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 IRAS 100 μ m intensities, specifically kg and kᵣ, we have constructed a substantial sample of Galactic cirri. This sample covers 561.25 deg² 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 kg and kᵣ 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 kg and kᵣ 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 kg and kᵣ 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.