Lars Mattsson

On the dust abundance gradients in late‐type galaxies – II. Analytical models as evidence for massive interstellar dust growth in SINGS galaxies

We use simple analytical models of the buildup of the dust component and compare these with radial dust distributions derived from observations of SINGS galaxies. The observations show that dust gradients are indeed typically steeper than the corresponding metallicity gradients and our models indicate very little dust destruction, but significant dust growth in the interstellar medium for most of these galaxies. Hence, we conclude that there is evidence for significant non-stellar dust production, and little evidence for dust destruction due to supernova (SN) shock waves. We find that dust is reprocessed rather than destroyed by shocks from SNe. Finally, we argue that dust abundances derived using standard methods may be overestimated, since even very ‘generous’ estimates of the metallicity result in dust-to-metals ratios above unity in several cases, if the dust abundances given in the literature are taken at face value.

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 Sloan Digital Sky Survey (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, anomalously high N/O abundance 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 Hxa0ii 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 Hxa0ii 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 Hxa0ii regions discussed by Peimbert. Our work thus suggests that the high Te-based N/O abundance ratios found in SDSS galaxies may not be real. However, such an effect is not expected to be present in dwarf galaxies since they generally have a 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 ∼0.2xa0dex.

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 r tes (sS FR) variations with redshift in star-forming SDSS galaxies of di fferent masses. We find that the maximum value of the sS FR, sS FRmax, decreases when the stellar mass, MS, of a galaxy increases, and decreases with decreasing of redshift. The sS FRmax can exceed the time-averaged sS FRby about an order of magnitude for massive galaxies. The metall icity – redshift relations for subsamples of galaxies with sS FR= sS FRmaxand withsS FR= 0.1×sS FRmaxcoincide for massive (log(MS/M⊙) & 10.5, with stellar mass MS in solar units) galaxies and di ffer for low-mass galaxies. This suggests that there is no correlation betwee n oxygen abundance and sS FRin massive galaxies and that the oxygen abundance correlates w ith thesS FRin low-mass galaxies. We find evidence in favour of that the irregular galaxies show, on average, higher sS FR and lower oxygen abundances than the spiral galaxies of simi lar asses and that the mass – metallicity relation for spiral galaxies di ffers 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 – redshif t relation on thesS FRobserved for the low-mass SDSS galaxies. The mass – metallicity and lumin osity – metallicity relations obtained for irregular SDSS galaxies agree with correspond ing relations for nearby irregular galaxies with direct abundance determinations. We find that the aperture e ffect does not make a significant contribution to the redshift variation of oxyg en abundances in SDSS galaxies.

On the dust abundance gradients in late-type galaxies – I. Effects of destruction and growth of dust in the interstellar medium

We present basic theoretical constraints on the effects of destruction by supernovae (SNe) and growth of dust grains in the interstellar medium (ISM) on the radial distribution of dust in late-type galaxies. The radial gradient of the dust-to-metals ratio is shown to be essentially flat (zero) if interstellar dust is not destroyed by SN shock waves and all dust is produced in stars. If there is net dust destruction by SN shock waves, the dust-to-metals gradient is flatter than or equal to the metallicity gradient (assuming the gradients have the same sign). Similarly, if there is net dust growth in the ISM, then the dust-to-metals gradient is steeper than or equal to the metallicity gradient. The latter result implies that if dust gradients are steeper than metallicity gradients, that is, the dust-to-metals gradients are not flat, then it is unlikely dust destruction by SN shock waves is an efficient process, while dust growth must be a significant mechanism for dust production. Moreover, we conclude that dust-to-metals gradients can be used as a diagnostic for interstellar dust growth in galaxy discs, where a negative slope indicates dust growth.

SDSS galaxies with double-peaked emission lines: double starbursts or active galactic nuclei?

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.

Polynomial probability distribution estimation using the method of moments

We suggest a procedure for estimating Nth degree polynomial approximations to unknown (or known) probability density functions (PDFs) based on N statistical moments from each distribution. The procedure is based on the method of moments and is setup algorithmically to aid applicability and to ensure rigor in use. In order to show applicability, polynomial PDF approximations are obtained for the distribution families Normal, Log-Normal, Weibull as well as for a bimodal Weibull distribution and a data set of anonymized household electricity use. The results are compared with results for traditional PDF series expansion methods of Gram–Charlier type. It is concluded that this procedure is a comparatively simple procedure that could be used when traditional distribution families are not applicable or when polynomial expansions of probability distributions might be considered useful approximations. In particular this approach is practical for calculating convolutions of distributions, since such operations become integrals of polynomial expressions. Finally, in order to show an advanced applicability of the method, it is shown to be useful for approximating solutions to the Smoluchowski equation.

Counterpart method for abundance determinations in H II regions

We suggest a new way of the determining abundances and electron temperatures in HII regions from strong emission lines. Our approach is based on the standard assumption that HII regions with similar intensities of strong emission lines have similar physical properties and abundances. A counterpart for a studied HII region may be chosen among HII regions with well-measured abundances (reference HII regions) by comparison of carefully chosen combinations of strong line intensities. Then the abundances in the investigated HII region can be assumed to be the same as that in its counterpart. In other words, we suggest to determine the abundances in HII regions by precedent. To get more reliable abundances for the considered HII region, a number of reference HII regions is selected and then the abundances in the target HII region is estimated through extra-/interpolation. We will refer to this method of abundance determination as the counterpart method or, for brevity, the C method. We define a sample of reference HII regions and verify the validity of the C method. We find that this method produces reliable abundances. Finally, the C method is used to obtain the radial abundance distributions in the extended discs of the spiral galaxies M83, NGC4625 and NGC 628.