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Surface photometry of LSB edge-on galaxies
Dmitry Bizyaev1, and Serafim Ka jsin3
2

arXiv:astro-ph/0306190 v1 10 Jun 2003

ABSTRACT We present results of surface photometry for eleven edge-on galaxies observed with the 6-m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. The photometric scale length, the scale height, and the central surface brightness of the stellar disks for the galaxies were found using photometric cuts made parallel to the ma jor and the minor axes for each galaxy. We show that four galaxies in our sample that were visually classified as ob jects of lowest surface brightness in the Revised Flat Galaxies Catalog have LSB (low surface brightness) disks. Stellar disks of our LSB galaxies are thinner than HSB (high surface brightness) ones. There is a go o d correlation between the central surface brightness of the stellar disk and its ratio of vertical to radial scales. The ratio of the disk photometric scales enables us to estimate the mass of the spherical galactic subsystem using results from numerical mo deling. Combining our results with published rotation curves we determined the mass of dark halos for the galaxies in our sample. The LSB galaxies tend to harbor relatively more massive spherical subsystems than those of HSB's. Indeed, we found no systematic difference between our LSB and HSB ob jects in dark halo masses considering it separately from the bulge. At the same time, the inferred mass/luminosity ratio for the LSB disks appears to be systematically higher, when compared to the ratio for HSB ones.
1 2 3

Physics Department, University of Texas at El Paso, TX 79968 Sternberg Astronomical Institute, Moscow, 119899, Russia, dmbiz@sai.msu.ru

Special Astrophysical Observatory of RAS, pos. Nizhnij Arkhyz, 357147, Karachaevo-Cherkessia, Russia, ska j@rebus.sao.ru


­2­ 1. Introduction

One of the main features of the low surface brightness galaxies (LSB) which make them different from the "usual" high surface brightness (HSB) galaxies is that they are considered to be dark matter dominated ob jects. We conducted the study of a sample of several edge-on LSB and HSB galaxies in order to compare their photometric parameters (including the stellar disk thickness) and the relative mass of their dark halos.

2.

Sample of galaxies and observations

Our sample consists of several ob jects taken from the Revised Catalog of Flat Galaxies ([1], RFGC hereafter). All those galaxies included to the catalog are highly inclined ob jects. We selected four ob jects of faintest surface brightness class as candidates to the LSB galaxies (according to [2]), and seven ob jects with intermediate and high surface brightness class as "reference" HSB ob jects. The surface photometry observations were made with the 6-m telescope at the Special Astrophysical Observatory (Russia). The large aperture of the telescope provided us with high angular resolution in images (0".2/pix) together with a go o d sensitivity to the very faint regions of the galaxies. The chosen galaxies and the set of calibrating frames were taked during one observing run in December 2000 using R Johnson-Cousins photometric band. The combined and calibrated R images were used to obtain the radial scale length of a galactic disk (h), the disk vertical scale heights (z0 , assuming sech2 law), the "face-on" central surface brightness (µ0 ) of the disk (see [3] for the details), and the bulge to disk luminosity ratio Lb/Ld. The final value of µ0 was corrected for the extinction in our Galaxy (according to the LEDA). The fitting profiles have been convolved with the atmospheric smearing function. The distribution of µ0 values indicates the presence of two subsamples: those with µ0 values greater than 23.5 mag /ar csec2 which we defined as LSB and those with higher surface brightness designated HSB in this work. Note that all galaxies of the faintest surface brightness class (according to the RFGC) were included to our LSB ob jects. Although our sample enables us to HSB disks, the sample is very limited. galaxies whose photometric parameters authors used the similar passband and t compare of the photometric parameters of LSB and We also incorporated one more sample of edge-on have been published by Barteldrees etc. [4]. The he same fitting functions to extract the photometric


­3­ parameters.

3.

LSB versus HSB: the vertical scale height of galactic disk as a new feature to compare.

As it was shown in [2] and [5], the galaxies of lower surface brightness tend to show the higher z0 /h ratio. However, mostly HSB galaxies were considered in those previous studies. Here we show the dependence of z0 /h on the central surface brightness µ0 for our sample. Our galaxies are denoted by the squares in Fig.1. The open squares are for the HSB subsample whereas the filled ones designate the LSB galaxies. The galaxies taken from [4] are plotted as crosses. Futhermore, the near-infrared (K band) sample of edge-on galaxies from [5] is available for the comparison (the 2MASS sample hereafter). The systematic difference between the R and K images in z0 /h (1.4 times, see [6] for details), in the surface brightness (2.4 mag , see [7]), and the internal extinction (1.2 mag ) were taken into account. Fig.1 shows all three samples together. The 2MASS sample is denoted by the small filled triangles. A trend in Fig.1 is seen well, in average of 2 mag difference in µ0 leads to 1.5 change in the ratio of the scales. Note that there is no correlation seen when h or z0 were drawn against µ0 separately. Following [6], we calculated the ratio of total mass Mt to B-band disk luminosity LB : Mt = 0.5G 4h Vm 2 /LB . Here Vm is the maximum of rotation curve (taken from the LEDA). The value of LB is inferred from the absolute B-magnitude of a galaxy taken from the LEDA and corrected for the internal absorption. In Fig. 3a one can see the values of Mt /LB plotted versus the ratio z0 /h. The notation in the figure is the same as in Fig.1. The three curves in Fig.3a are the same as in Fig.2 which were recalculated for the values of the mass to luminosity ratio M /L of 1, 2 and 4. As it is seen in Fig.3a, most of the galaxies have a reasonable value of M /L between 1 and 4. The mass to luminosity ratio is higher systematically for our LSB galaxies in comparison with HSB ones. Since the luminosity ratio Lb/Ld reflects the mass ratio, the mass of the dark halo Mh can be estimated from the dependence in Fig. 3. We drew the ratio of dark to luminous matter Mh /(Mb + Md ) for our galaxies ishown n Fig.3b. Here we designate the bulge mass as Mb . We show the ratio of "dark" to "luminous" mass Mh /(Mb + Md ) for our galaxies. Here we designate the bulde mass as Mb . We assume that the bulge and the disk have roughly the same M /L. Surprisingly, there is no systematic difference between the values of Mh /(Mb + Md ) for the galaxies of different central surface brightness. We also enable to compare the mass of the spherical subsystem Ms for our galaxies. In


­4­ Fig. 3c we present how the ratio Ms /Md depends on the disk central surface brightness. We kept the same notation as in Fig. 1 and Fig. 3a. The figure indicates that the LSB galaxies do not have the most massive dark matter halos, but have the most massive spherical subsystem. That supports the result by Graham ([10]) that not all LSB galaxies are the dark-matter dominated ob jects.

4.

Conclusions

1) We present the results of the photometric observations made for the sample of edge-on galaxies. Our sample contains four LSB galaxies as well as seven HSB ones. The photometric disk scales (vertical and radial), the disk central surface brightness and the bulge to disk luminosity ratio were determined. 2) Stellar disks of our LSB galaxies are thinner than HSB ones. There is a go o d correlation between their central surface brightness and their vertical to radial scales ratio. 3) Our LSB galaxies tend to harbor the massive spherical subsystems as well as values of the mass-to-luminosity ratio in their disks when compared to the Nevertheless, the dark halo is not strictly the most massive subsystem in our The LSB galaxies appear to be the spherical dominated systems, but not the dominated" ones. Acknowledgments D.B. is supported by NASA/JPL through grant NRA-99-04-OSS-058. The pro ject was partially supported by Russian Foundation for Basic Research via grant 01-02-17597. We have made use of the LEDA database. REFERENCES
[1] Karachentsev, I., Karachentseva, V., Kudrya, Y., et al.// 1999, Bull. Of Sp ecial Astrophys. Obs., 1999, 47, 5, (RFGC) [2] Bizyaev, D.// 2000, Astron. Lett. 26, 219. [3] van der Kruit, P., Searle, L. // 1981, A&A, 95, 105 and 116 [4] Barteldrees, A., Dettmar, R.-J. // 1994, A&AS, 103, 475 [5] Bizyaev, D., Mitronova, S. // 2002, A&A, 389, 795

to havehigher HSB ob jects. LSB galaxies. "dark matter


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[6] Zasov, A., Bizyaev, D., Makarov, D. etc. // 2002, Astron.Lett. 28, 527 [7] de Jong, R. // 1996, A&A, 313, 45 [8] Zasov, A., Makarov, D., Mikhailova, E. // 1991, Astron.Lett., 17, 374 [9] Mikhailova, E., Khop erskov, A., Sharpak, S. // 2001, in Stellar Dynamics: From Classic to Modern, Ed. by L.Ossipkov and I.Nikiforov, p. 147 [10] Graham, A. // 2002, MNRAS, 334, 721


­6­ FIGURE CAPTIONS Fig.1. The vertical to radial scale ratio z0 /h is shown against the disk central surface density µ0 . The open squares are for the HSB subsample, the filled ones designate our LSB galaxies. The galaxies taken from [4] are shown by crosses, the 2MASS sample is designated by the small filled triangles. Fig.2. The ratio o d scales z0 /h is plotted versus the relative mass of the spherical galactic subsystem Ms /Md according to the numerical simulations described in [9]. Fig.3. a) The scales same notation applies recalculated for M /L of M/L is systematica ratio z0 /h and t here as in Fig.1. values of 1 (solid lly higher for our he total mass to disk luminosity rati Mt /LB . The The three curves show the mo del results (see Fig.2) curve), 2 (dash-dotted) and 4 (dashed). The value LSB galaxies.

b) The ratio of dark to luminous mass Mh /(Mb + M d) is plotted against the disk central surface brightness µ0 with the same notation as in Fig. 3a. There is no systematic difference in the relative dark matter mass between the LSB and HSB galaxies. c) The ratio of the spherical to disk mass Ms /Md is shown in dependence on the disk central surface brightness. The LSB galaxies have the more massive spherical subsystem when compared to HSB ones.


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Fig. 1.-- .


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Fig. 2.-- .


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Fig. 3.-- .