We study the origin of the wide distribution of angles between the angular momenta of the stellar and gas components, $alpha_{rm G,S}$, in early-type galaxies (ETGs). We use the GALFORM model of galaxy formation, set in the $Lambda$ cold dark matter
framework, and coupled it with a Monte-Carlo simulation to follow the angular momenta flips driven by matter accretion onto haloes and galaxies. We consider a gas disk to be misaligned with respect to the stellar body if $alpha_{rm G,S}>30$~degrees. By assuming that the only sources of misaligments in galaxies are galaxy mergers, we place a lower limit of $2-5$ per cent on the fraction of ETGs with misaligned gas/stellar components. These low fractions are inconsistent with the observed value of $approx 42pm 6$ per cent in ATLAS$^{rm 3D}$. In the more general case, in which smooth gas accretion in addition to galaxy mergers can drive misalignments, our calculation predicts that $approx 46$ per cent of ETGs have $alpha_{rm G,S}>30$~degrees. In this calculation, we find correlations between $alpha_{rm G,S}$ and stellar mass, cold gas fraction and star formation rate, such that ETGs with high masses, low cold gas fractions and low star formation rates are more likely to display aligned cold gas and stellar components. We confirm these trends observationally for the first time using ATLAS$^{rm 3D}$ data. We argue that the high fraction of misaligned gas discs observed in ETGs is mostly due to smooth gas accretion (e.g. cooling from the hot halo of galaxies) which takes place after most of the stellar mass of the galaxy is in place and comes misaligned with respect to the stellar component. Galaxies that have accreted most of their cold gas content prior to the time where most of the stellar mass was in place show aligned components.
We study the atomic (HI) and molecular hydrogen (H2) contents of early-type galaxies (ETGs) and their gas sources using the GALFORM model of galaxy formation. This model uses a self-consistent calculation of the star formation rate (SFR), which depen
ds on the H2 content of galaxies. We first present a new analysis of HIPASS and ATLAS3D surveys, with special emphasis on ETGs. The model predicts HI and H2 contents of ETGs in agreement with the observations from these surveys only if partial ram pressure stripping of the hot gas is included, showing that observations of neutral gas in `quenched galaxies place stringent constraints on the treatment of the hot gas in satellites. We find that ~90% of ETGs at z=0 have neutral gas contents supplied by radiative cooling from their hot halos, 8% were supplied by gas accretion from minor mergers that took place in the last 1Gyr, while 2% were supplied by mass loss from old stars. The model predicts neutral gas fractions strongly decreasing with increasing bulge fraction. This is due to the impeded disk regeneration in ETGs, resulting from both active galactic nuclei feedback and environmental quenching by partial ram pressure stripping of the hot gas.
We study the contribution of galaxies with different properties to the global densities of star formation rate (SFR), atomic (HI) and molecular hydrogen (H2) as a function of redshift. We use the GALFORM model of galaxy formation, which is set in the
LCDM framework. This model includes a self-consistent calculation of the SFR, which depends on the H2 content of galaxies. The predicted SFR density and how much of this is contributed by galaxies with different stellar masses and infrared luminosities are in agreement with observations. The model predicts a modest evolution of the HI density at z<3, which is also in agreement with the observations. The HI density is predicted to be always dominated by galaxies with SFR<1Msun/yr. This contrasts with the H2 density, which is predicted to be dominated by galaxies with SFR>10Msun/yr. Current high-redshift galaxy surveys are limited to detect carbon monoxide in galaxies with SFR>30Msun/yr, which in our model make up, at most, 20% of the H2 in the universe. In terms of stellar mass, the predicted H2 density is dominated by massive galaxies, Mstellar>10^10Msun, while the HI density is dominated by low mass galaxies, Mstellar<10^9Msun. In the context of upcoming neutral gas surveys, we suggest that the faint nature of the galaxies dominating the HI content of the Universe will hamper the identification of optical counterparts, while for H2, we expect follow up observations of molecular emission lines of already existing galaxy catalogues to be able to uncover the H2 density of the Universe.
