How Robust are the Inferred Density and Metallicity of the
  Circumgalactic Medium?
  Quantitative estimates of the basic properties of the circumgalactic
medium(CGM), such as its density and metallicity, depend on the spectrum of
incident UV background radiation. Models of UV background are known to have
large variations, mainly because they are synthesized using poorly constrained
parameters, which introduce uncertainty in the inferred properties of the CGM.
Here, we quantify this uncertainty using a large set of new UV background
models with physically motivated toy models of metal-enriched CGM. We find
that, the inferred density and metallicity of low-density ($10^{-5}$ cm$^{-3}$)
gas is uncertain by factors of 6.3 and 3.2, whereas high density ($10^{-3}$
cm$^{-3}$) gas by factors of 4 and 1.6, respectively. The variation in the
shape of the UV background models is entirely responsible for such a variation
in the metallicity while variation in the density arises from both
normalization and shape of the UV background. Moreover, we find a harder
(softer) UV background infers higher (lower) density and metallicity. We also
study warm-hot gas at $T= 10^{5.5}$ K and find that metallicity is robustly
estimated but the inferred density is uncertain by a factor of 3 to 5.4 for low
to high-density gas. Such large uncertainties in density and metallicity may
severely limit the studies of the CGM and demand better observational
constraints on the input parameters used in synthesizing UV background.
