Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mso.anu.edu.au/~kcf/surya/spiralstructure/Fuchs02 Äàòà èçìåíåíèÿ: Mon Jul 9 16:08:18 2007 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 07:02:02 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: spiral galaxy

Massive Disks in Low Surface Brightness Galaxies
Burkhard Fuchs
Astronomisches Rechen­Institut, M¨nchhofstr. 12-14, 69120 Heidelberg, Germany o Abstract. An up date of the set of low surface brightness galaxies is presented which can b e used to set constraints on the otherwise ambiguous decomp ositions of their rotation curves into contributions due to the various comp onents of the galaxies. The selected galaxies show all clear spiral structure and arguments of density wave theory of galactic spiral arms are used to estimate the masses of the galactic disks. Again these estimates seem to indicate that the disks of low surface brightness galaxies might b e much more massive than currently thought. This puzzling result contradicts stellar p opulation synthesis models. This would mean also that low surface brightness galaxies are not dominated by dark matter in their inner parts. Keywords: low surface brightness galaxies

arXiv:astro-ph/0209157v1 9 Sep 2002

1. Intro duction In a previous pap er (Fuchs 2002) I have describ ed how arguments of density wave theory of galactic spiral arms can b e used to set constraints on the otherwise ambiguous decomp osition of the rotation curves of low surface brightness galaxies (LSBGs). For this purp ose galaxies were selected which show clear spiral structure. These came mainly from the set of LSBGs for which high­resolution rotation curves have b een published by McGaugh et al. (2001). The same authors (de Blok et al. 2001) have also constructed dynamical models of the galaxies. The observed rotation curves were modeled as
2 2 vc (R) = vc, bulge 2 (R) + vc, disk 2 (R) + vc, is gas 2 (R) + vc, halo

(R) ,

(1)

where vc,bulge , vc,disk , vc,is gas , and vc,halo denote the contributions due to the bulge, the stellar disk, the interstellar gas, and the dark halo, resp ectively. De Blok et al. (2001) provide actually for each galaxy several models, one with zero bulge and disk mass, one model with a `reasonable' mass­to­light ratio, and a `maximum­disk' model with bulge and disk masses at the maximum allowed by the data. All fit the data equally well. Applying the density wave theory argument I confirmed essentially the maximum­disk models. This result is puzzling b ecause the mass­to­light ratios of these models are unaccountably high in view of stellar p opulation synthesis modeling of LSBGs (cf. Bell & de Jong 2001). On the other hand, this might indicate that LSBGs are less dark matter dominated than currently thought.
c 2007 Kluwer Academic Publishers. Printed in the Netherlands.

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Figure 1. Image of ESO 206­140 reproduced from Beijersb ergen et al. (1999).

At the time of writing of their pap er there was no surface photometry available for some of the LSBGs in the set of McGaugh et al. (2001), so that no dynamical models could b e constructed. I have insp ected the images of these galaxies and found four galaxies, ESO 14­40, ESO 206­ 140, ESO 301­120, and ESO 425­180, which can b e used for the present purp ose as well (cf. Fig. 1), and contrived to obtain surface photometry of the galaxies.

2. Up date of the Sample of LSBGs In the meantime calibrated surface photometry has b een published by Beijersb ergen et al. (1999) for ESO 14­40, ESO 206­149, and ESO 425­ 180. For ESO 301­120 C. M¨llenhoff has kindly provided uncalibrated o surface photometry by reducing the blue image of the galaxy retrieved from the Digitized Sky Survey (ESO) with his two­dimensional bulge and disk fitting code (M¨llenhoff & Heidt 2001). The reliability of the o bulge and disk parameters determined this way was carefully checked with LSBGs for which calibrated surface photometry is available. The rotation curves of the four ESO galaxies could b e then interpreted following the prescription describ ed in Fuchs (2002). The principal idea is that in galactic disks spiral arms are preferentially amplified with azimuthal wave lengths of the order of the critical wave length (cf. Fuchs 2002 and references therein). From the observed numb er of spiral arms one can thus estimate the critical wave lengths and from these the surface densities of the disks. This constrains the decomp osition of the rotation curves considerably. The resulting dynamical models of

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Massive Disks in LSBGs

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Table I. Extended Sample of LSBGs. vm F 568-1 F 568-3 F 568-6 F 568-V1 UGC 128 UGC 1230 UGC 6614 ESO 14-40 ESO 206-140 ESO 302-120 ESO 425-180
ax

vd

isk,max

Md

isk

Mi

sgas

M /LR 14 7 11 16 4 6 8 4 4 1.7 2.4 M /LR


140 100 300 113 131 103 210 272 119 89 140 km/s

115 87 260 89 102 77 160 205 93 70 120 km/s

42 18 740 15 43 16 130 250 27 11 62 109 M

3 2 50 2 9 8 35 83 10 11 38 109 M

the LSBGs are summarized in Table 1. The second column gives the p eaks of the observed rotation curves, while the third column gives the p eaks of the combined contributions due to the stellar disks and the interstellar gas. Mass estimates of the combined star and gas disks of the LSBGs are given in the fourth column. The masses of the gas disks are given separately in the fifth column. The gas masses of the ESO galaxies have not b een measured, but have b een estimated using a relation based on the I­band luminosities of LSBGs derived by Schomb ert et al. (2001). Similar to the other galaxies the inferred mass­to­light ratios of the four ESO galaxies turn out to b e also much higher than exp ected from p opulation synthesis models, although they range at the low end of the sp ectrum of mass­to­light ratios found for the sample of LSBGs studied here. A p ossible exception is ESO 302­120, which app ears also to b e extremely gas rich, Mgas Mdisk . As can b e seen from Table 1 all dynamical models are maximum disk models. Quillen & Pickering (1997) have derived mass­to­light ratios for the disks of F 568­6 and UGC 6614 by analyzing spiral arm induced p erturbations of the velocity fields of the interstellar gas in the galaxies. Following this different approach they find also high mass­to­light ratios of the disks which agree well with the estimates given in Table 1. Thus the augmented set of galaxies seems to confirm the evidence for massive disks in LSBGs. This does not imply, however, that the disks of LSBGs are more massive than the disks of comparable intrinsically bright galaxies. De Blok & McGaugh (1996) have p ointed out that

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the bright galaxy NGC 2403 is very similar to the LSBG UGC 128. Both have ab out the same physical size, their rotation curves have the same amplitudes, and b oth show well develop ed two­armed spiral structure. Thus the maximum disk of NGC 2403 has the same mass as the maximum disk of UGC 128. Only their mass­to­light ratios are different. But the nature of the dim comp onents in the disks of LSBGs remains still totally unclear. The relation of Schomb ert et al. (2001) implies that the gas mass of a LSBG is approximately prop ortional to the luminosity of the LSBG. Thus M /L M /Mgas , which might indicate that LSBGs with high mass­to­light ratios have consumed by star formation more of their interstellar gas.

Acknowledgements I am indebted to Claus M¨llenhoff for reducing images of LSBGs reo trieved from the Digitized Sky Survey.

References
Beijersb ergen, M. , de Blok, W. J. G. and van der Hulst, J. M. Surface Photometry of Bulge dominated Low Surface Brightness Galaxies. Astron. Astrophys., 351: 903­919, 1999. Bell, E. F. and de Jong, R. S. Stellar Mass­to­Light Ratios and the Tully­Fisher Relation. Astrophys. J., 550: 212­229, 2001. De Blok, W. J. G. and McGaugh, S. S. Does Low Surface Brightness mean Low Density?. Astrophys. J., 469: L89­L92, 1996. De Blok, W. J. G. , McGaugh, S. S. and Rubin V. High­Resolution Rotation Curves of Low Surface Brightness Galaxies. I I. Mass Models. Astron. J., 122: 2396­2427, 2001. Fuchs, B. Dim Matter in the Disks of Low Surface Brightness Galaxies. In H. V. Klap dor­Kleingrothaus and R. Viollier, editors, Proceedings of the Fourth International Workshop on Dark Matter in Astro­ and Particle Physics, Cap etown, South Africa, February 2002. Springer, Berlin, astro-ph/0204387. McGaugh, S. S. , Rubin V. and de Blok, W. J. G. High­Resolution Rotation Curves of Low Surface Brightness Galaxies. I. Data. Astron. J., 122: 2381­2395, 2001. M¨llenhoff, C. and Heidt, J. Surface Photometry of Spiral Galaxies in NIR: o Structural Parameters of Disks and Bulges. Astron. Astrophys., 368: 16­37, 2001. Quillen, A. C. and Pickering, T. E. Spiral Structure based Limits on the Disk Mass of the Low Surface Brightness Galaxies UGC 6614 and F 568­6. Astron. J., 113: 2075­2086, 1977. Schomb ert, J. M. , McGaugh, S. S. and Eder, J. A. Gas Mass Fractions and the Evolution of Low Surface Brightness Dwarf Galaxies. Astron. J., 121: 2420­2430, 2001.

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