Why are some galaxy disks extremely thin?

Some low surface brightness galaxies are known to have extremely thin stellar disks with the vertical to planar axes ratio 0.1 or less, often referred to as superthin disks. Although their existence is known for over three decades, the physical origin for the thin distribution is not understood. We model the stellar thickness for a two-component (gravitationally coupled stars and gas) disk embedded in a dark matter halo, for a superthin galaxy UGC 7321 which has a dense, compact halo, and compare with a typical dwarf galaxy HoII which has a non-compact halo. We show that while the presence of gas does constrain the disk thickness, it is the compact dark matter halo which plays the decisive role in determining the superthin disk distribution in low-mass disks. Thus the compact dark matter halo significantly affects the disk structure and this could be important for the early evolution of galaxies.Comment: Submitted to MNRA

A slow bar in the dwarf irregular galaxy NGC 3741

Using the Tremaine-Weinberg method, we measure the speed of the HI bar seen in the disk of NGC 3741. NGC 3741 is an extremely gas rich galaxy with an {H\,{\sc i}} disk which extends to about 8.3 times its Holmberg radius. It is also highly dark matter-dominated. Our calculated value of the pattern speed $\Omega_p$ is 17.1 $\pm$ 3.4 km $\textrm{s}^{-1}\textrm{kpc}^{-1}$. We also find the ratio of the co-rotation radius to the bar semi-major axis to be (1.6 $\pm$ 0.3), indicating a slow bar. This is consistent with bar models in which dynamical friction results in a slow bar in dark matter dominated galaxies.Comment: 7 pages, 4 figures, 2 tables Accepted for publlication in MNRA

Analyzing the cosmic web environment in the vicinity of grand-design and flocculent spirals with local geometric index

We explore the environment of a combined set of $367$ grand-design and $619$ flocculent spiral galaxies. We introduce a novel estimator called the \textit{local geometric index} to quantify the morphology of the local environment of these $986$ spirals. The local geometric index allows us to classify the environment of galaxies into voids, sheets, filaments, and clusters. We find that grand-designs are mostly located in dense environments like clusters and filaments ($\sim 78\%$), whereas the fraction of the flocculents lying in sparse environments like voids and sheets is significantly higher ($> 10\%$) than that of the grand-designs. A $p$-value $<$ $10 ^{-3}$ from a Kolmogorov-Smirnov test indicates that our results are statistically significant at $99.9\%$ confidence level. Further, we note that dense environments with large tidal flows are dominated by the grand-designs. On the other hand, low-density environments such as sheets and voids favor the growth of flocculents.Comment: Accepted for publication in JCAP. arXiv admin note: substantial text overlap with arXiv:2302.0808

The dynamical lineage of field ultra-diffuse galaxies

Ultra-diffuse galaxies (UDGs) exhibit morphological similarities with other low luminosity galaxies indicating a possible evolutionary connection. We investigate for a common dynamical origin of field UDGs with other low luminosity field galaxies, namely the low surface brightness galaxies (LSBs) and the dwarf irregulars (dIrrs). Obtaining the scaling relations between (i) central stellar surface density and exponential stellar disk scale length, (ii) stellar and atomic hydrogen mass, and (iii) stellar and dynamical mass for LSBs and dIrrs respectively, and superposing the data for UDGs on them, we note that UDGs evolve fairly akin to dIrrs. We next construct distribution function-based stellar-dynamical models of these galaxies. Comparison of the modelled radial-to-vertical velocity dispersion ratio, and the rotational velocity-to-total stellar velocity dispersion ratio also indicate that the stellar kinematics of UDGs and dIrrs are similar. Finally, we conducted a principal component analysis to identify the key parameters accounting for the variance in the structure and kinematical data for the respective galaxy populations. We observe that the total HI-mass mostly regulates the variance for the UDGs and dIrrs, while the ratio of radial-to-vertical velocity dispersion dominates the same in LSBs. We therefore conclude that field UDGs and dIrrs possibly share a common dynamical lineage.Comment: 12 pages, 3 figures, 1 table (Submitted