Spectrophotometry of the polar ring galaxy
IIZw71
Enrique Pérez-Montero1, Rubén Garcı́a-Benito1, and Ángeles I. Dı́az1
Departamento de Fı́sica Teórica de la Universidad Autónoma de Madrid
enrique.perez@uam.es, ruben.benito@uam.es, angeles.diaz@uam.es
Summary. In this contribution we summarize the preliminary results of the longslit spectroscopy of the ring of IIZw71. The spectral coverage and resolution used to
study the knots of star formation in this object allow the simultaneous determination
of the reddening, the metallicity and the star formation properties in each of the
individual knots. Besides, we compare our results with those obtained from the
various narrow and broad band photometry in the literature.
Polar ring galaxies (PRG) are made-up of two distinct systems : the host
galaxy and the polar ring. The host galaxy is a normal galaxy that gathers
stars in a rotating disk. It is surrounded by a rotating ring, that contains stars
and interstellar gas clouds that have made these objects to be catalogued
as Blue Compact Dwarf Galaxies (BCD). PRGs are thought to form as a
consequence of their interaction with another close companion. IIZw71 is a
PRG catalogued as BCD whose companion is IIZw70, situated at 23 kpc
(Cox et al., 2001). In fact, the connection between these two galaxies is well
documented with HI maps: massive HI streamer or gaseous bridge connecting
the dwarf galaxies (Balkowski et al. 1978). The observations of this object
in narrow filter at Hα wavelength show the existence of several knots of star
formation along the polar ring (Gil de Paz et al., 2003, Figure 1, at left). Our
aim is to study these bright knots of IIZw71 and derive their properties.
1 Observations and reduction
The long-slit spectrophotometric observations of IIZw71 were obtained using
the instrument ISIS of the WHT telescope in La Palma (Spain) during 1 night
in July, 8th 2005. Using the R600B grating with the EEV12 camera in the
blue arm and the R300R grating with the Marconi2 camera we had a spectral
coverage from 3500 Å to 1 micron with a spectral resolution of 0.45 Å per
pixel for the blue arm and 0.86 Å per pixel for the red arm with a 1 arcsec
2
Enrique Pérez-Montero, Rubén Garcı́a-Benito, and Ángeles I. Dı́az
width slit. The spectra for each knot were reduced using standard procedures
with the software package IRAF.
Unfortunately, there is not a perfect alignment of the position of the slit
with the direction of the polar axis of the galaxy. The spatial profile of the
slit along with the Hα fluxes measured in the spectra, once flux calibrated,
lead to the probable distribution plotted in left pannel of Figure 1. We have
analyzed the spectra in four knots of star formation, labelled from A to D but
we have only the certainty of a correct position of the aperture of the slit in
the knots C and D, while in A and B is slightly displaced from the centre of
the knot in the image of Hα. In right pannel of Figure 1 we show the blue
spectra for each of the observed knots.
1e-15
IIZw71 - Knot A
5e-16
0
-2 -1
Flux (erg cm s )
IIZw71 - Knot B
1e-15
0
IIZw71 - Knot C
1e-15
0
1e-15
IIZw71 - Knot D
5e-16
0
3800
4000
4200
4400
4600
4800
5000
Wavelength (Å)
Fig. 1. At left, Hα photometry of the PRG IIZw71 with the probable position of
the slit for each of the knots of star formation, labelled from A to D. At right, blue
spectra once reduced and flux calibrated for each of these knots.
2 Analysis of the spectra
The reddening coefficient (C(Hβ)) has been calculated assuming the galactic
extinction law of Miller & Mathews (1972) with Rv =3.2. C(Hβ) was obtained
in each case by performing a least square fit to the ratio between F (λ) and
F (Hβ) to the theoretical values computed by Storey & Hummer (1995). These
coefficients depend on electron temperature and density, so we have used an
iterative method to estimate them. We have taken electron density to be
equal to n([Sii]) in the four knots and electron temperature to be equal to
T([Oiii]) in knots B and C, with a good measurement of the auroral line of
λ [Oiii] 4363 Å, while in the other two knots we have assumed the mean
Spectrophotometry of the polar ring galaxy IIZw71
3
electron temperature obtained in B and C. The presence of an underlying
stellar population is evident in knots B, C and D, implying absorption features
that depress Balmer emission lines and do not allow the measurement of these
lines with an acceptable accuracy (Dı́az, 1988). In order to quantify the effect
of the underlying absorption on the measured emission line intensities, we have
performed a multi-gaussian fit to the absorption and emission components
seen in this galaxy. The difference between the measurements of the absorption
subtracted lines and the ones obtained with the use of the pseudocontinuum
is, for all Balmer lines, within the observational errors.
In Figure 2, we represent the fit to the quotients of the detected Balmer
lines (Hα, Hβ, Hγ and Hδ) and Hβ in relation to the theoretical expected
values and the extinction curve. The slope of this fit acts as an estimator of
the constant of reddening. The Galactic extinction for this object is negligible,
so we can be sure that internal reddening is the unique source of extinction.
Since we can see, only the knots well covered by the slit lead to values of
C(Hβ) clearly larger than 0. In fact, the knot A, the worst covered, gives a
negative value of the constant, but compatible with zero.
0,05
0,05
-log [(Iλ/IHβ)/((Iλ/IHβ)0]
0,00
0,00
-0,05
-0,10
A
-0,3 -0,2 -0,1 0,0
-0,05
0,1
0,2
0,10
B
-0,3 -0,2 -0,1 0,0
0,1
0,2
0,1
0,2
0,20
C
0,10
0,00
D
0,00
-0,10
-0,10
-0,3 -0,2 -0,1 0,0
0,1
0,2
-0,20
-0,3 -0,2 -0,1 0,0
f(λ) - f(Hβ)
Fig. 2. Comparison between observed and theoretical quotients of the intensities of
the Balmer lines and the extinction curve. The slope of the least squares fit gives
the constant of reddening for each of the observed knots.
As well as Hα photometry, we can compare as well photometric information in UBVR (Cairós et al., 2001), in the slit and the whole bright Hα knots.
Since we can see in Figure 3 the knots not properly covered by the slit (A and
B) show redder colours than the whole region.
The program CHORIZOS (A chi-square code for parameterized modelling
and characterization of photometry and spectroscopy; Maı́z-Apellániz, 2004)
4
Enrique Pérez-Montero, Rubén Garcı́a-Benito, and Ángeles I. Dı́az
Fig. 3. Comparison between the colour index B-R as measured in each knot in the
whole Hα knot and in the region covered by the slit in the spectroscopic observations.
allows to give an estimation of reddening, metallicity and mean age of stellar
population that better fits the measured colour indexes. In this case, we have
considered for the metallicity the lowest available value, which corresponds to
0.4 times the solar value, and that is closer to the values measured in the two
brightest knots. The best chi-square fits obtained from the values measured in
the same positions covered by the slit lead to values with higher uncertainties
than those measured in the whole star forming region. Besides, as we can see
in left pannel of Figure 4, only the knots well covered by the slit lead to values
of colour excess compatible with the reddening constants obtained from the
decrement of Balmer as measured from the spectroscopy.
Regarding the mean age of the stellar population, as we can see in right
pannel of Figure 4, the values found by the model in each knot are compatible
with the values which could be derived from the equivalent width of Hβ, which
is an estimator of the age of the ionising population.
3 Analisys of the chemical abundances
We have measured the strong nebular emission lines in the four knots and,
besides, [Oiii] 4363 in knots B and C, allowing the determination of the electron temperature T([Oiii]) and oxygen chemical abundance. Uncertainties in
the derivation of oxygen abundance are high, due mainly to the assumption
of electron temperatures in the inner zones of the nebula other than O2+ . In
these cases we have resorted to the relationship between T([Oiii]) and T([Oii])
from Pérez-Montero & Dı́az (2003) that takes explicitly into account the dependence of T([Oii]) on electron density, derived from the quotient of [SII]
lines. Electron temperatures and oxygen abundances for these two knots are
summarized in Table 1. Total oxygen abundances for these knots are plotted
in dashed-dotted lines showing the uncertainty in the four pannels of Figure
5. Knot C appears to have lower metallicity than B. This could be caused to
Spectrophotometry of the polar ring galaxy IIZw71
5
Fig. 4. Results from the best fits from CHORIZOS. At left, comparison between the
colour excesses obtained from the decrement of Balmer and those predicted by the
models. At right, the mean age of the stellar population in the whole region and the
regions covered by the slit. In parenthesis, in angstroms, the measured equivalent
width of Hβ.
its older stellar population (higher rates of ejection of metals into IGM) or to
a better slit coverage (effect of higher electron temperatures and hence, lower
abundances).
Table 1. Calculated electron temperatures and oxygen abundances.
Knot
A
B
n([Sii] (cm−3 )
T([Oiii] (K)
12+log(O+ /H+ )
12+log(O2+ /H+ )
12+log(O/H)
<120
—
—
—
—
<200
12300 ± 2200
7.84 ± 0.28
7.64 ± 0.25
8.05 ± 0.28
C
D
<210
<500
13800 ± 2100
7.62 ± 0.22 —
7.56 ± 0.19 —
7.89 ± 0.21 —
In order to shed some light in this point, we have to use empirical calibrators to compare with the values obtained in these and the other weaker knots
(A and D). These calibrators are used when no measurement of the electron
temperature is possible and are based on the direct calibration between the
relative intensities of some bright emission lines and the abundance of some
relevant ions present in the nebula. We have derived oxygen abundances from
the O23 (e.g. McGaugh et al., 1991), N2 (e.g. Denicoló et al., 2001), S23 (e.g.
Pérez-Montero & Dı́az, 2005) and O3N2 (e.g. Pettini & Pagel, 2004) parameters. They are plotted with solid lines in Figure 5. Since we can see, aperture
effects are clear in the case of knot A, while they are not so evident in the
case of knot B. On the other hand, the S23 agrees better with the abundances
measured using the electron temperature and with the prediction of a lower
metallicity in knot C. Nevertheless, due to the large uncertainties associated
6
Enrique Pérez-Montero, Rubén Garcı́a-Benito, and Ángeles I. Dı́az
to the abundances derived from both the direct method and the empirical
calibrations it is difficult to establish any difference between the metallicities
of each knot. Only, the effects of aperture in knot A are evident and prevents
from a correct usage of empirical calibrators in this knot.
8,8
8,0
D
C
A
O23
7,6
-30
12+log(O/H)
8,8
12+log(O/H)
B
8,4
-20
A
-10
0
10
20
30
8,0
N2
7,6
-20
-30
D
C
8,4
-30
8,0
8,8
B
-10
0
10
20
spatial distribution (arsecs)
B
8,4
D
C
A
S23
7,6
12+log(O/H)
12+log(O/H)
8,8
-20
-10
A
B
0
10
30
D
C
8,4
8,0
O3N2
7,6
30
20
-30
-20
-10
0
10
20
spatial distribution (arsecs)
30
Fig. 5. Oxygen abundances in the four knots of IIZw71. In dash-dotted line using
the direct method and the electron temperature of [Oiii] in knots B and C. In solid
line, using empirical calibrators: O2 3, N2, S2 3 and O3N2.
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