Galactic Cirri at High Galactic Latitudes. I. Investigating Scatter in Slopes between Optical and Far-infrared Intensities

Based on the slopes between DESI g , r and Infrared Astronomical Satellite (IRAS) 100 μ m intensities, specifically k _g and k _r , we have constructed a substantial sample of Galactic cirri. This sample covers 561.25 deg ^2 at high Galactic latitudes (∣ b ∣ ≥ 30°), allowing for a systematic study of the physical parameters of the Galactic cirrus on a large scale, such as g − r color, dust temperature, asymmetry factor, and albedo. The ratio of k _g and k _r is consistent with the diffuse Galactic starlight model, suggesting that the diffuse starlight within our own Galaxy serves as the primary illumination source for the cirrus. Both k _g and k _r decrease slowly with increasing Galactic latitudes and IRAS 100 μ m intensities, while they do not have a correlation with Galactic longitudes. The distribution of k _g and k _r confirms a significant scatter in the slopes, reaching a factor of 4–5. Such large scatter cannot be explained by the weak correlation between the slopes and Galactic latitudes and IRAS 100 μ m intensities. Instead, it is attributed to substantial variations in the intrinsic properties of the dust, e.g., asymmetry factor and albedo. We propose that the properties of dust particles play a critical role in the observed scatter in slopes, making them the primary contributing factors. Moreover, the variations in dust properties within the cirrus are localized rather than exhibiting large-scale gradients.