Spectroscopic observations of a sample of dwarf spiral galaxies. II- Abundance gradients
ABSTRACT The oxygen gradient of four dS galaxies has been determined using abundances for several HII regions determined with four different methods. The gradient slopes of the three non-barred galaxies in the sample are quite steep, larger than -0.2 dex/kpc, while the gradient of the barred galaxy is shallower, only -0.1 dex/kpc. Although these gradients are quite steep they are real, following all the galaxies the same trend. Moreover, the results obtained here agree with those marked by the late-type, non-dwarf spirals, particularly the relationship between the gradient and the absolute magnitude and the optical size for non-barred galaxies, and the surface density for barred ones. Comment: 26 pages, 10 figures, resubmitted to AJ on August 28th after minor referee suggestions
arXiv:1011.1013v1 [astro-ph.CO] 3 Nov 2010
Spectroscopic observations of a sample of dwarf spiral galaxies. II- Abundance
A.M. Hidalgo-G´ amez
Departamento de F´ ısica, Escuela Superior de F´ ısica y Matem´ aticas, IPN, U.P. Adolfo L´ opez
Mateos, C.P. 07738, Mexico city, Mexico
D. Ram´ ırez-Fuentes
Instituto de Astronom´ ıa, UNAM, Ciudad Universitaria, Aptdo. 70 264, C.P. 04510, Mexico City,
and J. J. G´ onzalez
Instituto de Astronom´ ıa, UNAM, Ciudad Universitaria, Aptdo. 70 264, C.P. 04510, Mexico City,
The oxygen gradient of four dS galaxies has been determined using abundances for several
Hii regions determined with four different methods. The gradient slopes of the three non-barred
galaxies in the sample are quite steep, larger than −0.2 dex/kpc, while the gradient of the barred
galaxy is shallower, only −0.1 dex/kpc. Although these gradients are quite steep they are real,
following all the galaxies the same trend. Moreover, the results obtained here agree with those
marked by the late-type, non-dwarf spirals, particularly the relationship between the gradient and
the absolute magnitude and the optical size for non-barred galaxies, and the surface density for
The existence of differences in the chemical abundances along the galactocentric distance is
a well known characteristic of spiral galaxies (see, among others, the works of Searle 1971 as well
as D´ ıaz 1989; Zaritsky et al. 1989; Walsh & Roy 1989; Vila-Costas & Edmunds 1992). Normally,
the central regions exhibit higher abundances than those located at the outskirts. Such gradients,
both nebular and stellar, have been obtained for our Galaxy (e.g. Edvardsson 2001 and references
therein), but not without controversy (Fu et al. 2009).
The gradients in metallicity are probably the results of billions of years of evolution.
the chemical elements are processed in the interior of the stars, such abundance gradients can be
related to the gas mass fraction and the star formation rate in the disc of spiral galaxies (Phillips
– 2 –
& Edmunds 1991) or to variations in the Initial Mass Function (G¨ usten & Mezger 1982). They
also could be related with the yield if the closed-box model is considered. A constant yield might
give variations in the gas fraction, and vice versa (J. V´ ılchez, 2010 private communication).
Several investigations studied the dependence of the gradient with different properties of the
galaxies, such as the morphological type, the absolute magnitude and the rotation velocity (e.g.
Zaritsky et al. 1994; Vila-Costas & Edmunds 1992). The main conclusion of these investigations
is that gradients in non-barred galaxies seem to be related to the mass surface density and to the
morphological type (late-type galaxies have steeper gradients than early-types) while the central
abundance are correlated with the galaxy mass. For barred galaxies the gradients seem to be related
to the length of the bar compared to the size of the disc and to the ellipticity (Martin & Roy 1994).
In addition, barred galaxies seem to show a flatter gradient than the non-barred ones (Pagel et
al. 1979; Alloin et al. 1981) for all morphological types. Again, this is not without controversy,
because other authors measured large gradients in barred galaxies (Martin & Roy 1994).
The relation obtained by Zaritsky et al. (1994) between the gradient and the morphological
type is quite interesting in the sense that it might indicate that Hii regions in galaxies of later
morphological types, as Sm and Im, should have important differences in metallicity. A similar
conclusion was held also by Kunth & Roy (1996), where they discussed the smoothening of the
gradients due to the shear in spiral galaxies, while irregular galaxies, because of their lack of
internal movements, might keep their local enrichments. This is contrary to observations, where
no significant variations are found in the chemical abundances among the irregular galaxies but
only nitrogen local enrichment by Wolf-Rayet stars (see Kobulnicky & Skillman 1997) and only
very few galaxies show differences in the abundances larger than 1σ (Hidalgo-G´ amez et al. 2001).
There are several reasons for the rejection of the possible existence of metallicity gradients in
irregular galaxies. The use of the semi-empirical methods in the abundance determination, the
large uncertainties in the metallicity values, the small size of the galaxies, etc, have been given for
the non acceptance of differences in the metal content throughout an irregular galaxy (e.g. Pilyugin
2001). A recent example is the Local irregular galaxy IC 10, for which Magrini & Goncalves (2009)
obtained differences in the oxygen abundance as large as 0.6 dex but they still said that ”there
is no indications of radial gradients. Therefore, there is no irregular galaxy with an accepted
gradient or variations in the oxygen abundance. On the contrary, there are several dozens of spiral
galaxies where abundance gradients have been determined, some of them as small as −0.01 dex/kpc.
The main reason argued for the acceptance of such small gradients is the large size of the spiral
galaxies, most of the time larger than 10 kpc. Therefore, although the gradient is small, it gives
an important difference between the central part and the outskirts of a certain galaxy. In addition
to this situation, there are low-mass, spiral galaxies which does not show any difference in the
abundance among the H´ii regions. The best studied one is NGC 1313 (Walsh & Roy 1997). Moll´ a
& Roy (1999) concluded that the absence of oxygen gradient in NGC 1313 is due to its low mass,
which resembles irregular galaxies. To our concern, NGC 1313 is the only late-type spiral galaxy
with no gradient, and therefore unique. If a large population of spiral galaxies with no gradient
– 3 –
are found they will be the transition between the normal late-type Sm with larger gradient, as
suggested by Zaritsky et al. (1994), and the Im galaxies with no gradients.
The main goal of the present investigation is to check if there are any other late-type, low-
mass, spiral galaxy without a metallicity gradient. The abundance gradient of four dwarf spiral (dS)
galaxies have been determined in order to check if their gradients increase with the morphological
type, as suggested by the relationship by Zaristky et al. (1994) or, on the contrary, they got
shallower approaching the distribution abundance in irregular galaxies. All the galaxies in the
sample are of low mass, both total and gas, low luminosity and small size, very similar to the
characteristics of NGC 1313. Moreover, their Hii regions are extended and their inclinations are
not excessive. none of them have previous spectroscopic studies. For more details about the galaxies
in the sample, the reader is referred to Hidalgo-G´ amez et al. (AJ, submitted; hereafter paper I).
In the next section, the determination of the chemical abundance gradients for each of the
four galaxies in the sample is presented. In Section 3, a discussion on the accuracy of the values is
given. The relationship between these gradients and other global parameters of the galaxies is also
discussed. Brief conclusions are presented in Section 4.
2.Gradient abundances in dwarf spiral galaxies
A total of 29 Hii regions in four dS galaxies were studied in a previous paper (Hidalgo-G´ amez
et al. AJ submitted; hereafter paper I). Two of the galaxies, UGC 5242 and UGC 5296, have a
small number of regions while for the other two, UGC 6205 and UGC 6377, the number of regions
are larger, of the order of 10. Although the number of Hii regions detected in these galaxies is
much smaller than in other spiral galaxies, they agree with the observation in other dS galaxies
(Hidalgo-G´ amez 2005; Reyes-P´ erez 2009). The gas mass of all of the galaxies under study is small,
about 109M⊙ (Hidalgo-G´ amez, unpublished), and similar to the gas content that the low-mass,
late-type spiral NGC 1313 (Walsh & Roy 1996). This galaxy, however is brighter but with a similar
chaotic spiral structure than those studied here.
Two observables are needed in order to determine the abundance gradient: the galactocentric
distance of each Hii and its chemical abundance. In order to obtain a proper distance from the
center of the galaxy to the Hii regions a zero point should be considered. This could be photometric
(the place with the maximum surface brightness) or dynamical (the place with the lowest rotation
velocity). In the present investigation, the first one is considered. From V and R images (obtained
by one of the authors at the 1.5m telescope of San Pedro M´ artir-OAN) a photometric center of the
galaxy was determined. To identify the Hii regions, Hα images of each galaxy were compared with
the images from the acquisition camera except for UGC 6277 for which no Hα image was available.
Due to the low number of Hii regions in this galaxies this was an easy task. Only in UGC 6377,
because the lack of the Hα image, there might be some problems i n the identification. Once the
Hii regions were identified, the Hα images were compared with the V and R ones, in order to
– 4 –
determine the photometric center in the Hα image. A distance was obtained considering the pixel
sizes of every CCD involved and the distance to the galaxy, which might be the greatest source of
uncertainties. Inclination corrections for each galaxy were applied, using the values tabulated by
NED at the 25 mag arcsec2.
The abundances of the Hii regions studied here were determined in paper I. Here, a brief
summary of the values are given but the reader is referred to paper I for details. In one Hii region,
the forbidden oxygen line at 4363˚ A is detected and, therefore the standard method, as described
in e.g. Osterbrock 1989, can be used to determine the chemical abundances. In the other Hii
regions, the semi empirical methods are needed. A total of four different semi empirical methods
are used. The results are that the abundances are in the range between 8.6 and 7.7 dex for most of
the Hii regions. For a study of the feasibility of the semi empirical methods the reader is referred
to Hidalgo-G´ amez & Ram´ ırez-Fuentes (2009).
In order to study as carefully as possible the gradients in these four galaxies, a gradient value
will be determined with each set of abundances obtained in paper I: the R23, the P, the N2, the N3
and the average abundances. In addition, a comparison among the values determined with each
method will be done. This is very interesting because it is believe the bending of the slope observed
in some galaxies, as M101 (Zaritsky 1992), is due to an artifacts in the metallicity determination
As discussed in paper I, each method has its own advantages and drawbacks: the R23 and
P used a larger number of Hii regions whereas only those Hii regions with the highest S/N are
used the N2 and N3 methods. The gradients will be obtained from a least-square fitting to all
the abundance values determined for the Hii regions for each galaxy. A more robust value of
the slope could be obtained from a bi-variate fitting: first considering the galactocentric distance
as independent and then treating the abundances as the independent variable (Elmergreen et
al. 1996). The fitted slope and constant are a combination of the values for each single fitting.
However, in this study a single square-fitting has been used because it provides a simple and quick
comparison with other investigations. In this statistical approach the inclusion of one single odd
point might lead to an important change in the slope and do not reflect the real tendencies (Vila-Co
stas & Edmunds 1992). Therefore, and in addition to this mathematical value of the gradient, we
have also considered the differences between the innermost and outermost regions (I/O in Table
1) in each galaxy as well as between the most and less metallic ones (L/M in Table 1). Finally,
a gradient can be obtained when averaging all the abundances from all the Hii regions located
in a certain range of distance from the center. Therefore, a single abundance is obtained for a
single galactocentric distance. The advantage of this last determination is that the possible odd
abundances or uncertainties in the galactocentric distance are smoothed out. We advance that the
agreement among all the values of the slopes/gradients is remarkable for most of the galaxies.
As said, two galaxies have a very small number of Hii regions detected and therefore, the
gradient values determined might be not very reliable. Indeed, for UGC 5296 and UGC 5242 the
– 5 –
total number of Hii regions is much smaller than the minimum number of data-points needed for a
reliable gradient determination according to Dutiful & Roy (1996). There are only five Hii regions
in UGC 5296 (R´ eyes-P´ erez 2009) and about eight in UGC 5242 (Hidalgo-G´ amez, in preparation).
Therefore, we are aware that the gradient values for these galaxies are less confident than for the
rest of the sample. Nevertheless, there are some other galaxies where a gradient is determined from
only 4 regions, as NGC 3351, NGC 5068 and NGC 4395 (Vila-Costas & Edmunds 1992).
UGC 6205 is the galaxy with the largest number of Hii regions detected in our sample: a total
of 11 regions were studied in this investigation. The gradient determined with the semi empirical
methods are shown in the first column of Table 1. Also, the values determined using the most and
less metallic regions, as well as the most internal and external ones are presented in rows 6 and 7,
respectively. Finally, the slope from the averaged distances is presented in row 8 (See below). If
the three latter values of the gradients are similar to those obtained with the least-squared fitting
might indicate the robustness of the determination. This is the situation for UGC 6205, with all
the values of the gradients ranging from −0.2 to −0.4 dex/kpc. An average value of −0.31 dex/kpc
with a dispersion of 0.03 will be considered.
Another remarkable results presented in Table 1 is the steepness of the gradient, being larger
than the values obtained for most of the galaxies studied so far (e.g. see Table 4 in Vila-Costas &
Edmunds 1992). Why? One might think that this galaxy shows very extreme abundance values,
but the most external region, located at 3.1 kpc, has a metallicity of 7.7 dex while the most internal
regions have metallicities of 8.7 dex. The difference is only of 1 dex, which is not so large. There
are other spiral galaxies with such difference in their abundances, or larger, as M81 (Garnett &
Shields 1987). The point here is that M81 is very much larger than UGC 6205. So, UGC 6205 has
the same difference in the abundance than other galaxies but the galactocentric distances of the
Hii are smaller. Therefore, so large gradient.
The next question to be addressed is which is the region responsible for such a steep gradi-
ent. One might think that those regions with low S/N are responsible for the slope because their
abundance value might not be so reliable. This is not true for several reasons: firstly, the gradients
obtained with the N2 and N3 methods, which included only those high S/N regions, give only
slightly shallower gradients. Moreover, the values determined without the least-squared fitting are
of the same order except when the gradient is determined from the most and less metallic regions
which is of −1.1 dex/kpc, and it will be ignored because of its doubtful meaning.
Another way to check the reliability of the steep gradient, is, as previously said, using an
average abundance for different ranges of galactocentric distances: 0-0.6 kpc, 0.6-1.2 kpc, 1.2-1.8
kpc, 1.8-2.4 kpc, 2.4-3.0 kpc, and 3.0-3.600 kpc. As said, the variations of the abundance are
smoothed and the low S/N regions will not be the responsible of the value of the gradient. These