Gas In Dark Halos Research Articles

Cross-correlation between the soft X-ray background and SZ Sky

While both X-ray emission and Sunyaev-Zel'dovich (SZ) temperature fluctuations are generated by the warm-hot gas in dark matter halos, the two observables have different dependence on the underlying physical properties, including the gas distribution. A cross-correlation between the soft X-ray background (SXRB) and the SZ sky may allow an additional probe on the distribution of warm-hot gas at intermediate angular scales and redshifts complementing studies involving clustering within SXRB and SZ separately. Using a halo approach, we investigate this cross-correlation analytically. The two contributions are correlated mildly with a correlation coefficient of ∼0.3, and this relatively low correlation presents a significant challenge for its detection. The correlation, at small angular scales, is affected by the presence of radiative cooling or preheating and provides a probe on the thermal history of the hot gas in dark halos. While the correlation remains undetectable with CMB data from the WMAP satellite and X-ray background data from existing catalogs, upcoming observations with CMB missions such as Planck, for the SZ side, and an improved X-ray map of the large scale structure, such as the one planned with DUET mission, may provide a first opportunity for a reliable detection of this cross-correlation.

Autocorrelation Function of the Soft X‐Ray Background Produced by Warm‐Hot Gas in Dark Halos

We calculate the angular two-point autocorrelation function (ACF) of the soft X-ray background (SXRB) produced by the warm-hot intergalactic medium (WHIM) associated with dark halos, motivated primarily by searching for missing baryons and distinguishing different physical processes of the WHIM in dark halos. We employ a purely analytic model for the halo population that is completely determined by the universal density profile and the Press-Schechter mass function. We then adopt a phenomenological approach to nongravitational processes of the WHIM such as preheating and radiative cooling. It shows that the power spectra of the SXRB predicted by three WHIM models, namely, the self-similar model, preheating model, and cooling model, demonstrate remarkably different signatures in both amplitude and shape, with the peak locations moving from l ? 4 ? 104 for the self-similar model to a smaller value of l ? (3-5) ? 103 when nongravitational processes are taken into account. The corresponding ACFs for preheating and cooling models become shallower too as compared to the prediction of the self-similar model. This may permit an effective probe of the physical processes of the WHIM in massive halos in conjunction with the observationally determined power spectrum or ACF of the SXRB from diffuse WHIM. However, a direct comparison of our theoretical predictions with existing data (e.g., the ACF determined from ROSAT observations) is still difficult because of the dominant contribution of active galactic nuclei in the soft X-ray sky. We discuss briefly the implications of our results for resolving the missing baryon problem in the local universe.

The photoionization effect of the ultraviolet background on the colour-magnitude relation of elliptical galaxies

ABSTRA C T We have examined the effects of the ultraviolet background radiation (UVB) on the colour ‐ magnitude relation (CMR) of elliptical galaxies in clusters of galaxies in the hierarchical clustering scenario by using a semi-analytic model of galaxy formation. In our model the UVB photoionizes gas in dark haloes and suppresses the cooling of the diffuse hot gas on to galaxy discs. By using a semi-analytic model without the effect of the UVB, Kauffmann & Charlot found that the CMR can be reproduced by strong supernova heating because such supernova feedback suppresses the chemical enrichment in galaxies, especially for small galaxies. We find that the CMR also becomes bluer because of the UVB, in a different way from the effect of supernova feedback. While supernova feedback suppresses the chemical enrichment by a similar mechanism to galactic winds, the UVB suppresses the cooling of the hot gas. This induces suppression of the metallicity of the intracluster medium (ICM). In our model we find that the existence of the UVB can plausibly account for an observed ICM metallicity that is equal to nearly 0.3 times the solar value, and that in this case we can reproduce the CMR and the metallicity of the ICM simultaneously.

On dwarf elliptical galaxies and the faint blue counts

The nature of the faint blue objects seen in deep images of the sky has been a source of great puzzlement. Their high surface density and weak clustering argues against their being either the progenitors or the merging components of the present-day bright galaxies. The faint blue counts are interpreted as being due to dwarf elliptical galaxies undergoing their initial starburst at z∼1. The starburst epoch is determined by the decline in the UV background, which results in the stably confined photoionized gas in dark haloes of M ∼ 10 9 M ⊙ being able to cool and settle in the centre of the haloes and undergo star formation