An excessively massive thick disc of the enormous edge-on lenticular galaxy NGC7572

Abstract: Galactic discs are known to have a complex multilayer structure. An in-depth study of the stellar population properties of the thin and thick components can elucidate the formation and evolution of disc galaxies. Even though thick discs are ubiquitous, their origin is still debated. Here we probe the thick disc formation scenarios by investigating NGC7572, an enormous edge-on galaxy having $R_{25}\approx 25$ kpc and $V_{\rm rot} \approx 370$ km s$^{-1}$, which substantially exceeds the Milky Way size and mass. We analysed DECaLS archival imaging and found that the disc of NGC7572 contains two flaring stellar discs (a thin and a thick disc) with similar radial scales. We collected deep long-slit spectroscopic data using the 6m Russian BTA telescope and analysed them with a novel technique. We first reconstructed a non-parametric stellar line-of-sight velocity distribution along the radius of the galaxy and then fitted it with two kinematic components accounting for the orbital distribution of stars in thin and thick discs. The old thick disc turned out to be 2.7 times as massive as the intermediate-age thin component, $1.6\times 10^{11}$ $\textrm{M}{\odot}$ vs. $5.9\times10^{10}$ $\textrm{M}{\odot}$, which is very unusual. The different duration of the formation epochs evidenced by the [Mg/Fe] values of +0.3 and +0.15 dex for the thick and thin discs respectively, their kinematics and the mass ratio suggest that in NGC7572 we observe a rapidly formed very massive thick disc and an underdeveloped thin disc, whose growth ended prematurely due to the exhaustion of the cold gas likely because of environmental effects.

• The paper employs photometric modeling and long-slit spectroscopy to reveal that NGC7572’s thick disc is 2.7 times as massive as the thin disc, indicating rapid formation over ~0.25 Gyr.
• It details a structural analysis using a sech^2 profile fit that shows both discs have similar radial scales with significant flaring, with the thick disc exhibiting twice the scaleheight of the thin disc.
• The paper compares stellar population ages and metallicities, finding older stars (10–13 Gyr) in the thick disc, which challenges standard galactic disc evolution models.

Overview of "An Excessively Massive Thick Disc of the Enormous Edge-on Lenticular Galaxy NGC7572"

The study of the lenticular galaxy NGC7572 provides critical insights into the formation and evolution of galactic thick discs, particularly in massive systems. This paper by Kasparova et al. deploys an in-depth spectrophotometric analysis of NGC7572 using archival imaging data from DECaLS and long-slit spectroscopic observations from the Russian 6m BTA telescope. The investigation focuses on understanding the structural characteristics and formation scenarios of the galaxy's thick disc, which presents unusual properties in contrast to standard models of galactic discs.

Key Findings and Methodology

1. Structural Analysis:
   • The galaxy NGC7572 is characterized by two primary stellar discs—thin and thick—each displaying significant flaring. Their radial scales are notably similar, with the thick disc being exceptionally massive.
   • The analysis employs a $\sech^2$ model to fit vertical surface brightness profiles, revealing notable flaring in both disc components. The scaleheight of the thick disc was observed to be twice that of the thin disc.
2. Spectroscopic Observations:
   • Using high-resolution spectroscopic data, the study reconstructs the line-of-sight velocity distribution (LOSVD) and delineates distinct kinematic profiles for the thin and thick discs.
   • The thick disc is found to be 2.7 times as massive as the thin disc and composed of an older stellar population with distinct [Mg/Fe] ratios, suggesting rapid formation over $\sim0.25$ Gyr compared to the longer formation timescale of the thin disc.
3. Stellar Population Characteristics:
   • The study identifies the thin and thick discs' stellar populations as having ages of approximately 5 to 8 Gyr and 10 to 13 Gyr, respectively. The metallicities are roughly solar for both components.
   • Variations in $\alpha$-elements suggest different formation epochs: [Mg/Fe] ratios strongly support a short formation period for the thick disc, aligning with scenarios of rapid formation at high redshift.

Implications for Galactic Formation Theories

• Unusual Thick Disc Mass: The excessively massive thick disc challenges the generalized assumptions about the relation between thick and thin disc masses, especially in high-mass systems. It suggests that underdeveloped thin discs might result from environmental constraints, such as gas exhaustion, due to interactions within dense clusters.
• Galaxy Formation Scenarios: The properties observed in NGC7572 support a rapid formation scenario for thick discs, potentially influenced by minor mergers in the early universe. This highlights the importance of environmental factors in galactic evolution models, suggesting they could truncate the growth of thin discs prematurely.
• Potential for Further Studies: The methodologies and findings emphasize the need for further studies incorporating both photometric and spectroscopic analyses in high inclination galaxies. Expanding such studies to more galaxies can elucidate universal versus environment-specific patterns in galactic disc evolution.

Conclusion and Future Directions

The comprehensive study of NGC7572’s thick disc provides significant insights into galactic structure in massive galaxies. The findings underline the critical interplay between galactic dynamics, environmental factors, and star formation history in shaping galactic components. Future research should aim to extend these methods to a broader range of galactic masses and environments to enhance our understanding of galaxy formation and evolution. This work poignantly demonstrates the utility of combining detailed spectroscopic and morphological analyses to untangle the complex history of galaxies.

By leveraging these insights and methodologies, researchers stand to gain a deeper understanding of the formation and evolution of thick disc components, informing broader theories in galaxy formation and evolution.