Astronomical Research - RAINER LÜTTICKE

I have written my diploma thesis at the Astronomisches Institut der Ruhr-Universität Bochum (AIRUB) about surface photometry of edge-on disk galaxies and therein specially investigated box- and peanut-shaped bulges (e.g. IC 2531).

From June 1996 to May 1999 I worked as Ph.D. student as a member of the "Magellanic Cloud and Other Dwarf Galaxies" research project. (Graduiertenkolleg in Bochum and Bonn [Homepage] ). There I explored the influence of interactions between disk galaxies and their satellites.

My Ph.D. thesis, "Box- and Peanut-Shaped Bulges in Edge-on Disk Galaxies" (Abstract), was finished in September 1999.

From September 1999 to December 1999 I was a member of the "Sonderforschungsbereich 191" and from January 2000 to September 2000 I was working as scientific assistent at AIRUB.

After that time I researched only a little bit in astronomy together with my former collegues of the AIRUB.

My current position is elearning expert in an educational project for plasma engineering at the University of Wuppertal.

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ESO 597-36 in Hickson Compact Group 87

Image from Hubble Space Telescope

Diploma thesis:

I investigated a representative sample of edge-on galaxies with box/peanut (b/p) bulges. Their morphological types range from S0 to Sc and the survey is based on optical CCD imaging in g, r, and i (Barteldrees & Dettmar, 1994, A&AS, 103, 475). I decomposed the galaxies in disk and bulge and modeled these components. Thereby I could determine the classical scale parameters of the disk and in addition the parameters describing objectively the b/p structures. The X-structure of the distribution of the stars in the b/p bulge became obviously in all galaxies. In several cases the subtraction of the modeled disks from the galaxies let me recognize - in addition to the bulge - a thin component in the resulting image.

The mean results are:

The ratio between the cut-off radius (Rmax; truncation of the radial light distribution of galactic disks) and the radial scale parameter (h) of the disks is independent of the morphological type and is nearly constant (Rmax/h = 3.0 +/- 1.0).
The ratio between h and the vertical scale parameter (zo) is on an average 4.1 +/- 2.3. But it is likely to be dependent on the morphological type (more later types have greater ratios).
The light distribution of the disks perpendicular to the major axes is described at best by a sech-function (early types) or exp-function (later types). The often used sech^2-function is in the most cases the worst alternative.
B/p bulges are a homogenous class. Thus the weakest boxes and most highly distorted peanuts are to be merely extrema within a more general distribution of b/p bulge distortions.
The angle between a branch of the X-structure and the major axis is 43 ± 9 degree.
The properties of the b/p structures can be described by their total excess luminosity in relation to the observed total bulge luminosity (EL/BL_TOT), the maximum of excess to bulge luminosity measured in radial intervals (EL/BL_MAX), and the minimum of the a4-parameter (a4_MIN) quantifying the degree of boxiness. All parameter are almost lineary correlated.
B/p bulges have on an averange values of a4_MIN = -3.2%, EL/BL_TOT = 1.8% and EL/BL_MAX = 6.8%. However, the range of the parameters is large, e.g. for EL/BL_MAX from 2% to 24%.
It is possible that the properties of the b/p-structure are correlated with the morphological type: on an average the b/p-parameters rise with the type.
The most thin components (s. a.) are presumable bars which are likely the origin of the b/p bulges.

NGC 2788A

Image from VLT

Abstract of my dissertation :

Box and peanut shaped (b/p) bulges are not that peculiar as it seemed in the past. The investigation of a complete sample of ~1350 edge-on disk galaxies derived from the RC3 ("Third Reference Catalogue of Bright Galaxies", de Vaucouleurs et al., 1991) reveals that 45% of all classifiable bulges are b/p shaped. The frequency of b/p bulges is for all morphological types from S0 to Sd above 40%. In particular, this is for the first time that such a large frequency of b/p bulges is reported for galaxies as late as Sd. A visual classification of the bulges in three types of b/p bulges or as a non-b/p type is presented. It is shown that dust extinction does almost not influence the boxiness of bulges. Several objective parameters describing b/p bulges are discussed and a new parameter measuring the depression at the minor axis in peanut bulges is introduced.

Using catalogue data, the Digitzed Sky Survey (DSS), multicolor CCD and NIR data, and N-body simulations it is concluded that two classes of b/p bulges with different origins exist. Around 5% of the b/p bulges belong to the class of "thick boxy bulges" which is for the first time introduced here. These bulges all show irregularities and asymmetries and some possess possible merger remnants and merging satellites, without any evidence for bars. Therefore it is very likely that these bulges are not dynamically settled and the boxy structures result from material accreted from infalling satellite companions (soft merging) (Binney & Petrou, 1985; Whitmore & Bell, 1988).

For most b/p bulges (~95%), however, it is concluded that they have their origin in instabilities and resonances animated by bars (Combes et al., 1990; Raha et al., 1991). The frequency distribution of barred galaxies concerning the morphological type is consistent with this scenario. Evidence for such a mechanism is presented by the blue color of b/p distortions compared to the bulge color. This detection indicates that the material building the b/p structure originates in the stellar disk. The strongest evidence is the very significant correlation between b/p bulges and bar signatures in cuts along and parallel to the major axis which is visible in the NIR images. Regarding the shape of bulges and the position of the maximum of the b/p distortion the investigated N-body simulations (Pfenniger & Friedli, 1991) for stars in barred potentials demonstrate that this theory and observations are consistent. In the literature the connection between bars and b/p bulges is also supported in particular from gas kinematics (e.g. Kuijken & Merrifield, 1995; Bureau, 1998).

The N-body simulations of barred galaxies and the newly found correlation of the ratio of the projected bar length and the bulge length with bulge type in the NIR reveal that the boxiness of the bulge mainly depends on the viewing angle of the bar. Therefore edge-on bars can be observed in galaxies with peanut bulges and it is for the first time possible to measure directly the ratio of the bar length to its thickness. Furthermore, the boxiness seems to be correlated with the strength of the bar.

Within this framework the question what may cause the bar becomes even more important. An analysis of the environment of galaxies with b/p bulges reveals that these galaxies have more companions and satellites and show more frequently interactions than the control samples. Therefore the favourable model scenario for the development of the large fraction of b/p bulges are resonances at a bar triggered by galaxy interaction or by an infalling satellite in an otherwise stable disk.

The proposed scenarios for the formation of b/p bulges in this thesis support theories of secular evolution of bulges in which galaxies evolve from SA over SB. A dissolution of the bar (evolution from SB back to SA) is caused by a sufficient mass concentration in the center of the bulge as N-body simulations in the literature show (e.g. Norman et al., 1996). Bulges grow through a b/p phase either by disk instabilities, accreted material, or by both. Along the Hubble sequence, galaxies evolve from later to earlier types.

Additionally, it can be concluded that the analysis of the b/p bulges reveals parts of a merger sequence which is marked by the effects of mergers. The smallest mergers let arise bars leading to b/p bulges and soft-merger are most likely the origin of the thick boxy bulges.

Main results:

In a complete sample of 734 edge-on disk galaxies with classifiable bulges derived from the RC3 out of ~1350 galaxies with a diameter larger than 2 arcmin and visually inspected with the DSS it is found that 45% of all bulges are b/p shaped. This high frequency was suggested earlier by Dettmar & Barteldrees (1988) and Dettmar (1989), but only from a small sample of CCD images. Dividing the b/p bulges in three classes 4% of all bulges are peanut bulges, 16% boxy bulges, and the remaining 25% are weakly box shaped bulges.
The frequency of b/p bulges is in all morphological types (S0-Sd) > 40%. This is the first time that such a large fraction of b/p bulges is found in galaxies as late as Sd.
In the context of objective classification of b/p bulges the two angles theta, the opening angle of the b/p isophotes to the major axis, and psi are parameters of a peanut bulge. Psi is a new introduced parameter measuring the depression at the minor axis of peanut bulges. Typical angles for peanut bulges are psi = 3 and theta = 40 degree. There is no useful parameter for an objective classification of all type of b/p bulges. The best objective parameter up to now is the minimum of the a4 parameter.
A blue color of b/p distortions compared to the bulge color (in two Sb galaxies) is detected for the first time. This detection is an indication for the connection between disk and b/p structure, since disks tend to be bluer than bulges.
Investigating the large scale environment (cluster membership and density parameters) no correlation with b/p bulges is found. However, all investigations of the small scale environment point to a connection between the existence of b/p bulges and satellites. Additionally, galaxies with peanut bulges show at a significant level more often interactions compared to a control sample.
Inspecting the sample of edge-on galaxies from the RC3 and additional CCD data of further galaxies a class of 20 galaxies with prominent, large, and boxy bulges are identified. This new class is called "thick boxy bulges" and the prototype is IC 4745. The fraction of these galaxies is ~5% of all galaxies with b/p bulges.
While the galaxies with "normal" b/p bulges show only few indications for accretion events, the thick boxy bulges all show irregularities and asymmetries and some even possess substructures which could be possible merger remnants or merging satellites. The luminosity distribution of these bulges is not described best by a r^(1/4)-law, but frequently by r^(1/2). Furthermore, the fraction of small nearby projected satellites is increased in the sample of galaxies with thick boxy bulges. The relative size of these bulges is too large to be explained by bars and no thick boxy bulge shows a bar signature in the NIR.
The investigations in the NIR reveal that the detection of b/p bulges is not strongly influenced by dust. In general the shape of bulges in the optical and in the NIR is the same.
Cuts along and parallel to the major axis in the optical for early type galaxies, less affected by dust, as well as in the NIR for all types show the strong correlation between bars and b/p bulges. This correlation is the first time verified in a large sample of galaxies (N=61 in the NIR). The investigation of the bar signatures in the NIR reveals also a correlation between prominent b/p bulges and strong bar signatures pointing to a dependence of the boxiness of a galaxy on the bar strength.
The frequency distribution of barred galaxies in respect to the morphological type is similar to the frequency distribution of galaxies with b/p bulges. Additionally, the modified frequencies of bulge types in simulated highly inclined galaxies with different viewing angles of a bar are consistent with the observed frequencies. These results support the interpretation that bars viewed under different angles in edge-on galaxies are the main explanation of b/p bulges with different shapes.
The ratio of the projected bar length (BAL) to the length of the bulge (BUL) correlates with the bulge type. The maximal ratios of the galaxies with peanut bulges are consistent with ratios observed in face-on galaxies, while the results of the investigated N-body simulations do not fit to these observations. Thick bars and boxy bulges are two different components revealed by their different lengths. The correlation of BAL/BUL with the bulge type is the first observational evidence for an explanation of the different degree of boxiness by the viewing angle of a bar.
The ratio of BAL to the length of the b/p distortion (BPL) has a mean value for the observed peanut bulges of 2.8 ± 0.3. This value is consistent with the investigated N-body simulations whereas a ratio of 2.5 ± 0.2 can be derived for an edge-on galaxy with an edge-on bar. In the NIR observations the coincidence of the positions of the maximum of the b/p distortion and the limits of the central bulge (CBU/BPL = 1 ± 0.1) is found for the first time.
Interpreting peanut bulges as galaxies with a bar seen edge-on the ratio of bar length to thickness, BAL/BAT = 14 ± 4, can be directly measured for the first time.
The list of classified bulges, of peanut bulges, and of thick boxy bulges are a new base for future observations of b/p bulges and will replace older lists in the literature.

The old observatory in Meudon/Paris (place of a conference in Sep 98).

Pictures of my observation trips ?

Very interesting topics in astronomy ;-)

luett@astro.ruhr-uni-bochum.de

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