arXiv subjects

L. S. Pilyugin

Publications and source records attributed to L. S. Pilyugin.

At least 37 records · Page 2Linked to original sources

arXiv2017

Breaks in surface brightness profiles and radial abundance gradients in the discs of spiral galaxies

We examine the relation between breaks in the surface brightness profiles and radial abundance gradients within the optical radius in the discs of 134 spiral galaxies from the CALIFA survey. The distribution of the radial abundance (in logarithmic scale) in each galaxy was fitted by simple and broken linear relations. The surface brightness profile was fitted assuming pure and broken exponents for the disc. We find that the maximum absolute difference between the abundances in a disc given by broken and pure linear relations is less than 0.05 dex in the majority of our galaxies and exceeds the scatter in abundances for 26 out of 134 galaxies considered. The scatter in abundances around the broken linear relation is close (within a few percent) to that around the pure linear relation. The breaks in the surface brightness profiles are more prominent. The scatter around the broken exponent in a number of galaxies is lower by a factor of two or more than that around the pure exponent. The shapes of the abundance gradients and surface brightness profiles within the optical radius in a galaxy may be different. A pure exponential surface brightness profile may be accompanied by a broken abundance gradient and vise versa. There is no correlation between the break radii of the abundance gradients and surface brightness profiles. Thus, a break in the surface brightness profile does not need to be accompanied by a break in the abundance gradient.

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arXiv2016

On the influence of the environment on galactic chemical abundances

We examine the influence of the environment on the chemical abundances of late-type galaxies with masses of 10^9.1 M_sun - 10^11 M_sun using data from the Sloan Digital Sky Survey(SDSS). We find that the environmental influence on galactic chemical abundances is strongest for galaxies with masses of 10^9.1 M_sun to 10^9.6 Msun. The galaxies in the densest environments may exceed the average oxygen abundances by about 0.05 dex (the median value of the overabundances for 101 galaxies in the densest environments) and show higher abundances in nitrogen by about 0.1. The abundance excess decreases with increasing galaxy mass and with decreasing environmental density. Since only a small fraction of late-type galaxies is located in high-density environments these galaxies do not have a significant influence on the general X/H - M relation. The metallicity - mass relations for isolated galaxies and for galaxies with neighbors are very similar. The mean shift of non-isolated galaxies around the metallicity - mass relation traced by the isolated galaxies is less than 0.01 dex for oxygen and less than 0.02 dex for nitrogen. The scatter in the galactic chemical abundances is large for any number of neighbor galaxies (at any environmental density), i.e., galaxies with both enhanced and reduced abundances can be found at any environmental density. This suggests that environmental effects do not play a key role in evolution of late-type galaxies as was also concluded in some of the previous studies.

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arXiv2016

Oxygen abundance maps of CALIFA galaxies

We construct maps of the oxygen abundance distribution across the disks of 88 galaxies using CALIFA data release 2 (DR2) spectra. The position of the center of a galaxy (coordinates on the plate) were also taken from the CALIFA DR2. The galaxy inclination, the position angle of the major axis, and the optical radius were determined from the analysis of the surface brightnesses in the SDSS $g$ and $r$ bands of the photometric maps of SDSS data release 9. We explore the global azimuthal abundance asymmetry in the disks of the CALIFA galaxies and the presence of a break in the radial oxygen abundance distribution. We found that there is no significant global azimuthal asymmetry for our sample of galaxies, i.e., the asymmetry is small, usually lower than 0.05 dex. The scatter in oxygen abundances around the abundance gradient has a comparable value, $\lesssim 0.05$ dex. A significant (possibly dominant) fraction of the asymmetry can be attributed to the uncertainties in the geometrical parameters of these galaxies. There is evidence for a flattening of the radial abundance gradient in the central part of 18 galaxies. We also estimated the geometric parameters (coordinates of the center, the galaxy inclination and the position angle of the major axis) of our galaxies from the analysis of the abundance map. The photometry-map-based and the abundance-map-based geometrical parameters are relatively close to each other for the majority of the galaxies but the discrepancy is large for a few galaxies with a flat radial abundance gradient.

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arXiv2016

New calibrations for abundance determinations in HII regions

Simple relations for deriving the oxygen abundance in HII regions with intensities of the three strong emission lines R_2, R_3, and N_2 (R calibration) or S_2, R_3, and N_2 (S calibration) in their spectra are suggested. A sample of 313 reference HII regions of the counterpart method is used as calibrating data points. Relations for the determination of nitrogen abundances, the R calibration, are also constructed. We find that the oxygen and nitrogen abundances in high-metallicity HII regions can be estimated using the intensities of the two strong lines R_2 and N_2 (or S_2 and N_2 for oxygen) only. The corresponding two-dimensional relations are provided. There are considerable advantages of the suggested calibration relations as compared to the existing ones. First, the oxygen and nitrogen abundances estimated through the suggested calibrations agree with the Te-based abundances within ~0.1 dex over the whole metallicity range, i.e., the relative accuracy of the calibration-based abundances is 0.1 dex. Although we constructed distinct relations for high- and low-metallicity objects, the separation between these two can be simply obtained from the intensity of the N_2 line. Moreover, the applicability ranges of the high- and low-metallicity relations overlap for adjacent metallicities, i.e., the transition zone disappears. Second, the oxygen abundances produced by the two suggested calibrations are in remarkable agreement with each other. In fact, the R-based and S-based oxygen abundances agree within ~0.05 dex in the majority of cases for more than three thousand HII region spectra.

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arXiv2016

Study of young stellar groupings in HII regions based on the spectral and photometric data

We studied 102 star forming regions in seven spiral galaxies (NGC 628, NGC 783, NGC 2336, NGC 6217, NGC 6946, NGC 7331, and NGC 7678) on the basis of complex spectroscopic, photometric (UBVRI) and spectrophotometric (H alpha line) observations. Using data on the chemical composition and absorption in HII regions, obtained from spectroscopic observations, and using evolutionary models, we estimated physical parameters (ages and masses) of young stellar groupings embedded in HII regions. We found that the gas extinction, A(gas), which determined from the Balmer decrement, does not correspond in some cases to the absorption A(stars) in the young stellar associations (complexes). This is due to the spatial offset relative HII cloud the stellar group related to him. It has been found that the condition A(gas) = A(stars) does not satisfied for the star forming regions, in which: 1) the contribution to the total emission of gas in the B and/or V bands is higher than 40%, and 2) EW(H alpha) > 1500A. Extinction A(V) in studied star forming regions corrected for the Galactic absorption A(V)Gal ranges from 0 to 3 mag with a mean value A(V)-A(V)Gal = 1.18+-0.84 mag. We estimated masses and ages for 63 star forming regions. The regions have ages from 1 to 10 Myr, the most part of them are younger than 6 Myr. The derived masses of young stellar groupings range from 10^4Msun in the nearby galaxies NGC 628 and NGC 6946 to 10^7Msun in the most distant NGC 7678. More than 80% of groupings have masses between 10^5Msun and 10^6Msun. The lowest mass estimate of 1x10^4Msun for the objects in NGC 628 and NGC 6946 belongs to the mass interval of the youngest Galactic open clusters.

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arXiv2015

On the central abundances of Active Galactic Nuclei and Star-forming Galaxies

We examine the relation between oxygen abundances in the narrow-line regions (NLRs) of active galactic nuclei (AGNs) estimated from the optical emission lines through the strong-line method (the theoretical calibration of Storchi-Bergmann et al.(1998)), via the direct Te-method, and the central intersect abundances in the host galaxies determined from the radial abundance gradients. We found that the Te-method underestimates the oxygen abundances by up to ~2 dex (with average value of ~0.8 dex) compared to the abundances derived through the strong-line method. This confirms the existence of the so-called "temperature problem" in AGNs. We also found that the abundances in the centres of galaxies obtained from their spectra trough the strong-line method are close to or slightly lower than the central intersect abundances estimated from the radial abundance gradient both in AGNs and Star-forming galaxies. The oxygen abundance of the NLR is usually lower than the maximum attainable abundance in galaxies (~2 times the solar value). This suggests that there is no extraordinary chemical enrichment of the NLRs of AGNs.

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arXiv2015

Oxygen abundance distributions in six late-type galaxies based on SALT spectra of HII regions

Spectra of 34 H II regions in the late-type galaxies NGC1087, NGC2967, NGC3023, NGC4030, NGC4123, and NGC4517A were observed with the South African Large Telescope (SALT). In all 34 H II regions, oxygen abundances were determined through the "counterpart" method (C method). Additionally, in two H II regions in which the auroral lines were detected oxygen abundances were measured through the classic Te method. We also estimated the abundances in our H II regions using the O3N2 and N2 calibrations and compared those with the C-based abundances. With these data we examined the radial abundance distributions in the disks of our target galaxies. We derived surface-brightness profiles and other characteristics of the disks (the surface brightness at the disk center and the disk scale length) in three photometric bands for each galaxy using publicly available photometric imaging data. The radial distributions of the oxygen abundances predicted by the relation between abundance and disk surface brightness in the W1 band obtained for spiral galaxies in our previous study are close to the radial distributions of the oxygen abundances determined from the analysis of the emission line spectra for four galaxies where this relation is applicable. Hence, when the surface-brightness profile of a late-type galaxy is known, this parametric relation can be used to estimate the likely present-day oxygen abundance in its disk.

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arXiv2015

On the radial abundance gradients in disks of irregular galaxies

We determine the radial abundance distributions across the disks of fourteen irregular galaxies of the types Sm and Im (morphological T types T = 9 and T =10) as traced by their HII regions. The oxygen and nitrogen abundances in HII regions are estimated through the Te method or/and with the counterpart method (C method). Moreover, we examine the correspondence between the radial abundance gradient and the surface brightness profile. We find that irregular galaxies with a flat inner profile (flat or outwardly increasing surface brightness in the central region) show shallow (if any) radial abundance gradients. On the other hand, irregular galaxies with a steep inner profile (with or without a bulge or central star cluster) usually show rather steep radial abundance gradients. This is in contrast to the widely held belief that irregular galaxies do not usually show a radial abundance gradient.

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arXiv2014

The Abundance Properties of Nearby Late-Type Galaxies. \\ II. The Relation between Abundance Distributions and Surface Brightness Profiles

The relations between oxygen abundance and disk surface brightness (OH-SB relation) in the infrared W1 band are examined for a nearby late-type galaxies. The oxygen abundances were presented in Paper I. The photometric characteristics of the disks are inferred here using photometric maps from the literature through bulge-disk decomposition. We find evidence that the OH - SB relation is not unique but depends on the galactocentric distance r (taken as a fraction of the optical radius R_25) and on the properties of a galaxy: the disk scale length h and the morphological T-type. The parametric OH - SB relation reproduces the observed data better than a simple, one-parameter relation; the deviations resulting when using our parametric relation are smaller by a factor of ~1.4 than that the simple relation. The influence of the parameters on the OH - SB relation varies with galactocentric distance. The influence of the T-type on the OH - SB relation is negligible at the centers of galaxies and increases with galactocentric distance. In contrast, the influence of the disk scale length on the OH -- SB relation is maximum at the centers of galaxies and decreases with galactocentric distance, disappearing at the optical edges of galaxies. Two-dimensional relations can be used to reproduce the observed data at the optical edges of the disks and at the centers of the disks. The disk scale length should be used as a second parameter in the OH - SB relation at the center of the disk while the morphological T-type should be used as a second parameter in the relation at optical edge of the disk. The general properties of the abundance - surface brightness relations are similar for the three considered bands B, K, and W1.

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arXiv2014

The Abundance Properties of Nearby Late-Type Galaxies.I. The Data

We investigate the oxygen and nitrogen abundance distributions across the optical disks of 130 nearby late-type galaxies using around 3740 published spectra of HII regions. We use these data in order to provide homogeneous abundance determinations for all objects in the sample, including HII regions in which not all of the usual diagnostic lines were measured. Examining the relation between N and O abundances in these galaxies we find that the abundances in their centres and at their isophotal R_25 disk radii follow the same relation. The variation in N/H at a given O/H is around 0.3 dex. We suggest that the observed spread in N/H may be partly caused by the time delay between N and O enrichment and the different star formation histories in galaxies of different morphological types and dimensions. We study the correlations between the abundance properties (central O and N abundances, radial O and N gradients) of a galaxy and its morphological type and dimension.

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arXiv2013

The metallicity - redshift relations for emission-line SDSS galaxies: examination of the dependence on the star formation rate

We analyse the oxygen abundance and specific star formation rates (sSFR) variations with redshift in star-forming SDSS galaxies of different masses. We find that the maximum value of the sSFR, sSFRmax, decreases when the stellar mass, Ms, of a galaxy increases, and decreases with decreasing of redshift. The sSFRmax can exceed the time-averaged sSFR by about an order of magnitude for massive galaxies. The metallicity - redshift relations for subsamples of galaxies with sSFR = sSFRmax and with sSFR = 0.1sSFRmax coincide for massive (log(Ms/Mo) > 10.5, with stellar mass Ms in solar units) galaxies and differ for low-mass galaxies. This suggests that there is no correlation between oxygen abundance and sSFR in massive galaxies and that the oxygen abundance correlates with the sSFR in low-mass galaxies. We find evidence in favour of that the irregular galaxies show, on average, higher sSFR and lower oxygen abundances than the spiral galaxies of similar masses and that the mass - metallicity relation for spiral galaxies differs slightly from that for irregular galaxies. The fact that our sample of low-mass galaxies is the mixture of spiral and irregular galaxies can be responsible for the dependence of the metallicity - redshift relation on the sSFR observed for the low-mass SDSS galaxies. The mass - metallicity and luminosity - metallicity relations obtained for irregular SDSS galaxies agree with corresponding relations for nearby irregular galaxies with direct abundance determinations. We find that the aperture effect does not make a significant contribution to the redshift variation of oxygen abundances in SDSS galaxies.

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arXiv2012

Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios

We have compared the oxygen and nitrogen abundances derived from global emission-line SDSS spectra of galaxies using (1) the Te method and (2) two recent strong line calibrations: the ON and NS calibrations. Using the Te method, anomously high N/O abundances ratios have been found in some SDSS galaxies. To investigate this, we have Monte Carlo simulated the global spectra of composite nebulae by a mix of spectra of individual components, based on spectra of well-studied HII regions in nearby galaxies. We found that the Te method results in an underestimated oxygen abundance (and hence in an overestimated nitrogen-to-oxygen ratio) if HII regions with different physical properties contribute to the global spectrum of composite nebulae. This effect is somewhat similar to the small-scale temperature fluctuations in HII regions discussed by Peimbert. Our work thus suggests that the high Te-based N/O abundances ratios found in SDSS galaxies may not be real. However, such an effect is not expected to be present in dwarf galaxies since they have generally an uniform chemical composition. The ON and NS calibrations give O and N abundances in composite nebulae which agree with the mean luminosity-weighted abundances of their components to within 0.2 dex.

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arXiv2011

SDSS galaxies with double-peaked emission lines: double starbursts or AGNs?

With the aim of investigating galaxies with two strong simultaneous starbursts, we have extracted a sample of galaxies with double-peaked emission lines in their global spectra from the SDSS spectral database. We then fitted the emission lines Halpha, Hbeta, [OIII]5007, [NII]6584, [SII]6717 and [SII]6731 of 129 spectra by two Gaussians to separate the radiation of the two (blue and red) components. A more or less reliable decomposition of the all those emission lines have been found for 55 spectra. Using a standard BPT classification diagram, we have been able to divide the galaxies from our sample into two subsamples: Sample A consisting of 18 galaxies where both components belong to the photoionised class of objects, and Sample B containing 37 galaxies which show non-thermal ionisation (AGNs). We have examined the properties of the blue and red components, and found that the differences between radial velocities of components lie within 200 - 400 km/s for galaxies of both subsamples. The equivalent number of ionising stars is in the range 10^4 - 10^5 O7V stars for each component in the galaxies of Sample A. We have estimated the oxygen and nitrogen abundances as well as the electron temperatures for each component using the recent NS-calibration and from global spectra for galaxies from Sample A using both the NS and ON-calibration. We have found that the global oxygen abundance is typically in between the measured abundances of individual components for our sample of galaxies, and that both calibrations provide consistent global abundances. Finally, we suggest the classical O/H -- N/O diagram is used to test the reliability of the dividing lines between starburst-like objects and AGNs in the so-called BPT diagram.

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arXiv2010

Abundance determination in HII regions from spectra without the [OII]3727+3729 line

We suggest an empirical calibration for determination of oxygen and nitrogen abundances and electron temperature in HII regions where the [OII]3727+3729 line (R_2) is not available. The calibration is based on the strong emission lines of OIII, NII, and SII (NS calibration) and derived using the spectra of HII regions with measured electron temperatures as calibration datapoints. The NS calibration makes it possible to derive abundances for HII regions in nearby galaxies from the SDSS spectra where R_2 line is out of the measured wavelength range, but can also be used for the oxygen and nitrogen abundances determinations in any HII region independently whether the nebular oxygen line [OII]3727+3729 is available or not. The NS calibration provides reliable oxygen and nitrogen abundances for HII regions over the whole range of metallicities.

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arXiv2010

The redshift evolution of oxygen and nitrogen abundances in emission-line SDSS galaxies

The oxygen and nitrogen abundance evolutions with redshift and galaxy stellar mass in emission-line SDSS galaxies are investigated. This is the first such study for nitrogen abundances, and it provides an additional constraint for the study of the chemical evolution of galaxies. We have devised a criterion to recognize and exclude from consideration AGNs and star-forming galaxies with large errors in the line flux measurements. To select star-forming galaxies with accurate line fluxes measurements, we require that, for each galaxy, the nitrogen abundances derived with various calibrations based on different emission lines agree. Using this selection criterion, subsamples of star-forming galaxies have been extracted from catalogs of the MPA/JHU group. We found that the galaxies of highest masses, those with masses > 10^11.2 M_sun, have not been enriched in both oxygen and nitrogen over the last 3 Gyr: they have formed their stars in the so distant past that these have returned their nucleosynthesis products to the interstellar medium before z=0.25. The galaxies in the mass range from 10^11.0 M_sun to 10^11.2 M_sun do not show an appreciable enrichment in oxygen, but do show some enrichment in nitrogen: they also formed their stars before z=0.25 but later in comparison to the galaxies of highest masses; these stars have not returned nitrogen to the interstellar medium before z=0.25 because they have not had enough time to evolve. This suggests that stars with lifetimes of 2-3 Gyr contribute to the nitrogen production. Finally, galaxies with masses < 10^11 M_sun show enrichment in both oxygen and nitrogen during the last 3 Gyr: they have undergone appreciable star formation and have converted up to 20% of their mass into stars over this period.

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arXiv2009

The electron temperatures of SDSS high-metallicity giant extragalactic HII regions

Spectra of high-metallicity (12+log(O/H) > 8.2) HII regions where oxygen auroral lines are measurable in both the O+ and O++ zones, have been extracted from the Data Release 6 of the Sloan Digital Sky Survey (SDSS). Our final sample consists of 181 SDSS spectra of HII regions in galaxies in the redshift range from ~0.025 to ~0.17. The t_2,O-t_3,O diagram is examined. In the SDSS HII regions, the electron temperature t_2,O is found to have a large scatter at a given value of the electron temperature t_3,O. The majority of the SDSS HII regions lie below the t_2,O-t_3,O relation derived for HII regions in nearby galaxies, i.e. the positions of the SDSS HII regions show a systematic shift towards lower t_2,O temperatures or/and towards higher t_3,O temperatures. The scatter and shift of the SDSS HII regions in the t_2,O-t_3,O diagram can be understood if they are composite nebulae excited by two or more ionizing sources of different temperatures.

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arXiv2009

On the electron temperatures in high-metallicity HII regions

The electron temperatures of high-metallicity (12+log(O/H) > 8.2) HII regions have been studied. The empirical ff relations which express the nebular-to- auroral [OIII] line ratio Q_3,O (as well as the nebular-to-auroral [OII] line ratio Q_2,O, and the nebular-to-auroral [NII] line ratio Q_2,N) in terms of the nebular R_3 and R_2 line fluxes in spectra of high-metallicity HII regions are derived, and the electron temperatures t_3,O, t_2,O, and t_2,N in a number of extragalactic HII regions are also determined. Furthermore, the t_2 - t_3 diagram is discussed. It is found that there is a one-to-one correspondence between t_2 and t_3 electron temperatures for HII regions with a weak nebular R_3 lines (logR_ < 0.5). The derived t_2,N - t_3,O relation for these HII regions is similar to commonly used t_2 - t_3 relations. The HII regions with a strong nebular R_3 line flux (logR_3 > 0.5) do not follow this relation. A discrepancy between t_2,N and t_2,O temperatures is found, being the t_2,N temperatures systematically lower than t_2,O ones. The differences are small at low electron temperatures and increases with increasing electron temperatures up to 10% at t=1. The uncertainties in the atomic data may be the cause of this discrepancy.

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arXiv2007

The Oxygen Abundance of Nearby Galaxies from Sloan Digital Sky Survey Spectra

We have derived the oxygen abundance for a sample of nearby galaxies in the Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least two independent spectra of one or several HII regions with a detected [OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line is out of the observed wavelength range, we propose a method to derive (O/H)_ff abundances using the classic Te method coupled with the ff relation. (O/H)_7325 abundances have also been determined, based on the [OII]7320,7330 line intensities, and using a small modification of the standard Te method. The (O/H)_ff and (O/H)_7325 abundances have been derived with both the one- and two-dimensional t_2 - t_3 relations. It was found that the (O/H)_ff abundances derived with the parametric two-dimensional t_2 - t_3 relation are most reliable. Oxygen abundances have been determined in 29 nearby galaxies, based on 84 individual abundance determinations in HII regions. Because of our selection methods, the metallicity of our galaxies lies in the narrow range 8.2 < 12 + log (O/H) < 8.4. The radial distribution of oxygen abundances in the disk of the spiral galaxy NGC 4490 is determined for the first time.

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