Printed by BAR STRENGTHS IN EARLY-TYPE DISK GALAXIES R. Buta (U. Alabama), E. Laurikainen (U....
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printed by www.postersession.com BAR STRENGTHS IN EARLY-TYPE DISK GALAXIES R. Buta (U. Alabama), E. Laurikainen (U. Oulu), H. Salo (U. Oulu), J. H. Knapen (IAC, Tenerife), T. Speltincx (U. Oulu), D. L. Block (U. Witwatersrand) Why study bar strengths in S0 galaxies? Bars are a major morphological feature spirals and S0s have in common Bars in early-type galaxies usually have little gas or star formation and are good cases to compare with sophisticated n- body simulations such as those of Athanassoula (2003) The distribution of bar strengths in S0s may have clues to S0 evolution and origin. What is the “Near-Infrared S0 Survey” (NIRS0S)? (http://bama.ua.edu/~rbuta/nirs0s/nirs0s.html) a statistically well-defined database of near-IR (2.2 micron K s -band) images of ~200 S0-Sa galaxies having total blue magnitude B T <= 12.5 and isophotal axis ratio logR 25 <= 0.35. The Survey asks: How strong do S0 bars get compared to spiral bars? How does the distribution of S0 bar strengths compare to spirals? What characterizes the morphology of early-type galaxy bars? The Survey is complemented by optical B, V images to facilitate derivation of orientation parameters, and the sample crosses the “spiral/S0 divide” (types S0/a to Sa) to evaluate continuity of bar properties. Bulge-to-total light ratios of S0-Sa galaxies - based on the detailed 2D decompositions, SDSS i-band images confirm that these early types have a lower bulge- to-total light ratio than previously thought. Previous estimates (e.g., Simien and de Vaucouleurs 1986) gave a ratio of 0.5-0.6; instead, we obtained <B/T> = 0.30+/-0.18 for 53 galaxies when the contributions of bars and ovals are taken into account. This agrees with the value of 0.25+/-0.10 obtained by Laurikainen et al. (2005, 2006) using the same multi-component approach with K s -band images. At least 4 of our SDSS NIRS0S galaxies are unusual star- forming early-types having no measurable bulge. If these are included, we get <B/T>=0.28+/-0.21 (n=57). Pseudobulges in S0-Sa galaxies - The mean Sersic index for 53 SDSS NIRS0S galaxies is <n>=1.9+/-0.6, again in agreement with Laurikainen et al. (2005 2006), implying a preponderance of “pseudobulges” (Kormendy and Kennicutt 2004). Can SDSS i-band images be used for bar strength studies? - Fig. 4 compares Q b estimates from SDSS i-band images with estimates made from K s -band images. The good agreement (<Q b (SDSS i)-Q b (K s )> = -0.003+/-0.045) suggests that SDSS i-band images are a reasonable substitute for K s -band images, at least for early-type galaxies with little dust. Bar Strengths in S0-Sa galaxies – With both SDSS and published K s -band bar strengths from Buta et al. (2006) and Laurikainen et al. (2006), we now have a NIRS0S subsample large enough to provide a distribution of bar strengths for early-type galaxies that may be compared to that for spirals (Fig. 5). When restricted to the 54 S0s currently in the total sample, we find no cases having Q b > 0.25. This is in contrast to the extended tail which reaches Q b ~ 0.7 for both OSUBGS (Eskridge et al. 2002) and NIRS0S early-type spirals. - g- and i-band images (Fig. 1) downloaded from SDSS data release website, cleaned of foreground and background objects, and sky-subtracted. IRAF routine ELLIPSE used to derive orientation parameters, surface brightness, and color profiles (Fig. 2). 2D Decompositions (Laurikainen et al. 2005; Fig. 3) - bulges modeled as spheres using Sersic r 1/n law - disks modeled as exponentials - bars modeled as Ferrers function Bar Strength Calculation (Laurikainen et al. 2004) - bar strength computed as Q b =F T max /F 0R , where F T max is the maximum tangential force and F 0R is the mean radial force (Combes and Sanders 1981), inferred from near-IR images assuming a constant mass-to-light ratio. For these early-type galaxies, spiral arm torques are minimal. These are the same procedures used to analyze previously obtained NIRS0S K s -band images. Athanassoula, E. 2003, MNRAS, 341, 1179 Block, D. L., Bournaud, F., Combes, F., Puerari, I., and Buta, R. 2002, A & A, 394, L35 Buta, R. and Block, D. L. 2001, ApJ, 550, 243 Buta, R., Laurikainen, E., Salo, H., Block, D., & Knapen, J. 2006, AJ, 132, 1859 Buta, R., Vasylyev, S., Salo, H., & Laurikainen, E. 2005, AJ, 130, 506 Eskridge, P. et al. 2002, ApJS, 143, 73 Gunn, J. E. et al. 1998, AJ, 116, 3040 Kormendy, J. and Kennicutt, R. 2004, ARAA, 42, 603 Laurikainen, E., Salo, H., Buta, R., and Vasylyev, S. 2004, MNRAS, 355, 1251 Laurikainen, E., Salo, H., and Buta, R. 2005, MNRAS, 362, 1319 Laurikainen, E., Salo, H., Buta, R., Knapen, J., et al. AJ, 132, 2634 Simien, F. and de Vaucouleurs, G. 1986, ApJ, 302, 154 York, D. G. et al. 2000, AJ, 120, 1579 - the SDSS provides a homogeneous database for bar strength studies of galaxies - for 53 early-type SDSS galaxies, the mean Sersic index is <n>=1.9+/-0.6 and the mean bulge-to-total light ratio is 0.30+/-0.18, comparable to results from recent K s -band decomposition studies (Laurikainen et al. 2005, 2006) -the distribution of bar strengths in S0 galaxies (SDSS + published sources) differs from spirals in lacking an extended tail of high values. The difference could mean that S0 galaxies are not accreting any external gas (Block et al. 2002), and hence are not showing the effects of regenerative bars, or that S0s are simply thicker and have higher surface brightness background disks and bulges compared to spirals. and distribution of the SDSS archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, NASA, NSF, the US Dept. of Energy, the Japanese Monbukagakusho, and the Max Planck Society. Funding for the Ohio State University Bright Galaxy Survey was provided by grants from the NSF (grants AST 92-17716 and AST 96-17006), with additional funding from the Ohio State University. Generous allocations of observing time on the NOT, ESO-NTT, WHT and CTIO-Blanco telescope are gratefully acknowledged. Obtaining K s -band images requires a significant investment of telescope time. Can SDSS (York et al 2000; Gunn et al. 1998) i-band (0.8 micron) images be used effectively for our survey? We have prepared SDSS g- and i-band images of a subsample of ~60 NIRS0S galaxies, to: Compare the effectiveness of using SDSS i-band images for bar strength studies Extract parameters for the bulges and disks using ellipse fits and 2D decompositions Deproject images and Fourier-analyze bar intensity profiles Derive bar strengths using the gravitational torque method (GTM, Laurikainen et al. 2004) Combine SDSS i-band and K s -band bar strengths to get a distribution of bar strengths for S0s BACKGROUND OBJECTIVES OF THIS STUDY MATERIALS AND METHODS RESULTS CONCLUSIONS BIBLIOGRAPHY Fig. 1: i-band (left) and g-i color index (right) images of four NIRS0S Fig. 1: i-band (left) and g-i color index (right) images of four NIRS0S galaxies, NGC 4203 (SA(s)0 galaxies, NGC 4203 (SA(s)0 o ), 4293 [(R)SB(s)0/a], 4612 [(R)SAB0 ), 4293 [(R)SB(s)0/a], 4612 [(R)SAB0 o ], and ], and 4754 (SB0 4754 (SB0 - ). The color index maps are coded such that red features are ). The color index maps are coded such that red features are light and blue features are dark. light and blue features are dark. Fig. 2: IRAF routine ELLIPSE was used to derive orientation parameters (left), Fig. 2: IRAF routine ELLIPSE was used to derive orientation parameters (left), surface brightness profiles (middle), and color index profiles relative to the surface brightness profiles (middle), and color index profiles relative to the mean color index (right) for each galaxy. No zero points have been applied to the mean color index (right) for each galaxy. No zero points have been applied to the surface brightness profiles. surface brightness profiles. Fig 3: 2D bar/disk/bulge Fig 3: 2D bar/disk/bulge decomposition of NGC 4754. Blue=bar decomposition of NGC 4754. Blue=bar model, red=bulge model, green=disk model, red=bulge model, green=disk model, light blue=total model. model, light blue=total model. These are used to deproject images These are used to deproject images for bar strength analyses. for bar strength analyses. Fig. 5: Distribution of Q Fig. 5: Distribution of Q b b values for 147 OSUBGS values for 147 OSUBGS spirals (top panel, from Buta et al. 2005) and 93 spirals (top panel, from Buta et al. 2005) and 93 NIRS0S galaxies (bottom panels), of which 54 are NIRS0S galaxies (bottom panels), of which 54 are S0 galaxies and 39 are early-type spirals. S0 galaxies and 39 are early-type spirals. Fig. 4: Comparison between Fig. 4: Comparison between Q Q b values derived from K values derived from K s - - band images and SDSS i-band band images and SDSS i-band images for 15 galaxies. images for 15 galaxies.
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Transcript of Printed by BAR STRENGTHS IN EARLY-TYPE DISK GALAXIES R. Buta (U. Alabama), E. Laurikainen (U....
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BAR STRENGTHS IN EARLY-TYPE DISK GALAXIESR. Buta (U. Alabama), E. Laurikainen (U. Oulu), H. Salo (U. Oulu), J. H. Knapen (IAC, Tenerife), T. Speltincx (U. Oulu), D.
L. Block (U. Witwatersrand)
Why study bar strengths in S0 galaxies?Bars are a major morphological feature spirals and S0s have in commonBars in early-type galaxies usually have little gas or star formation and are good cases to compare with sophisticated n-body simulations such as those of Athanassoula (2003)The distribution of bar strengths in S0s may have clues to S0 evolution and origin.
What is the “Near-Infrared S0 Survey” (NIRS0S)? (http://bama.ua.edu/~rbuta/nirs0s/nirs0s.html)
a statistically well-defined database of near-IR (2.2 micron Ks-band) images of ~200 S0-Sa galaxies having total blue magnitude BT <= 12.5 and isophotal axis ratio logR25 <= 0.35. The Survey asks: How strong do S0 bars get compared to spiral bars? How does the distribution of S0 bar strengths compare to spirals? What characterizes the morphology of early-type galaxy bars? What are the near-IR properties of S0 bulges and disks?
The Survey is complemented by optical B, V images to facilitate derivation of orientation parameters, and the sample crosses the “spiral/S0 divide” (types S0/a to Sa) to evaluate continuity of bar properties.
Bulge-to-total light ratios of S0-Sa galaxies - based on the detailed 2D decompositions, SDSS i-band images confirm that these early types have a lower bulge-to-total light ratio than previously thought. Previous estimates (e.g., Simien and de Vaucouleurs 1986) gave a ratio of 0.5-0.6; instead, we obtained <B/T> = 0.30+/-0.18 for 53 galaxies when the contributions of bars and ovals are taken into account. This agrees with the value of 0.25+/-0.10 obtained by Laurikainen et al. (2005, 2006) using the same multi-component approach with Ks-band images. At least 4 of our SDSS NIRS0S galaxies are unusual star-forming early-types having no measurable bulge. If these are included, we get <B/T>=0.28+/-0.21 (n=57).
Pseudobulges in S0-Sa galaxies - The mean Sersic index for 53 SDSS NIRS0S galaxies is <n>=1.9+/-0.6, again in agreement with Laurikainen et al. (2005 2006), implying a preponderance of “pseudobulges” (Kormendy and Kennicutt 2004).
Can SDSS i-band images be used for bar strength studies? - Fig. 4 compares Qb estimates from SDSS i-band images with estimates made from Ks-band images. The good agreement (<Qb(SDSS i)-Qb(Ks)> = -0.003+/-0.045) suggests that SDSS i-band images are a reasonable substitute for Ks-band images, at least for early-type galaxies with little dust.
Bar Strengths in S0-Sa galaxies – With both SDSS and published Ks-band bar strengths from Buta et al. (2006) and Laurikainen et al. (2006), we now have a NIRS0S subsample large enough to provide a distribution of bar strengths for early-type galaxies that may be compared to that for spirals (Fig. 5). When restricted to the 54 S0s currently in the total sample, we find no cases having Qb > 0.25. This is in contrast to the extended tail which reaches Qb ~ 0.7 for both OSUBGS (Eskridge et al. 2002) and NIRS0S early-type spirals.
- g- and i-band images (Fig. 1) downloaded from SDSS data release website, cleaned of foreground and background objects, and sky-subtracted. IRAF routine ELLIPSE used to derive orientation parameters, surface brightness, and color profiles (Fig. 2).
2D Decompositions (Laurikainen et al. 2005; Fig. 3)
- bulges modeled as spheres using Sersic r1/n law- disks modeled as exponentials- bars modeled as Ferrers function
Bar Strength Calculation (Laurikainen et al. 2004)
-bar strength computed as Qb=FTmax/F0R, where FT
max is the maximum tangential force and F0R is the mean radial force (Combes and Sanders 1981), inferred from near-IR images assuming a constant mass-to-light ratio. For these early-type galaxies, spiral arm torques are minimal.
These are the same procedures used to analyze previously obtained NIRS0S Ks-band images.
Athanassoula, E. 2003, MNRAS, 341, 1179Block, D. L., Bournaud, F., Combes, F., Puerari, I., and Buta, R. 2002, A & A, 394, L35Buta, R. and Block, D. L. 2001, ApJ, 550, 243Buta, R., Laurikainen, E., Salo, H., Block, D., & Knapen, J. 2006, AJ, 132, 1859Buta, R., Vasylyev, S., Salo, H., & Laurikainen, E. 2005, AJ, 130, 506Eskridge, P. et al. 2002, ApJS, 143, 73Gunn, J. E. et al. 1998, AJ, 116, 3040Kormendy, J. and Kennicutt, R. 2004, ARAA, 42, 603Laurikainen, E., Salo, H., Buta, R., and Vasylyev, S. 2004, MNRAS, 355, 1251Laurikainen, E., Salo, H., and Buta, R. 2005, MNRAS, 362, 1319Laurikainen, E., Salo, H., Buta, R., Knapen, J., et al. AJ, 132, 2634Simien, F. and de Vaucouleurs, G. 1986, ApJ, 302, 154York, D. G. et al. 2000, AJ, 120, 1579
- the SDSS provides a homogeneous database for bar strength studies of galaxies- for 53 early-type SDSS galaxies, the mean Sersic index is <n>=1.9+/-0.6 and the mean bulge-to-total light ratio is 0.30+/-0.18, comparable to results from recent Ks-band decomposition studies (Laurikainen et al. 2005, 2006)-the distribution of bar strengths in S0 galaxies (SDSS + published sources) differs from spirals in lacking an extended tail of high values. The difference could mean that S0 galaxies are not accreting any external gas (Block et al. 2002), and hence are not showing the effects of regenerative bars, or that S0s are simply thicker and have higher surface brightness background disks and bulges compared to spirals.
This work was supported by NSF Grant AST 05-07140 and the Academy of Finland. Funding for the creation and distribution of the SDSS archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, NASA, NSF, the US Dept. of Energy, the Japanese Monbukagakusho, and the Max Planck Society. Funding for the Ohio State University Bright Galaxy Survey was provided by grants from the NSF (grants AST 92-17716 and AST 96-17006), with additional funding from the Ohio State University. Generous allocations of observing time on the NOT, ESO-NTT, WHT and CTIO-Blanco telescope are gratefully acknowledged.
Obtaining Ks-band images requires a significant investment of telescope time. Can SDSS(York et al 2000; Gunn et al. 1998) i-band (0.8 micron) images be used effectively for our survey? We have prepared SDSS g- and i-band images of a subsample of ~60 NIRS0S galaxies, to:
Compare the effectiveness of using SDSS i-band images for bar strength studies Extract parameters for the bulges and disks using ellipse fits and 2D decompositions Deproject images and Fourier-analyze bar intensity profiles Derive bar strengths using the gravitational torque method (GTM, Laurikainen et al. 2004) Combine SDSS i-band and Ks-band bar strengths to get a distribution of bar strengths for S0s
BACKGROUND
OBJECTIVES OF THIS STUDY
MATERIALS AND METHODS
RESULTS
CONCLUSIONS
BIBLIOGRAPHY
Fig. 1: i-band (left) and g-i color index (right) images of four NIRS0S galaxies, NGC 4203 (SA(s)0Fig. 1: i-band (left) and g-i color index (right) images of four NIRS0S galaxies, NGC 4203 (SA(s)0oo), ), 4293 [(R)SB(s)0/a], 4612 [(R)SAB04293 [(R)SB(s)0/a], 4612 [(R)SAB0oo], and 4754 (SB0], and 4754 (SB0--). The color index maps are coded such that red ). The color index maps are coded such that red features are light and blue features are dark.features are light and blue features are dark.
Fig. 2: IRAF routine ELLIPSE was used to derive orientation parameters (left), surface brightness profiles (middle), and Fig. 2: IRAF routine ELLIPSE was used to derive orientation parameters (left), surface brightness profiles (middle), and color index profiles relative to the mean color index (right) for each galaxy. No zero points have been applied to the color index profiles relative to the mean color index (right) for each galaxy. No zero points have been applied to the surface brightness profiles.surface brightness profiles.
Fig 3: 2D bar/disk/bulge decomposition of NGC Fig 3: 2D bar/disk/bulge decomposition of NGC 4754. Blue=bar model, red=bulge model, 4754. Blue=bar model, red=bulge model, green=disk model, light blue=total model. These are green=disk model, light blue=total model. These are used to deproject images for bar strength analyses.used to deproject images for bar strength analyses.
Fig. 5: Distribution of QFig. 5: Distribution of Qb b values for 147 OSUBGS spirals (top panel, values for 147 OSUBGS spirals (top panel,
from Buta et al. 2005) and 93 NIRS0S galaxies (bottom panels), of from Buta et al. 2005) and 93 NIRS0S galaxies (bottom panels), of which 54 are S0 galaxies and 39 are early-type spirals.which 54 are S0 galaxies and 39 are early-type spirals.
Fig. 4: Comparison between QFig. 4: Comparison between Qbb values values
derived from Kderived from Kss-band images and SDSS -band images and SDSS
i-band images for 15 galaxies.i-band images for 15 galaxies.
