Survey of kinematics of ultra-flat galaxies with Russian 6-meter telescope

Aleksandra Antipova 1 , Dmitry Makarov 1 , Aleksandr Mosenkov 3 , Vladimir Reshetnikov 2

  • 1 The Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz, Russian Federation
  • 2 St.Petersburg State University, St.Petersburg, Russian Federation
  • 3 Central Astronomical Observatory, St.Petersburg, Russian Federation

Abstract

We present the results of long-slit spectroscopy for 90 ultra-flat galaxies obtained on the Russian 6-m telescope BTA with the SCORPIO spectrograph. Ultra-flat galaxies are disk galaxies of late morphological types (Sc-Sd) with a large axis ratio (a/b>10). These galaxies are visible at a very large angle to the line of sight, almost edge-on. For these galaxies, we obtained rotation curves using the H-alpha line. We use a Polyex function for approximation of the rotation curves. We found the systematic heliocentric redshift, the dynamic radius, amplitude, and slope of the outer part of the rotation curve. In addition, we present the photometric results of these galaxies using Pan-STARRS data. Obtained results, including spectra and rotation curves, will be available in the database of edge-on galaxies. Here we briefly describe the structure of this database.

ROTATION CURVES AND DECOMPOSITION OF EDGE-ON GFLAXIES

Edge-on galaxies are the only objects outside of our Galaxy where it is possible to study the vertical distribution of matter in the disks directly. It allows us to study the three-dimensional distribution of matter in galaxies. Among this class of objects, super-thin galaxies with an axis ratio of a/b>10 deserve special attention. The existence of a large population of pure disk super-thin galaxies is difficult to explain in the framework of the modern hierarchical model of galaxy formation, since the constant processes of merging and absorption should lead to “heating” of the galaxy disk. It is believed that the existence of super-thin galaxies with a ratio of the visible axes a/b>10 is possible only if there is a massive dark gala around them (Zasov et al. 1991, Sov. Astron. Lett., 17, 374; Sotnikova et al. 2006, Astron. Lett., 32, 649; Khoperskov et al., 2010, Astron. Nachr., 331, 731) Therefore, the study of super-thin disks is extremely important for understanding the physical processes of the formation and evolution of galaxies in the Universe.

Our sample includes galaxies with the axis ratio (a/b)O > 10 on blue POSS plates and (a/b)E >= 8.5 on the red ones. We selected only galaxies with declination > -25 degree and the major axis a<4 arcmin. We excluded from the sample any galaxy with known measurements on its internal kinematics (rotation curves or HI-line width) except some cases for understanding the sistematics and cross-correlations. The observations are carrying out using 6-meter telescope with the focal image reducer SCORPIO.  The grating VPH 1800 allows us to measure the redshift of emission lines with precision of 5-10 km/s. Our galaxy sample is based on a catalog of flat galaxies (RFGC, Karachentsev et al. 1999, Bull. Spec. Astrophys. Obs., 47, 5).

Spectrum of the galaxy RFGC3563 after performing the procedures of primary reduction and linearization. 

Spectral data reduction was performed in a standard way using the ESO MIDAS procedures. It includes the bias subtraction, the correction for small-scale sensitivity variations of the CCD matrix, the cosmic rays removal, and the lianerization of spectra.

The galaxies' rotation curves are constructed measuring H-α line velocities along the slit. We fit the rotation curves with the polyex - function (M. Persic, P. Salucci and F. Stel, Mon. Not. Roy. Astron. Soc. 281, 27 (1996)) to determine the parameters of the rotation curve.

The original polyex function is applied only to already folded rotation curves. We are upgrading this feature. Our polyex function is symmetrical and takes into account the effect of line broadening due to the cosmological redshift.

cz = cz0 - (1+cz/c)*V0*(1-exp(-abs(R)/Rpe))*(1+alpha*abs(R)/Rpe)*sign(R) ,

where

cz - the observed redshift multiplied by the speed of light.

cz0 - cz of the center of the galaxy.

V0 - the characteristic velocity of the rotation curve in the polyex model.

Rpe - the dynamic radius of the galaxy in the polyex model.

alpha - the slope of the outer part of the rotation curve.

R = (r-R0) - the distance from the center of the galaxy along the gap.

r - coordinate of this point along the slit

R0 - the position of the center of the galaxy.

To take into account the symmetry of the rotation curve function along the gap relative to the center of the galaxy, a sign (R) function was added - which returns "-1" for R <0 and "+1" for R> 0. This equation is valid for the rotation curve growing along the slit. If the speed of the galaxy decreases along the gap, then the formula is corrected by an additional constant.

As a first approximation, to determine the center and amplitude of the rotation curve, we used the fitting of the observed rotation curve with its centrally symmetric copy. For this we used a cubic spline.

The left panel shows the observed rotation curve of the RFGC3563 galaxy (gray circles) and its approximation by the polyex function (black solid line). The right panel shows the same rotation curve, but folded relative to its center, found using the polyex function. White circles are the moving part of the rotation curve, Gray circles are the moving part of the rotation curve.

Using the polyex function for the galaxy RFGC3563, we determined the following parameters:
cz0 = 8971.62 ± 1.15
V0 = 93.72 ± 9.69
R0=137.39 ± 0.18
Rpe=5.88 ± 0.87
alpha=0.06 ± 0.02

We obtained the same processing and parameters for 90 galaxies from our sample. We compare the galaxy speeds we received with the radio data:

<x> = -5.5 + -2.9 Sigma = 19.8 N (measurements) = 46 N (unique galaxies) = 23

The distribution of the sample galaxies by the heliocentric radial velocity (cz0). N is the number of galaxies in the range cz0. Median redshift of the galaxies in our sample is 8979 km/s that roughly corresponds to 120 Mpc.

Modern mass surveys of the sky, for example, Pan-STARRS, allow extensive research into a large number of galaxies. Photometry of flat galaxies is a non-trivial task due to the specific shape and distribution of light in edge - on galaxies. Photometry of edge - on galaxies requires a separate approach, taking into account the specifics of these objects. 

 

We decompose the galaxy sample described above using the Pan-starrs sky survey.

 

The decomposition of galaxies was carried out by a program written by Aleksander Mosenkov (mosenkovAV@gmail.com), based on the software package Galfit (Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2002, AJ, 124, 266) and IMFIT (Erwin P., 2015, ApJ, 799, 226). In simplest use, Galfit allows one to fit an ellipsoid model to light profiles in an image. For more complicated situations, it can model highly detailed shapes that are curved, irregular, lopsided, ringed, truncated, or have spiral arms. One can mix and match these features within a single component model, or can add them to other components to create complex shapes. IMFIT provides an ability specify a set of one or more 2D image functions (e.g., elliptical exponential, elliptical Sérsic, circular Gaussian) which are added together to generate a model image. This model image is then matched to the input image by adjusting the 2D function parameters via nonlinear minimization of the total χ2

Fitting the image with a 2D model using program of Aleksander Mosenkov will allow you to evaluate the exponential radial and vertical scale of the disc, its model integral magnitude, the contribution of the bulge, thin and thick discs.

We show the approximation with a two-component model (“edgedisk” + “sersic” model, see work Peng, CY, Ho, LC, Impey, CD, & Rix, H.-W. 2002, AJ, 124, 266) of a galaxy profile by the example of RFGC3563. Bright star formation regions and background stars and galaxies were masked when approximating. The upper-left panel shows the galaxy region from Pan-starrs in the filter g. The approximating model is represented on the upper right tab of the panel, and the result of subtracting the model from the original frame.

THE EDGE-ON GALAXY DATABASE

Galaxies inlined on a large angle to the line-of sight, practically edge-on, play a special role in observational astrophysics. Edge-on galaxies provide a unique opportunity to directly observe the distribution of dust, thin and thick stellar disks, distortion of the disk in the outer parts of galaxies. The study of kinematics does not require correction for the inclination, which is one of the main uncertainties in the Tully-Fisher relation.
Modeling the rotation curves taking into account the distribution of visible matter allows us to impose restrictions on the distribution of dark matter, which is the key point for the theory of formation and evolution of galaxies. Due to the fact that the orientation of the spin of the edge-on galaxies is known with high accuracy, we can determine the orientation of galaxy rotation with respect to the elements of the large-scale structure of the Universe, which helps us to understand the nature of the angular momentum of galaxies. Therefore, the need to create a database of edge-on galaxies, which would contain numerous information about these objects, is obvious.

The structure of the Edge-on Galaxy Database, which describes the rotation curves of galaxies, consists of several levels interconnected.

Level 1 consists of a table containing general information on a galaxy where each object is associated with an unique id which is used for the object identification in other tables.
Level 2 describes sets of rotation curves observed in a uniform manner. Usually, one publication contains one dataset, but in some cases it can be splitted between several publications. The table contains information about the data presentation: the folding flag indicates that the rotation curve is folded with respect to the center of the galaxy; the relative velocity flag shows that the velocity is measured relative to the systematic redshift of the galaxy; the zcorrected flag specifies that the measurements were corrected for the cosmological broadening; the paflag contains information about accuracy of the position angle determination.
Level 3 describes a specific observation and consists of one table, which indicates which dataset it belongs to, which line was used for measurements, the shift relative to the center, the systematic speed of the galaxy, the unique number of the rotation curve, the PGC number of the galaxy, contains a link to the HyperLeda database.
Level 4 collected the measurements and consists of two tables for the rotation velocity and the velocity dispersion of the stellar populations. The first one gives the measured velocity and its error along the slit while the second one has the similar structure but for the velocity dispersion.

ACKNOWLEDGEMENTS

Development of The Edge-on Galaxy Database is supported by the RFBR (grant 19-32-90244).

Data analysis is supported by  the RFBR (grant 19-32-50129\19).

D. M. is grateful the Russian Science Foundation (grant 19–12–00145).