Long-slit spectrophotometry of the multiple knots of the polar ring galaxy IIZw71

Abstract

Aims. The blue compact dwarf galaxy IIZw71 is catalogued as a probable polar-ring galaxy, and along its long axis it has several very luminous knots showing recent episodes of star formation. Our main aim is to study the physical properties, the stellar content, and the kinematics in the brightest knots of the polar ring. Methods. We carried out long-slit spectroscopic observations of the polar ring in the spectral range 3500-10 000 Å taken with the William Herschel Telescope (WHT). The spectroscopic observations complemented by the available photometry of the galaxy in narrow Hα filters. Results. We measured the rotation curve of the ring, from which we infer a ratio M/L B ≈ 3.9 inside the star-forming ring. We measured the auroral [Oiii] line in the two brightest knots, allowing us to measure oxygen, sulphur, nitrogen, argon, and neon chemical abundances following the direct method. Different empirical calibrators were used to estimate the oxygen abundance in the two faintest knots where the temperature sensitive lines could not be measured. The metallicities obtained are very similar for all the knots, but lower than previously reported in the literature from integrated spectra. The N/O abundance, as derived from the N 2 O 2 parameter (the ratio of the [Nii] and [Oii] intensities), is remarkably constant over the ring, indicating that local polution processes are not conspicuous. Using synthetic stellar populations (SSPs) calculated with the code STARLIGHT, we studied the age distribution of the stellar populations in each knot, finding that in all of them there is a combination of a very young population with less than 10 Myr, responsible for the ionisation of the gas, with other populations older than 100 Myr, probably responsible for the chemical evolution of the knots. The small differences in metallicity and the age distributions among the different knots are indicative of a common chemical evolution, probably related to the process of interaction with the companion galaxy IIZw70. Finally, we calculated the SFR in the different knots from the Hα luminosities. The combined SFR rate for the ring amounts to half of the integrated SFR for this galaxy reported by previous authors. The average surface SFR is also higher than the average reported value of HII in polar rings by more than an order of magnitude.

Abundance determination of multiple star-forming regions in the H ii galaxy SDSS J165712.75+321141.4

“…This difference is greater than the estimated observational errors and is similar (or even smaller) to what is found in other works with the spatial resolution of knots that belong to H ii galaxies or blue compact dwarf galaxies (see e.g. Izotov et al 1997; Vílchez & Iglesias‐Páramo 1998; Kehrig et al 2008; Cairós et al 2009a; Pérez‐Montero et al 2009; García‐Benito et al 2010). However, in general, these differences were attributed to the observational uncertainties (pointing errors, seeing variations, etc.)…”

“…8, different empirical calibrations give results that are not in complete agreement with direct measurements and the goodness of the result even changes between knots when using the same calibrator. Pérez‐Montero et al (2009) found a very similar behaviour for the star‐forming knots of IIZw71, except for the Ar 3 /O 3 parameter, which is a better estimator of the oxygen abundances for the knots in SDSS J1657. For two knots in Mrk 1418, Cairós et al (2009a) derived the oxygen abundances from the observed [O iii ] λ 4363 Å auroral emission line and compared them with those abundance values derived using the N2 and O3N2 empirical parameters, and found a similar discrepancy, with the empirically derived values being slightly larger.…”

Chemical abundances of the PRGs UGC 7576 and UGC 9796

“…From the average values of L(H α ) 2.36 × 10 36 erg/s for UGC 7576 and L(H α ) 5.33 × 10 35 erg/s for UGC 9796, we have obtained an average SFR ∼ 1.9 × 10 −5 M /yr and SFR ∼ 4.2 × 10 −6 M /yr, respectively. These values are significantly lower than those obtained for NGC 4650A (Spavone et al 2010), which is SFR ∼ 0.06 M /yr, and for IIZw71 (Pérez-Montero et al 2009), which is SFR ∼ 0.035 M /yr.…”

“…We also compared our new results with those obtained by Pérez-Montero et al (2009) for IIZw71, a blue compact dwarf (2001), versus radius. The superimposed lines are the linear best fit derived by Pilyugin et al (2006); the red line represents the best fit to the abundance of oxygen-rich spirals, while the blue line is those related to ordinary spirals.…”

High-resolution spectroscopy of the blue compact dwarf galaxy Haro 15 - II. Chemodynamics

“…This occurs when the object is very compact, or even extended but with few star‐forming knots, and when good spatial and spectral resolution together with a simultaneous wide spectral coverage are required (see e.g. Hägele et al 2006, 2007, 2008, 2009, 2010, 2011; Cumming et al 2008; Hägele 2008; López‐Sánchez & Esteban 2009b, 2010a; Pérez‐Montero et al 2009; Firpo et al 2010; Paper I; López‐Sánchez 2010; López‐Sánchez & Esteban 2010b).…”