 69.C-0203(A), 71.B-0509(A) and 383.B-0271(A).
- ... IRAF
- IRAF is the Image Reduction and Analysis Facility distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation (NSF).
- ...
website
- http://www.sdss.org/DR7/products/value_added/index.html
All Tables
Table 1: Extinction-corrected narrow emission-line fluxes.
Table 2: Physical conditions and element abundances.
Table 3: Comparison of photoionisation models with observations.
Table 4: Comparison of abundances in Tol 2240-384 with the solar values.
Table 5: Narrow components of strong emission lines in the UVES spectrum.
Table 6: Broad components of strong emission lines.
All Figures
![]() |
Figure 1:
Flux-calibrated VLT/UVES spectrum of Tol 2240-384, obtained on
23 August 2009, corrected for the redshift of z
= 0.07595 [ESO program 383.B-0271(A)] (upper spectrum).
The lower spectrum is the upper spectrum downscaled by a factor of 100.
The scale of the ordinate is that for the upper spectrum.
Note the broad emission in the hydrogen line H |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Flux-calibrated and redshift-corrected archival VLT/FORS1
medium-resolution ( left) and low-resolution (
right) spectra of Tol 2240-384 obtained on 12
September 2002 [ESO program 69.C-0203(A)] (upper spectra). The
lower spectra are the upper spectra downscaled by a factor
of 100. The scale of the ordinate is that for the upper
spectra. Note the strong broad emission in the hydrogen line H |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Left: deconvolved R image
of Tol 2240-384 revealing two high-surface brightness regions
separated by 1
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Surface brightness profiles (SBPs) of Tol 2240-384 in U,
B, and R computed with
method iv in Papaderos et al.
(2002).
The point spread function (PSF) in the B band,
derived from two well-exposed nearby stars in the field of view, is
shown with the gray curve. The straight line shows a linear fit to the
host galaxy of Tol 2240-384 for radii |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
The Baldwin-Phillips-Terlevich (BPT) diagram (Baldwin
et al. 1981) for narrow emission lines. Plotted are
the |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Decomposition of strong emission line profiles into two Gaussian
components in the UVES spectrum of Tol 2240-384 for:
a) [O II]
|
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Decomposition of the H |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
The dependence of the theoretical H |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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