"Structure and dynamics of the central regions of disk galaxies", por Jairo Méndez Abreu PDF Imprimir


Jairo Méndez Abreu
Tesis leída en julio de 2008



Trabajo dirigido por:
Alfonso Aguerri (IAC) y Enrico Maria Corsini (Dipartimento Astronomia, Università di Padova),  realizada en cotutela entre las dos universidades (Padova y La Laguna)


In this thesis, we focus on the structural components observed in the central regions of disk galaxies, namely bulges and bars. We aim to better understand the formation and evolution scenarios which could lead to the bulges and bars observed in the nearby universe. This study was performed from an observational point of view, by analyzing either the structural and photometric properties of bulges and bars, their kinematics, and stellar population characteristics.

The first part of the thesis is mainly focussed on the structure, photometry and stellar populations of bulges.

We present a new fitting algorithm (GASP2D) to perform two-dimensional photometric decomposition of the galaxy surface-brightness distribution. We adopted a Sérsic and an exponential profile to describe the surface-brightness distribution of the bulge and disk, respectively. Both components are characterized by elliptical and concentric isophotes with constant (but possibly different) ellipticity and position angles.


The structural parameters of the bulges and disks of a magnitude-limited sample of 148 S0-Sb galaxies were derived from the Two Micron All Sky Survey  (2MASS) $J-$band images by means of GASP2D. We found that the surface-brightness radial profiles of larger bulges are more centrally peaked than those of the smaller bulges. Larger bulges have a lower effective surface-brightness and higher absolute luminosities. They reside in larger disks, as revealed by the correlation between central velocity dispersion and disk scale-length. This reveals a strong coupling between bulges and disks. This has been interpreted as an indication of the formation of  bulges via  the secular evolution of their host disks. However, the bulges of the sample galaxies follow the same fundamental plane, Faber-Jackson, and photometric plane relationships as those found for elliptical galaxies, supporting the idea that bulges and ellipticals formed in the same way. These results indicate that the scaling relations alone are not enough to clearly distinguish between bulges formed by early dissipative collapse, merging or secular evolution.

The probability distribution function (PDF) of the equatorial ellipticity of the bulges was derived from the distribution of the observed ellipticities of bulges and misalignments between bulges and disks. About 80% of bulges in unbarred lenticular and early-to-intermediate spiral galaxies are  not oblate but triaxial ellipsoids. Their mean axial ratio in the equatorial plane is =0.85. Their  PDF is not significantly dependent on morphology, light concentration or luminosity. The scenarios proposed until now in which bulges have assembled from mergers and/or have grown  over long times through disk secular evolution have to be tested against this derived PDF of bulge intrinsic ellipticities.

Photometry and long-slit spectroscopy are presented for 14 cluster disk galaxies. The structural parameters of the galaxies are derived from the R-band images by means of GASP2D. The rotation curves, velocity dispersion profiles and Hb, Mg, and Fe line-strengths are measured from the spectra obtained along the major-axis of galaxies. Correlations between the central values of Mgb, Fe, Hb, and sigma are found. Three classes of bulges are identified. The youngest bulges (~2 Gyr) with ongoing star formation, intermediate-age bulges (4-8 Gyr) have solar metallicity, and old bulges (~10 Gyr) have high metallicity. Most of the sample bulges display solar alpha/Fe ratio, no gradient in age, and a negative gradient of metallicity. This metallicityf metallicity radial profile favors a scenario with bulge formation via dissipative collapse. This implies strong inside-out formation that should give rise to a negative gradient in the alpha/Fe ratio too. But, no gradient is measured in the alpha/Fe radial profiles for all the galaxies, except for NGC 1366. The bulge of NGC 1292 is the most reliable pseudobulge of our sample. The properties of its stellar population are consistent with a slow buildup within a scenario of secular evolution.

Photometry and long-slit spectroscopy are presented for a sample of 6 galaxies with a low surface-brightness (LSB) stellar disk and  for 4 galaxies with a high surface-brightness disk. The characterizing parameters of the bulge and disk components were  derived by means of GASP2D. The stellar and ionized-gas kinematics were measured along the major and minor-axis in half of the sample galaxies, whereas the other half was observed only along two diagonal axes. Spectra along two diagonal axes were obtained also for one of the objects with major and minor-axis spectra. The kinematic measurements extend in the disk region out to a surface-brightness level ~24  mag arcsec^-2, reaching in all cases the flat part of the rotation curve.  The  stellar kinematics turns out to be more regular and symmetric than the ionized-gas kinematics,  which often shows the presence of non-circular, off-plane, and non-ordered motions. This raises the question about the reliability of the use of the ionized gas as the tracer of the circular velocity  in the modeling of the mass distribution, in particular in the central regions of LSB galaxies.

The second part of  the thesis is  mainly focussed on  the structure, photometry, and frequency of bars in nearby galaxies.

The fraction of barred disk galaxies and their properties was studied in a sample of  ~3000 galaxies from the Sloan Digital Sky Survey (SDSS). This represents a volume limited sample with all non-interacting galaxies located in the redshift range 0.01<0.04, brighter than M_r=-20, and with i <60 degrees. The bar fraction was determined by two methods: ellipse fitting and Fourier analysis. Both methods were tested and calibrated with extensive simulations of barred and non-barred galaxies similar to the observed ones. The fraction of barred galaxies in our sample turned out to be 45%. We split the sample into different morphological types, finding that 32% of S0s, 55% of early-type spirals, and 52% of late-type spirals, are barred galaxies. Neither the bar strength nor the bar length correlates with the local galaxy density, indicating that local environment does not play an important role in bar formation and evolution. In contrast, internal galaxy properties (such as the central light concentration and disk size) determine bar properties. Thus, galaxies with high central light concentrations (i.e., the S0 galaxies) have fewer and weaker bars than late-type ones and the larger bars reside in larger galaxies.

A new version of the fitting algorithm GASP2D was developed to perform two-dimensional photometric decomposition of galaxy surface-brightness distributions taking into account the presence of bars. We adopted a S\'ersic and an exponential profile to describe the surface-brightness distribution of the bulge and disk, respectively. The bar surface-brightness distribution can be described by either flat, elliptical or Ferrers profiles. The bulge and the disk are characterized by elliptical and concentric isophotes with constant (but  possibly  different) ellipticity and position angles. On the contrary, the bar component is built on a frame of generalized ellipses, where boxy and disky shapes are allowed. These ellipses have constant ellipticity and are concentric to those of the other components. We demonstrate the ability of GASP2D v2.0 to recover the correct photometric structural parameters for the bulge, disk and bar components. In addition, we confirm the capacity of the code to work efficiently with the data from the SDSS.

We presented the results of high resolution absorption-line spectroscopy of 3 face-on galaxies, NGC98, NGC600, and NGC1703 with the aim of searching for box/peanut (B/P)-shaped  bulges. These observations tested and confirmed, for the first time the prediction that face-on B/P-shaped bulges can be recognized by two minima in the profile along the bar's major axis of the fourth Gauss-Hermite moment h4 of the line-of-sight velocity distribution (LOSVD). In NGC98, a clear double minimum in h4 is present along the major axis of the bar and before the end of the bar, as predicted. In contrast, in NGC600, which is also a barred galaxy but lacks a substantial bulge, we did not find any significant kinematic signature for a B/P-shaped bulge. In NGC1703, which is an  unbarred control galaxy, we found no evidence of a B/P bulge. We also show directly that the LOSVD  is broader at the location of the h4 minimum in NGC98 than elsewhere. This more direct method avoids possible artifacts associated with the degeneracy between the measurement of line-of-sight velocity dispersion and h4.

The conclusions of this thesis can be summarized in three main points:

(1) The typical scaling relations for galaxies cannot be used alone to distinguish between the different formation scenarios proposed until now since different models are able nowadays to reproduce these relations. Measuring the intrinsic structural properties represents an useful step forward in this field and sets new constraints that should be reproduce by numerical simulations for galaxy formation;

(2) the kinematic and stellar population properties of bulges give strong additional constraints needed to unveil their formation mechanism;

(3) the link between bulges and bars appears to be fundamental to understand the formation of both components. The existence of boxy/peanut bulges and their relation with bars can be now investigated by a new kinematic diagnostic that can be applied in face-on galaxies. This result opens a new line of research which will help to give a more general scheme of bulge/bar formation and evolution.




CONTACTO:  jairo 'at' iac.es