Cold gas and dust: Hunting spiral-like structures in early-type galaxies (2001.08087v1)

Abstract: Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body. Dust is frequently found in ETGs as well. The goal of our study is to understand the link between dust and cold gas in nearby ETGs as a function of HI content. We analyse deep optical $g-r$ images obtained with the MegaCam camera at the Canada-France-Hawaii Telescope for a sample of 21 HI-rich and 41 HI-poor ETGs. We find that all HI-rich galaxies contain dust seen as absorption. Moreover, in 57 percent of these HI-rich galaxies, the dust is distributed in a large-scale spiral pattern. Although the dust detection rate is relatively high in the HI-poor galaxies ($\sim$59 percent), most of these systems exhibit simpler dust morphologies without any evidence of spiral structures. We find that the HI-rich galaxies possess more complex dust morphology extending to almost two times larger radii than HI-poor objects. We measured the dust content of the galaxies from the optical colour excess and find that HI-rich galaxies contain six times more dust (in mass) than HI-poor ones. In order to maintain the dust structures in the galaxies, continuous gas accretion is needed, and the substantial HI gas reservoirs in the outer regions of ETGs can satisfy this need for a long time. We find that there is a good correspondence between the observed masses of the gas and dust, and it is also clear that dust is present in regions further than 3~Reff. Our findings indicate an essential relation between the presence of cold gas and dust in ETGs and offer a way to study the interstellar medium in more detail than what is possible with HI observations.

• The paper reveals that a significant fraction of early-type galaxies contain cold gas and dust, finding dust in 71% of the sample and spiral dust patterns often in hi-rich galaxies through optical and radio data analysis.
• The presence of dust, especially in spiral forms, challenges the traditional view of early-type galaxies as passive systems and suggests dynamic processes like external gas accretion contribute to maintaining these structures.
• These findings emphasize the importance of external accretion processes in early-type galaxy evolution and highlight the value of combining deep optical imaging with radio data to understand complex interstellar medium structures.

Analyzing Cold Gas and Dust Structures in Early-Type Galaxies

The paper by Yıldız et al. investigates the interplay between cold gas and dust within early-type galaxies (ETGs), emphasizing the peculiar presence of spiral-like structures. The paper leverages new optical imaging from the MegaCam camera on the Canada-France-Hawaii Telescope to assess the distribution of dust and cold gas in a sample of nearby ETGs. This work adds to the contemporary understanding of ETGs, typically dominated by old stellar populations and often considered devoid of significant interstellar medium (ISM) activity.

Key Findings and Methodology

The authors analyze optical $g-r$ images for two distinct groups of ETGs: those rich in neutral hydrogen (\hi) and those poor in \hi. They identify that approximately 50% of all nearby ETGs contain some cold gas, often distributed beyond the stellar regions of the galaxy. Notably, they discover dust in 71% of their sample, with a high incidence of spiral patterns in \hi-rich galaxies (57%).

Their methodology involves model calculations to determine dust-free color profiles, from which they derive color excess maps. These are used to estimate dust masses, revealing that \hi-rich galaxies contain significantly more dust than their \hi-poor counterparts. Moreover, the paper infers that continuous gas accretion from large \hi reservoirs might be necessary to maintain observed dust structures.

Implications for Understanding ETGs

The presence of dust, particularly in a spiral form traditionally associated with later-type galaxies, suggests dynamic internal processes or past accretion events. These findings challenge the conventional paradigm that ETGs are largely passive systems lacking substantial ISM. They suggest that the presence of spiral-like dust structures in ETGs could be indicative of recent or ongoing interactions with the external environment, potentially due to accretion of material from mergers or other external sources.

Practical and Theoretical Implications

The paper's results impact the broader understanding of galaxy evolution, particularly highlighting the importance of external accretion processes in maintaining ISM structures within ETGs. They also raise questions about the mechanisms through which such large reserves of \hi can influence the morphologies of dust and possible star formation activities.

Technologically, this research advocates for the utility of deep optical imaging combined with radio data to unravel complex ISM structures in galaxies. By combining such observational techniques, researchers can gain insights into the less understood aspects of ETG behavior, laying the foundation for an enriched comprehension of their evolutionary pathways.

Future Research Directions

Future investigations could explore the specific processes leading to gas and dust accretion in ETGs, possibly through simulations focusing on minor mergers and external gas inflow. There is also potential to paper the molecular gas components in conjunction with models predicting ISM evolution in ETGs, shedding light on their ability to sustain extended dust lanes.

Moreover, assessing the implications of these findings for galaxy modeling, including aspects like dust grain size distributions and their impact on observables such as infrared emission properties, could provide a deeper understanding of the dust lifecycle within galaxies.

In sum, Yıldız et al. present compelling evidence of dynamic ISM phenomena in ETGs, encouraging a reevaluation of the evolutionary processes traditionally associated with these galaxies. This paper serves as a significant step towards reconciling observational anomalies with theoretical frameworks concerning the maturation of massive elliptical and lenticular systems in the cosmic landscape.