K-DRIFT Science Theme: Galactic Cirrus Clouds and Circumgalactic Medium

TL;DR

The paper addresses the challenge of studying the circumgalactic medium and extended halos in the presence of foreground Galactic cirrus and extremely low surface brightness features. It assesses the Korea Astronomy and Space Science Institute's K-DRIFT telescope design and multi-band imaging strategy, arguing that wide-field, low-systematic-background observations—especially when combined with narrowband Hα and UV data—can separate cirrus from extragalactic signals and reveal multiphase halo gas and halo dust. It synthesizes current knowledge on Galactic cirrus diagnostics, extraplanar halo dust, intracluster dust, and Hα-traced gas in ram-pressure stripping, and outlines concrete observing modes and science cases for K-DRIFT, including jellyfish tails and DIG halos. The study highlights the potential impact on understanding metal enrichment, gas flows, and the connection between CGM and ICL, providing a framework for a systematic LSB census of galaxy environments with K-DRIFT.

Abstract

In this paper, we review the extended halo material and the circumgalactic medium (CGM), including both dust and gas, and discuss promising science cases that could be realized using the KASI Deep Rolling Imaging Fast Telescope (K-DRIFT). Scattered starlight from cirrus clouds in our Galaxy poses one of the major challenges to studying the low surface brightness features of extragalactic sources. Therefore, it is essential to investigate how to discriminate extragalactic sources from the cirrus cloud features. At the same time, interstellar dust clouds themselves are fundamental to understanding dust properties and the interstellar radiation field, both of which are essential for studies of chemical evolution and star formation in our Galaxy. Measuring the reddening of background sources, such as quasars, with K-DRIFT, which benefits from its broad field of view and accurate background subtraction, allows for effective detection of extended dust in galactic halos, the CGM, and intracluster space. Observations of the H-alpha emission lines can be used to identify signatures of star formation activity within galaxies, as well as the environmental effects acting on them. Galactic winds driven by active galactic nuclei and starbursts can be traced through H-alpha emission. Strong ram pressure stripping effectively removes the interstellar medium (ISM) from galaxies. The stripped ISM becomes ionized or dissociated through mixing with the hot intracluster medium (ICM), forming H-alpha tails. The surface brightness of these H-alpha tails correlates not only with the presence of star formation in the tails but also the mixing stage of the stripped ISM and ICM. The H-alpha survey with K-DRIFT will enable the investigation of the evolutionary stages of ram pressure stripped galaxies in cluster environments, as well as the multiphase gas reservoir around galaxies and in the CGM.

Figures (8)

• Figure 1: Molleview projection map showing the locations and angular sizes of cirrus clouds in Galactic coordinates, with coordinate grids marked at 30$^\circ$ intervals. The data is taken from the catalog of dust clouds compiled by Dutra2002, revealing a total of 5004 cirrus clouds. The size of the circles represents the rough, angular sizes of individual clouds.
• Figure 2: Interstellar radiation field of Mathis1983_ISRF and Draine2011_book. AB magnitudes (left panel) and fluxes (right) in the SDSS $u$, $g$, $r$, $i$, and $z$ bands are shown as orange circles.
• Figure 3: Color maps and color-color diagrams. Top: Color maps obtained from Monte-Carlo simulations. Bottom: Color-color diagrams for the models. In the bottom panels, simulation results are shown in green dots. The black crosses represent the observational results from roman2020. The red diamonds denote the original colors of the ISRF of our Galaxy Mathis1983_ISRFDraine2011_book. The purple and orange contours denote the 1$\sigma$ and 2$\sigma$ significance levels for the colors measured from LSB galaxies in the Stripe82 regions roman2017roman2020. In the model calculations, the $i$- and $z$-band fluxes of the ISRF were lowered by a factor of 1.1 to match the observed color-color diagrams.
• Figure 4: GALEX FUV and NUV maps of NGC 891. The images were rotated such that the major axis of the disk is horizontal. Contour levels are $I=250, 450, 700, 1000, 1500, 2500, 4000$, and 7000 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ Å$^{-1}$. Concentric contours and white regions are artifacts due to the masking of foreground stars. The abscissa ($x$) and ordinate ($z$) denote the physical coordinates along the major and minor axes, respectively, in units of kpc, calculated assuming a distance of 9.5 Mpc to NGC 891. Right ascension and declination coordinates are also indicated by dotted lines. Adopted from Seon2014.
• Figure 5: H$\alpha$ emission of a simulated RPS galaxy after encountering a strong ICM wind lee26. The green and blue contours denote the distribution of all stars and of stars younger than 20 Myr, respectively. Numerous star-forming clumps are visible in the near tail at 300 Myr. The star-forming regions coincide with bright H$\alpha$ cores, but only a small fraction of the H$\alpha$ emission correlates with young stars in the tail.