The vertical structure of the stellar disk in NGC 551

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
² Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Indore 453552, India
³ Indian Institute of Astrophysics, Bangalore 560034, India
⁴ Department of Physics, Indian Institute of Science, Bangalore 560012, India
⁵ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
⁷ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, 04510 México, D.F., Mexico

^⋆ Corresponding authors: harshal1908.hr@gmail.com, naren@iiti.ac.in

Received: 18 April 2024
Accepted: 3 June 2025

Abstract

Aims. This paper aims to self-consistently determine the 3D density distribution of the stellar disk in NGC 551 and to utilize it to study the observational signatures of two-component stellar disks (thin and thick) in galaxies.

Methods. Assuming that the baryonic disks are in hydrostatic equilibrium, we solved the Poisson-Boltzmann equation to estimate the 3D density distribution in the stellar disk of NGC 551. Unlike in previous studies, we used integral-field spectroscopic observations to estimate the stellar velocity dispersion. A 3D dynamical model of the stellar disk was built using these density solutions and the observed rotation curve. Using this model, we generated simulated surface brightness maps and compared them with observations to verify the consistency of our modeling. Furthermore, the dynamical model was inclined to 90° to produce an edge-on surface density map of the galaxy. We further investigated this map by fitting different 2D functions and plotting vertical cuts in a logarithmic scale to infer observational signatures of two-component disks in galaxies.

Results. We estimated the vertical stellar velocity dispersion in NGC 551 using an iterative method and obtained results consistent with the formalism employed in the Disk Mass Survey. Through dynamical modeling of the stellar disk in NGC 551, we produced moment maps, which reasonably matched the observations, indicating consistent modeling. We examined the simulated edge-on model by taking vertical cuts and decomposing them into multiple Gaussian components. We find that an artificial double Gaussian component arises due to the line-of-sight integration effect, even for a single-component disk. This indicates that decomposing vertical intensity cuts into multiple Gaussian components is an unreliable method for identifying multicomponent disks. Instead, an up-bending break, visible in the plot of the vertical cuts in the logarithmic scale for a two-component disk, serves as a more reliable indicator, which is absent in the case of a single-component disk. We performed 2D fitting on the edge-on surface density map using the product of a scaled modified Bessel function (for the radial profile) and a sech² function (for the vertical profile) to estimate the stellar disk's structural parameters. We find that these traditional methods systematically underestimate the scale length and flattening ratio of the stellar disk. Therefore, we suggest using detailed modeling to accurately deduce the structural parameters of stellar disks in galaxies.