the standard $\Lambda$CDM cosmology, two modes of (radial) gas accretion occur. Gas entering the potential well of a galaxy gets shocked to the virial temperature of the dark matter halo at an early epoch of infall, and then gradually cools and falls onto the central galaxy (Rees & Ostriker 1977; Silk 1977; Binney 1977, White &Rees 1978). Gas that has never been shock heated close to virial temperature falls onto the galaxy from outside the virial radius along filamentary structures (e.g. Kere[š]{} et al. 2005). This latter mode is termed “cold mode” accretion, and is believed to be the dominant way of gas accretion for galaxies in low mass dark matter halos (Kere[š]{} et al. 2005, Dekel & Birnboim et al. 2006). Numerical studies characterise the filaments of cold mode accretion as cool and clumpy clouds surrounded by ionised gas (Kere[š]{} & Hernquist 2009).

There have been many observational attempts to trace the cold mode accreting gas around galaxies. Most of the evidence comes from observing ionised gas at intermediate to high redshift. QSO absorption lines in Mg II with an inflowing velocity feature have been found to be prevalent at a distance of tens of kpc around star forming 