K-Drift Advances Galactic Cirrus & Circumgalactic Medium Studies with Fast Imaging

Researchers are tackling the complex problem of distinguishing between faint extragalactic signals and foreground Galactic cirrus clouds, a crucial step in mapping the circumgalactic medium (CGM). Kwang-il Seon, Jaehyun Lee, and Jongwan Ko, from the Korea Astronomy and Space Science Institute, alongside Woowon Byun, Jaewon Yoo, Kyungwon Chun et al., present a compelling case for utilising the KASI Deep Rolling Imaging Fast (K-DRIFT) telescope to probe this extended halo material. Their work promises to significantly advance our understanding of dust properties, star formation, and chemical evolution within our Galaxy, as well as the distribution and behaviour of gas surrounding galaxies , offering unprecedented insights into the interplay between galaxies and their environments. By meticulously measuring reddening and observing H-alpha emission, K-DRIFT will allow scientists to trace galactic winds, identify ram pressure stripping, and ultimately, unlock the secrets held within the multiphase gas reservoirs of the CGM.

The team achieved this by developing methods to effectively discriminate between these features, simultaneously advancing our understanding of interstellar dust properties and the interstellar radiation field crucial for tracing chemical evolution and star formation. Measuring the reddening of distant quasars with K-DRIFT’s broad field of view and precise background subtraction capabilities allows for the detection of extended dust within galactic halos, the CGM, and even intracluster space. Experiments show that observations of H-alpha emission lines provide a powerful tool for identifying star formation activity within galaxies and assessing the environmental influences acting upon them.

Galactic winds, driven by both active galactic nuclei and intense starbursts, can be traced directly through the distribution of H-alpha emission, revealing the dynamics of gas outflow. The research establishes that a dedicated H-alpha survey with K-DRIFT will enable detailed investigations into the evolutionary stages of galaxies undergoing ram pressure stripping in cluster environments. This work opens the possibility of mapping the multiphase gas reservoir surrounding galaxies and within the CGM with unprecedented detail. By characterizing the properties of the CGM, scientists prove that it plays a critical role in galactic evolution, mediating interactions between galaxies and the larger intergalactic medium. Understanding gas flows, inflows sustaining star formation and outflows regulating it, is paramount to understanding how galaxies evolve over cosmic timescales, and this study provides a pathway to achieving that understanding.

K-DRIFT Dust and Gas Mapping via Quasar Reddening

Addressing a key observational challenge, the research team developed a strategy to differentiate extragalactic sources from foreground cirrus clouds, which obscure low surface brightness features. This discrimination is achieved through precise background subtraction facilitated by K-DRIFT’s broad field of view, enabling effective detection of extended dust within galactic halos, the CGM, and intracluster space. Researchers measure the reddening of background quasars, a technique that allows for mapping the distribution of dust with unprecedented sensitivity. Experiments employ observations of H-alpha emission lines to identify star formation activity within galaxies and to trace galactic winds driven by active galactic nuclei and starbursts.

The study pioneered a method to analyse H-alpha tails formed when ram pressure stripping removes interstellar medium (ISM) from galaxies, subsequently ionising or dissociating it through mixing with the hot intracluster medium (ICM). The research harnessed data from GALEX FUV and NUV observations of NGC 891, rotating the images to align the disk’s major axis horizontally and analysing contour levels up to 7000 photons cm−2s−1 sr−1 Å−1. This approach enables the team to investigate the vertical extent of extraplanar dust, comparing it to H-alpha emissions and revealing a multiphase nature to the halo material. Scientists are combining radiative transfer models with multi-wavelength observations, including far-infrared and optical broadband data, to improve measurements of extraplanar dust and trace its properties.

Furthermore, the study anticipates utilising deep optical images from K-DRIFT to reveal filamentary absorption features created by optically thick dust clouds, structures indicative of galactic fountains, across a larger galaxy sample. This deep imaging survey will enable a statistical analysis correlating the amount of filamentary dust with host galaxy properties like star formation rate. The team also investigates the presence of dust in the ICM and IGM, estimating extinction values of AV ∼0.2, 0.3 mag for clusters like Coma and AB ∼0.2 mag for the Abell catalog, potentially revealing its impact on the thermal state and structure formation of the universe.

Cirrus clouds show distinct bluer optical colours

Scientists have achieved a crucial breakthrough in distinguishing between extragalactic sources and foreground cirrus clouds, a long-standing challenge in studying faint astronomical features. The research team developed a dust radiative transfer model, utilising algorithms from Seon et al. (2014) and Seon & Draine (2016), to simulate the optical properties of cirrus clouds and their impact on observed colours. Results demonstrate that cirrus clouds exhibit bluer colours, specifically, a range of 0.55 ≲g −r ≲0.74 and 0.01 ≲r −i ≲0.33, as identified by Smirnov et al. (2023), due to the wavelength-dependent scattering of light by dust grains. This colour difference arises because shorter wavelengths are scattered more efficiently, leading to a bluer appearance for the clouds compared to the redder light from distant galaxies.

Experiments revealed that the spectral energy distribution of the interstellar radiation field (ISRF) closely matches established models by Mathis et al. (1983) and Draine (2011), as depicted in Figure 2 of the study. Monte Carlo radiative transfer calculations, performed using the MoCafe code developed by Seon (2015) and Seon & Draine (2016), produced colour maps, g −r, r −i, and i−z, that accurately reproduce observational data from Román et al. (2020). Researchers have identified over 5000 cirrus clouds, and are developing methods, including machine learning techniques analysing optical colours, to distinguish these from extragalactic sources, leveraging differences in their spectral properties. This work highlights the importance of accurately mapping dust distribution in galactic halos and the CGM, as well as studying the multiphase gas surrounding galaxies. The authors acknowledge limitations in spatial resolution from existing far-infrared surveys, and suggest future research should focus on utilising K-DRIFT’s capabilities for detailed observations of these phenomena, potentially revealing previously unseen features in the CGM and galactic halos.