We analyse long‐slit spectral observations of 14 Wolf–Rayet (WR) galaxies from the sample of Schaerer, Contini & Pindao. All 14 galaxies show broad WR emission in the blue region of the spectrum, consisting of a blend of N iiiλ4640, C iiiλ4650, C ivλ4658 and He iiλ4686 emission lines, which is a spectral characteristic of WN stars. Broad C ivλ5808 emission, termed the red bump, is detected in nine galaxies and C iiiλ5996 is detected in six galaxies. These emission features are due to WC stars. We derive the numbers of late WN and early WC stars from the luminosity of the blue and red bumps, respectively. The number of O stars is estimated from the luminosity of the Hβ emission line, after subtracting the contribution of WR stars. The Schaerer & Vacca models predict that the number of WR stars relative to O stars, NWR/NO, increases with metallicity. For low‐metallicity galaxies, the results agree with predictions of evolutionary synthesis models for galaxies with a burst of star formation, and indicate an initial mass function (IMF) slope −2 ≲Γ≲− 2.35 in the low‐metallicity regime. For high‐metallicity galaxies our observations suggest a Salpeter IMF (Γ=−2.35) and an extended short burst. The main possible sources of error are the adopted luminosities for single WCE and WNL stars. We also report, for the first time, on NGC 450 as a galaxy with WR characteristics. For NGC 450, we estimate the number of WN and WC stars. The number ratio NWR/NO, and the equivalent widths of the blue bump, EWλ4686, and of the red bump, EWλ5808, in NGC 450 are also in good agreement with the instantaneous burst model prediction for WR galaxies.
Interactions between galaxies are very common. There are special types of interactions that produce systems called polar ring galaxies (PRGs), composed of a lenticular, elliptical or spiral host galaxy, surrounded by a ring of stars and gas, orbiting in an approximately polar plane.In this paper, we study AM 2020-504, a PRG with an elliptical host galaxy, and a narrow and well-defined ring, probably formed by accretion of material from a donor galaxy, collected by the host galaxy. Our observational study is based on BVRI broad-band imagery as well as long-slit spectroscopy in the wavelength range 4100-8600 Å, performed at the 1.6-m telescope at the Observatório do Pico dos Dias, Brazil. We have estimated a redshift of z = 0.01683, corresponding to a heliocentric radial velocity of 5045 ± 23 km s −1 . The (B − R) colour map shows that the ring is bluer than the host galaxy, which indicates that the ring is a younger structure. Standard diagnostic diagrams have been used to classify the main ionizing source of selected emission-line regions (nucleus, host galaxy and ring). We find that the ring regions are mainly ionized by massive stars, while the nucleus presents the characteristics of an active galactic nucleus. Using two empirical methods, we have found oxygen abundances for the H II regions located in the ring in the range 12+log(O/H) = 8.3-8.8 dex. We have also found the presence of an oxygen gradient across the ring, and it appears that AM 2020-504 follows the metallicity-luminosity relation of spiral galaxies. These results support the accretion scenario for this object and they rule out cold accretion as the source for the H I gas in the polar ring.
When analysing H II regions, a possible source of systematic error on empirically derived quantities, such as the gas temperature and the chemical composition, is the limited size of the slit used for the observations. In order to evaluate this type of systematic error, we use the photoionization code AANGABA to create a virtual photoionized region and mimic the effect of a slit observation. A grid of models was built varying the ionizing radiation spectrum emitted by a central stellar cluster, as well as the gas abundance. The calculated line surface brightness was then used to simulate slit observations and to derive empirical parameters using the usual methods described in the literature. Depending on the fraction of the object covered by the slit, the empirically derived physical parameters and chemical composition can be different from those obtained from observations of the whole object. This effect is mainly dependent on the age of the ionizing stellar cluster. The low-ionization lines, which originate in the outer layers of the ionized gas, are more sensitive to the size of the area covered by the slit than the highionization forbidden lines or recombination lines, because these lines are mainly produced closer to the inner radius of the nebula. For a slit covering 50 per cent or less of the total area, the measured [O III], [O II] and [O I] line intensities are less than 78, 62 and 58 per cent of the total intensity for a young H II region (t < 3 Myr); for older objects the effect due to the slit is less significant. Regarding the temperature indicator T [O III] , the slit effects are small (usually less than 5 per cent) because this temperature is derived from [O III] high-ionization lines. On the other hand, for the abundance (and temperature) indicator R 23 , which depends also on the [O II] line, the slit effect is slightly higher. Therefore, the systematic error due to slit observations on the O abundance is low, being usually less than 10 per cent, except for H II regions powered by stellar clusters with a relative low number of ionizing photons between 13.6 and 54.4 eV, which create a smaller O ++ emitting volume. In this case, the systematic error on the empirical O abundance deduced from slit observations is more than 10 per cent when the covered area is less than 50 per cent. method (with some improvements) is very convenient for obtaining a first evaluation of the chemical composition of large samples of objects such as planetary nebulae, H II regions and active galactic nuclei. The emitting gas is assumed to have high-and low-ionized zones, characterized by the temperature indicated, respectively, by the [O III] and [N II] line intensity ratios. The electron density is usually derived from the [S II] line ratio (see, for instance, Osterbrock 1989). The various ions of the elements present in the gas are assumed to be in either of these regions, following their ionization potential.As photoionization codes became friendly and available to users (such as the well-known CLOUDY), several authors...
Aims. We report first results of an investigation of the tidally disturbed galaxy system AM 546-324, whose two principal galaxies 2MFGC 04711 and AM 0546-324 (NED02) were previously classified as interacting doubles. This system was selected to study the interaction of ellipticals in a moderately dense environment. We provide spectral characteristics of the system and present an observational study of the interaction effects on the morphology, kinematics, and stellar population of these galaxies. Methods. The study is based on long-slit spectrophotometric data in the range of ∼4500−8000 Å obtained with the Gemini MultiObject Spetrograph at Gemini South (GMOS-S). We have used the stellar population synthesis code STARLIGHT to investigate the star formation history of these galaxies. The Gemini/GMOS-S direct r-G0303 broad band pointing image was used to enhance and study fine morphological structures. The main absorption lines in the spectra were used to determine the radial velocity. Results. Along the whole long-slit signal, the spectra of the Shadowy galaxy (discovered by us), 2MFGC 04711, and AM 0546-324 (NED02) resemble that of an early-type galaxy. We estimated redshifts of z = 0.0696, z = 0.0693 and z = 0.0718, corresponding to heliocentric velocities of 20 141 km s −1 , 20 057 km s −1 , and 20 754 km s −1 for the Shadowy galaxy, 2MFGC 04711 and AM 0546-324 (NED02), respectively. The central regions of 2MFGC 04711 and AM 0546-324 (NED02) are completely dominated by an old stellar population of 2 × 10 9 < t ≤ 13 × 10 9 yr and do not show any spatial variation in the contribution of the stellar-population components. Conclusions. The observed rotation profile distribution of 2MFGC 04711 and AM 0546-324 (NED02) can be adequately interpreted as an ongoing stage of interaction with the Shadowy galaxy as the center of the local gravitational potential-well of the system. The three galaxies are all early-type. The extended and smooth distribution of the material in the Shadowy galaxy is a good laboratory to study direct observational signatures of tidal friction in action.
Aims. We report the first study of the peculiar ring galaxy HRG 54103 which was previously classified as a Saturn-like type galaxy. Methods. The study is based on low resolution spectroscopy and photometric observations in the optical band to highlight the characteristics of this almost isolated galaxy. The colour distribution of HRG 54103 was examined through direct CCD BVRI Kron-Cousins system imagery. Color-color diagrams of the bulge and ring are displayed and further compared with the star-forming ring galaxy HRG 2302.Results. The results of image enhancement of the morphological structure of this galaxy are discussed. The nuclear emission-line spectrum resembles that of a Seyfert2/LINER object, with z = 0.022 and heliocentric V = 6483 ± 18 km s −1 , in agreement with the literature. The nuclear, bulge and ring section radial velocities along the ring major axis show a peculiar distribution, which together and the disk-ring asymmetry suggest a possible merger event in the recent past of this object, which could also have caused its formation.
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