SYSTEMATIC CONTINUUM ERRORS IN THE Lyα FOREST AND THE MEASURED TEMPERATURE-DENSITY RELATION

Continuum fitting uncertainties are a major source of error in estimates of the temperature-density relation (usually parameterized as a power-law, T proportional to Delta(gamma-1)) of the intergalactic medium through the flux probability distribution function (PDF) of the Ly alpha forest. Using a simple order-of-magnitude calculation, we show that few percent-level systematic errors in the placement of the quasar continuum due to, e. g., a uniform low-absorption Gunn-Peterson component could lead to errors in gamma of the order of unity. This is quantified further using a simple semi-analytic model of the Ly alpha forest flux PDF. We find that under(over) estimates in the continuum level can lead to a lower (higher) measured value of gamma. By fitting models to mock data realizations generated with current observational errors, we find that continuum errors can cause a systematic bias in the estimated temperature-density relation of approximate to -0.1, while the error is increased to sigma(gamma) approximate to 0.2 compared to sigma(gamma) approximate to 0.1 in the absence of continuum errors.