Browsing by Author "Laurikainen, E."
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Item AINUR: Atlas of Images of NUclear Rings(Oxford University Press, 2010-03-11) Comeron, S.; Knapen, J. H.; Beckman, J. E.; Laurikainen, E.; Salo, H.; Martinez-Valpuesta, I.; Buta, R. J.; Instituto de Astrofisica de Canarias; Universidad de la Laguna; University of Oulu; University of Alabama TuscaloosaWe present the most complete atlas of nuclear rings to date. We include 113 rings found in 107 galaxies, six of which are elliptical galaxies, five are highly inclined disc galaxies, 18 are unbarred disc galaxies and 78 are barred disc galaxies. Star-forming nuclear rings occur in 20 +/- 2 per cent of disc galaxies with types between T = -3 and T = 7. We aim to explore possible relationships between the size and morphology of the rings and various galactic parameters. We also aim to establish whether ultra-compact nuclear rings are a distinct population of nuclear rings or if they are merely the low-end tail of the nuclear ring size distribution. We produce colour index and structure maps, as well as H alpha and Pa alpha continuum-subtracted images from Hubble Space Telescope archival data. We derive ellipticity profiles from H-band Two-Micron All-Sky Survey images in order to detect bars and find their metric parameters. We measure the non-axisymmetric torque parameter, Q(g), and search for correlations between bar and ring metric parameters, and Q(g). Our atlas of nuclear rings includes star-forming and dust rings. Nuclear rings span a range from a few tens of parsecs to a few kiloparsecs in radius. Star-forming nuclear rings can be found in a wide range of morphological types, from S0 to Sd, with a peak in the distribution between Sab and Sb and without strong preference for barred galaxies. The ellipticities of rings found in disc galaxies range from c(r) = 0 to c(r) = 0.4, assuming that nuclear rings lie in the galactic plane. Dust nuclear rings are found in elliptical and S0 galaxies. For barred galaxies, the maximum radius that a nuclear ring can reach is a quarter of the bar radius. We found a nearly random distribution of position angle offsets between nuclear rings and bars. There is some evidence that nuclear ring ellipticity is limited by bar ellipticity. We confirm that the maximum relative size of a star-forming nuclear ring is inversely proportional to the non-axisymmetric torque parameter, Q(g) ('stronger bars host smaller rings') and that the origin of nuclear rings, even the ones in non-barred hosts, is closely linked to the existence of dynamical resonances. Ultra-compact nuclear rings constitute the low-radius portion of the nuclear ring size distribution. We discuss implications for the lifetimes of nuclear rings and for their origin and evolution.Item BARS, OVALS, AND LENSES IN EARLY-TYPE DISK GALAXIES: PROBES OF GALAXY EVOLUTION(IOP Publishing, 2009-01-21) Laurikainen, E.; Salo, H.; Buta, R.; Knapen, J. H.; University of Oulu; University of Alabama Tuscaloosa; Instituto de Astrofisica de CanariasThe origin of S0 galaxies is discussed in the framework of early mergers in a cold dark matter cosmology, and in a scenario where S0s are assumed to be former spirals stripped of gas. From an analysis of 127 early-type disk galaxies (S0-Sa), we find a clear correlation between the scale parameters of the bulge (r(eff)) and the disk (h(R)), a correlation which is difficult to explain if these galaxies were formed in mergers of disk galaxies. However, the stripping hypothesis, including quiescent star formation, is not sufficient to explain the origin of S0s either, because it is not compatible with our finding that S0s have a significantly smaller fraction of bars (46% +/- 6%) than their assumed progenitors, S0/a galaxies (93% +/- 5%) or spirals (64%-69%). Our conclusion is that even if a large majority of S0s were descendants of spiral galaxies, bars and ovals must play an important role in their evolution. The smaller fraction particularly of strong bars in S0 galaxies is compensated by a larger fraction of ovals/lenses (97% +/- 2% compared to 82%-83% in spirals), many of which might be weakened bars. We also found massive disklike bulges in nine of the S0 galaxies, which might have formed at an early gas-rich stage of galaxy evolution.Item Counter-winding spiral structure in ESO 297-27(IOP Publishing, 2008-07-31) Grouchy, R. D.; Buta, R.; Salo, H.; Laurikainen, E.; Speltincx, T.; University of Alabama Tuscaloosa; University of OuluESO 297-27, an intermediate-to-late-type spiral galaxy, exhibits a rare counter-winding spiral structure in which two sets of nonoverlapping arms open in opposite senses. We imaged ESO 297-27 along with 41 other galaxies for a survey of the measurable properties of nonbarred ringed galaxies, which is the category to which the galaxy is assigned in the Catalogue of Southern Ringed Galaxies. We analyzed broadband, narrowband, and spectroscopic data of ESO 297-27 to measure and update its physical parameters. Using a rotation curve obtained from slit spectra of the galaxy's Ha emission in combination with the Hubble-de Vaucouleurs dust asymmetry method, we determined that a single inner spiral arm is leading while at least three weak outer arms are trailing. We believe that none of the galaxy's catalogued companions are strong candidates for distorting the galaxy gravitationally. However, an unclassified companion may have perturbed ESO 297-27 in the past, but velocity data are needed to confirm this. To date, we have identified ESO 297-27 as only the second clear-cut example of a counter-winding spiral pattern, suggesting either that such patterns are very transient or that extraordinary circumstances are needed to produce them. It is important to emphasize that counter-rotation is not necessarily synonymous with a counter-winding spiral structure. Currently, there are no known cases of counter-rotation that show counter-winding spirals as strong as what is seen in ESO 297-27.Item DECREASED FREQUENCY OF STRONG BARS IN S0 GALAXIES: EVIDENCE FOR SECULAR EVOLUTION?(IOP Publishing, 2010-08-26) Buta, R.; Laurikainen, E.; Salo, H.; Knapen, J. H.; University of Alabama Tuscaloosa; University of Oulu; University of Turku; Instituto de Astrofisica de Canarias; Universidad de la LagunaUsing data from the Near-Infrared S0 Survey of nearby, early-type galaxies, we examine the distribution of bar strengths in S0 galaxies as compared to S0/a and Sa galaxies, and as compared to previously published bar strength data for Ohio State University Bright Spiral Galaxy Survey spiral galaxies. Bar strengths based on the gravitational torque method are derived from 2.2 mu m K(s)-band images for a statistical sample of 138 (98 S0, 40 S0/a, Sa) galaxies having a mean total blue magnitude < BT > <= 12.5 and generally inclined less than 65 degrees. We find that S0 galaxies have weaker bars on average than spiral galaxies in general, even compared to their closest spiral counterparts, S0/a and Sa galaxies. The differences are significant and cannot be entirely due to uncertainties in the assumed vertical scale heights or in the assumption of constant mass-to-light ratios. Part of the difference is likely simply due to the dilution of the bar torques by the higher mass bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of gas accretion according to this theory. If S0s are stripped former spirals, or else are evolved from former spirals due to internal secular dynamical processes which deplete the gas as well as grow the bulges, then the weaker bars and the prevalence of lenses in S0 galaxies could further indicate that bar evolution continues to proceed during and even after gas depletion.Item Fourier dissection of early-type galaxy bars(IOP Publishing, 2006-09-26) Buta, R.; Laurikainen, E.; Salo, H.; Block, D. L.; Knapen, J. H.; University of Alabama Tuscaloosa; University of Oulu; University of Witwatersrand; University of HertfordshireThis paper reports on a near-infrared survey of early-type galaxies designed to provide information on bar strengths, bulges, disks, and bar parameters in a statistically well-defined sample of S0-Sa galaxies. Early-type galaxies have the advantage that their bars are relatively free of the effects of dust, star formation, and spiral structure that complicate bar studies in later type galaxies. We describe the survey and present results on a detailed analysis of the relative Fourier intensity amplitudes of bars in 26 early-type galaxies. We also evaluate the "symmetry assumption'' of these amplitudes with radius, used recently for bar-spiral separation in later type galaxies. The results show a wide variety of radial Fourier profiles of bars, ranging from simple symmetric profiles that can be represented in terms of a single Gaussian component to both symmetric and asymmetric profiles that can be represented by two overlapping Gaussian components. More complicated profiles than these are also found, often due to multiple barlike features including extended ovals or lenses. Based on the gravitational bar torque indicator Q(b), double-Gaussian bars are stronger on average than single-Gaussian bars, at least for our small sample. We show that published numerical simulations in which the bar transfers a large amount of angular momentum to the halo can account for many of the observed profiles. The range of possibilities encountered in models seems well-represented in the observed systems.Item The kinematically measured pattern speeds of NGC 2523 and NGC 4245(IOP Publishing, 2007-07-26) Treuthardt, P.; Buta, R.; Salo, H.; Laurikainen, E.; University of Alabama Tuscaloosa; University of OuluWe have applied the Tremaine-Weinberg continuity equation method to derive the bar pattern speed in the SB(r) b galaxy NGC 2523 and the SB(r) 0/a galaxy NGC 4245 using the calcium triplet absorption lines. These galaxies were selected because they have strong inner rings which can be used as independent tracers of the pattern speed. The pattern speed of NGC 2523 is 26.4 +/- 6.1 kms(-1) kpc(-1), assuming an inclination of 49.7 degrees and a distance of 51.0 Mpc. The pattern speed of NGC 4245 is 75.5 +/- 31.3 km s(-1) kpc(-1), assuming an inclination of 35.4 degrees and a distance of 12.6 Mpc. The ratio of the corotation radius to the bar radius of NGC 2523 and NGC 4245 is 1.4 +/- 0.3 and 1.1 +/- 0.5, respectively. These values place the bright inner rings near and slightly inside the corotation radius, as predicted by barred galaxy theory. Within the uncertainties, both galaxies are found to have fast bars that likely indicate dark halos of low central concentration. The photometric properties, bar strengths, and disk stabilities of both galaxies are also discussed.Item Near-infrared atlas of S0-Sa galaxies (NIRS0S)(Wiley-Blackwell, 2011-12-11) Laurikainen, E.; Salo, H.; Buta, R.; Knapen, J. H.; University of Oulu; University of Turku; University of Alabama Tuscaloosa; Instituto de Astrofisica de Canarias; Universidad de la LagunaWe present an atlas of Ks-band images of 206 early-type galaxies, including 160 S0-S0/a galaxies, 12 ellipticals and 33 Sa galaxies (+ one later type). The majority of the atlas galaxies belong to a magnitude-limited (mB = 12.5 mag) sample of 185 Near-InfraRed S0 Survey galaxies. To ensure that misclassified S0s are not omitted, 25 ellipticals from the Third Reference Catalogue of Bright Galaxies classified as S0s in the Carnegie Atlas were included in the sample. The observations were carried out using 34 m class telescopes with subarcsecond pixel resolution (similar to 0.25 arcsec), and were obtained in good seeing conditions (full width at half-maximum similar to 1 arcsec). The images are 23 mag deeper than Two-Micron All-Sky Survey images, allowing the detection of faint outer discs in S0s. Both visual and photometric classifications are made, largely following the classification criteria of de Vaucouleurs. Special attention is paid to the classification of lenses, which are coded in a more systematic manner than in any of the previous studies. A new lens type, called a barlens, is introduced, possibly forming part of the bar itself. Also, boxy/peanut/x-shaped structures are identified in many barred galaxies, even though the galaxies are not seen edge-on, indicating that vertical thickening is not enough to explain these structures. Photometric classification includes detection of exponential outer discs or other structures not directly visible in the images, but becoming clear in unsharp masking or residual images in decompositions. In our photometric classification, nuclear bars are assigned for 15 galaxies, which are overshadowed by bulges in visual classification. The mean Hubble stage in the near-infrared is found to be similar to that in the optical. We give dimensions of structure components, and radial profiles of the position angles and ellipticities, and show deviations from perfect elliptical isophotes. Shells and ripples, generally assumed to be manifestations of recent mergers, are detected only in six galaxies. However, multiple lenses appear in as much as 25 per cent of the atlas galaxies, which is a challenge to the hierarchical evolutionary picture of galaxies. Such models need to explain how the lenses were formed and then survived in multiple merger events that galaxies may have suffered during their lifetimes.Item Overview of the Near-IR S0 Galaxy Survey (NIRS0S)(Hindawi, 2011) Laurikainen, E.; Salo, H.; Buta, R.; Knapen, J. H.; University of Oulu; University of Turku; University of Alabama Tuscaloosa; Instituto de Astrofisica de Canarias; Universidad de la LagunaAn overview of the results of the near-IR S0 galaxy survey (NIRS0S) is presented. NIRS0S is a magnitude- (m(B) <= 12.5 mag) and inclination-(<65 degrees) limited sample of similar to 200 nearby galaxies, mainly S0s, but includes also Sa and E galaxies. It uses deep K-s-band images, typically reaching a surface brightness of 23.5 mag arcsec(-2). Detailed visual and photometric classifications were made, for the first time coding also the lenses in a systematic manner. As a comparison sample, a similar sized spiral galaxy sample with similar image quality was used. Based on our analysis, the Hubble sequence was revisited: following the early idea by van den Bergh we suggested that the S0s are spread throughout the Hubble sequence in parallel tuning forks as spirals (S0a, S0b, S0c, etc.). This is evidenced by our improved bulge-to-total (B/T) flux ratios in the S0s, reaching small values typical to late-type spirals. The properties of bulges and disks in S0s were found to be similar to those in spirals and, also, the masses and scale parameters of the bulges and disks to be coupled. It was estimated that the spiral bulges brighter than -20 mag in K-band are massive enough to be converted into the bulges of S0s merely by star formation. Bars were found to be fairly robust both in S0s and spirals, but, inspite of that, bars might evolve significantly within the Hubble sequence.Item Photometric scaling relations of lenticular and spiral galaxies(Oxford University Press, 2010-06-11) Laurikainen, E.; Salo, H.; Buta, R.; Knapen, J. H.; Comeron, S.; University of Oulu; University of Turku; University of Alabama Tuscaloosa; Instituto de Astrofisica de CanariasPhotometric scaling relations are studied for S0 galaxies and compared with those obtained for spirals. New two-dimensional multi-component decompositions are presented for 122 early-type disc galaxies, using deep K-s-band images. Combining them with our previous decompositions, the final sample consists of 175 galaxies (Near-Infrared Survey of S0s, NIRS0S: 117 S0s + 22 S0/a and 36 Sa galaxies). As a comparison sample we use the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS) of nearly 200 spirals, for which similar multi-component decompositions have previously been made by us. The improved statistics, deep images and the homogeneous decomposition method used allow us to re-evaluate the parameters of the bulges and discs. For spirals we largely confirm previous results, which are compared with those obtained for S0s. Our main results are as follows. (1) Important scaling relations are present, indicating that the formative processes of bulges and discs in S0s are coupled [e.g. M-K(0)(disc) = 0.63 M-K(0)(bulge) -9.3], as has been found previously for spirals [for OSUBSGS spirals M-K(0)(disc) = 0.38 M-K(0)(bulge) -15.5; the rms deviation from these relations is 0.5 mag for S0s and spirals]. (2) We obtain median r(eff)/h(r)(0)similar to 0.20, 0.15 and 0.10 for S0, S0/a-Sa and Sab-Sc galaxies, respectively: these values are smaller than predicted by simulation models in which bulges are formed by galaxy mergers. (3) The properties of bulges of S0s are different from the elliptical galaxies, which are manifested in the M-K(0)(bulge) versus r(eff) relation, in the photometric plane (mu(0), n, r(eff)), and to some extent also in the Kormendy relation ((eff) versus r(eff)). The bulges of S0s are similar to bulges of spirals with M-K(0)(bulge) < -20 mag. Some S0s have small bulges, but their properties are not compatible with the idea that they could evolve to dwarfs by galaxy harassment. (4) The relative bulge flux (B/T) for S0s covers the full range found in the Hubble sequence, even with 13 per cent having B/T< 0.15, typical for late-type spirals. (5) The values and relations of the parameters of the discs [h(r)(0), M-K(0)(disc), mu(0)(0)] of the S0 galaxies in NIRS0S are similar to those obtained for spirals in the OSUBSGS. Overall, our results support the view that spiral galaxies with bulges brighter than -20 mag in the K band can evolve directly into S0s, due to stripping of gas followed by truncated star formation.Item Properties of bars and bulges in the Hubble sequence(Wiley-Blackwell, 2007-09-17) Laurikainen, E.; Salo, H.; Buta, R.; Knapen, J. H.; University of Oulu; University of Alabama Tuscaloosa; Instituto de Astrofisica de Canarias; University of HertfordshireProperties of bars and bulges in the Hubble sequence are discussed, based on an analysis of 216 disc galaxies of S0-Sm types (S0s from the Near-Infrared S0 Survey and spirals from the Ohio State University Bright Spiral Galaxy Survey). For this purpose we have collected, and completed when necessary, the various analyses we have previously made separately for early- and late-type galaxies. We find strong evidence of pseudo-bulges in all Hubble types. Pseudo-bulges are disc-like structures formed by secular evolutionary processes in galaxies. Similar to spirals, the early-type disc galaxies (S0-S0/a) have on average relatively exponential bulges with Sersic index n < 2, and 56 per cent of them show disc-like fine structures in the region of the bulge. For some of the galaxies there is also kinematic evidence of pseudo-bulges. If S0-S0/a galaxies were once spirals, stripped of their gas, then redistributed gas and star formation in the disc would be a natural explanation for all pseudo-bulges in the Hubble sequence. However, it is difficult to explain how the bulges of S0 galaxies, which typically include about 30 per cent of the total galaxy mass, were formed by secularly induced central star formation. A more likely explanation is that pseudo-bulges in barred early-type galaxies are a combination of secularly induced star formation and the central steepening of the old stellar distribution. Bulges in non-barred early-type galaxies could be either classical merger-built bulges, or pseudo-bulges formed by similar processes as in barred galaxies, but in response to massive ovals or lenses (70 per cent of S0-S0/a galaxies have ovals/lenses). Observational support for the outlined picture comes from the fact that bars in early-type galaxies seem more evolved: their bars are long and massive and frequently (40 per cent) have ansae morphologies. In this scenario it would be possible also to explain why barred early-type galaxies (preferentially pseudobulges) have slightly smaller B/T flux ratios than the non-barred early-type galaxies (mostly classical bulges).Item RING STAR FORMATION RATES IN BARRED AND NONBARRED GALAXIES(IOP Publishing, 2010-05-10) Grouchy, R. D.; Buta, R. J.; Salo, H.; Laurikainen, E.; National Optical Astronomy Observatory; University of Alabama Tuscaloosa; University of Oulu; University of TurkuNonbarred ringed galaxies are relatively normal galaxies showing bright rings of star formation in spite of lacking a strong bar. This morphology is interesting because it is generally accepted that a typical galactic disk ring forms when material collects near a resonance, set up by the pattern speed of a bar or bar-like perturbation. Our goal in this paper is to examine whether the star formation properties of rings are related to the strength of a bar or, in the absence of a bar, to the non-axisymmetric gravity potential in general. For this purpose, we obtained Ha emission line images and calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA galaxies and four weakly barred SAB galaxies with rings. For comparison, we combine our new observations with a re-analysis of previously published data on five SA, seven SAB, and 15 SB galaxies with rings, three of which are duplicates from our sample. With these data, we examine what role a bar may play in the star formation process in rings. Compared to barred ringed galaxies, we find that the inner ring SFRs and H alpha+[N II] equivalent widths in nonbarred ringed galaxies show a similar range and trend with absolute blue magnitude, revised Hubble type, and other parameters. On the whole, the star formation properties of inner rings, excluding the distribution of H II regions, are independent of the ring shapes and the bar strength in our small samples. We confirm that the deprojected axis ratios of inner rings correlate with maximum relative gravitational force Q(g); however, if we consider all rings, a better correlation is found when a local bar forcing at the radius of the ring, Q(r), is used. Individual cases are described and other correlations are discussed. By studying the physical properties of these galaxies, we hope to gain a better understanding of their placement in the scheme of the Hubble sequence and how they formed rings without the driving force of a bar.Item Statistics of the structure components in S0s: implications for bar-induced secular evolution(Oxford University Press, 2013-02-26) Laurikainen, E.; Salo, H.; Athanassoula, E.; Bosma, A.; Buta, R.; Janz, J.; University of Oulu; University of Turku; Centre National de la Recherche Scientifique (CNRS); CNRS - National Institute for Earth Sciences & Astronomy (INSU); UDICE-French Research Universities; Aix-Marseille Universite; University of Alabama Tuscaloosa; Ruprecht Karls University HeidelbergThe fractions and dimension of bars, rings and lenses are studied in the near-infrared (IR) S0 galaxy survey, which is a sample of similar to 200 early-type disc galaxies, mainly S0s. We find evidence that multiple lenses in some barred S0s are related to bar resonances in a similar manner as the inner and outer rings, for which the outer/inner length ratio is similar to 2. Inner lenses in the non-barred galaxies normalized to galaxy diameter are clearly smaller than those in the barred systems. Interestingly, these small lenses in the non-barred galaxies have similar sizes as barlenses (lens-like structures embedded in a bar), and therefore might actually be barlenses in former barred galaxies, in which the outer, more elongated bar component has been destroyed. We also find that fully developed inner lenses are on average a factor of 1.3 larger than bars, whereas inner rings have similar sizes as bars. The fraction of inner lenses is found to be constant in all family classes (A, AB, B). Nuclear bars appear most frequently among the weakly barred (AB) galaxies, which are consistent with the theoretical models by Maciejewski & Athanassoula. Similar-sized bars as the nuclear bars were detected in seven 'non-barred' S0s. Galaxy luminosity does not uniquely define the sizes of bars or bar-related structures, neither is there any upper limit in galaxy luminosity for bar formation. Although all the family classes cover the same range of galaxy luminosity, the non-barred (A) galaxies are on average 0.6 mag brighter than the strongly barred (B) systems. Overall, our results are consistent with the idea that bars play an important role in the formation of the structure components of galaxies. The fact that multiple lenses are common in S0s, and that at least the inner lenses can have very old stellar populations, implies that the last destructive merger or major gas accretion event, must have taken place at a fairly high redshift.