Lyman-alpha Cooling Emission from Galaxy Formation

Abstract: Recent studies have shown that galaxies accrete most of their baryons via the cold mode, from streams with temperatures T~10^4-10^5 K. At these temperatures, the streams should radiate primarily in the Lya line and have therefore been proposed as a model to power the extended, high-redshift objects known as Lya blobs and other high-redshift Lya sources. We introduce a new Lya radiative transfer code, aRT, and apply it to cosmological hydrodynamical simulations. We address physical and numerical issues that are critical to making accurate predictions for the cooling luminosity, but that have been mostly neglected or treated simplistically so far. We highlight the importance of self-shielding and of properly treating sub-resolution models in simulations. Most existing simulations do not self-consistently incorporate these effects, which can lead to order-of-magnitude errors in the predicted cooling luminosity. Using a combination of post-processing ionizing radiative transfer and re-simulation techniques, we develop an approximation to the consistent evolution of the self-shielded gas. We quantify the dependence of the Lya cooling luminosity on halo mass at z=3 for the simplified problem of pure gas accretion. While cooling in massive halos (without additional energy input from star formation and AGN) is in principle sufficient to produce L_alpha~10^43-10^44 erg s^-1 blobs, this requires including energy released in gas of density sufficient to form stars, but which is kept 100% gaseous in our optimistic estimates. Excluding emission from such dense gas yields lower luminosities by up to one to two orders of magnitude at high masses, making it difficult to explain the observed Lya blobs with pure cooling. Resonant scattering produces diffuse Lya halos, even for centrally concentrated emission, and broad double peaked line profiles. [Abridged]

Lyα Cooling Emission from Galaxy Formation

The paper systematically investigates Lyα cooling emission from galaxy formation, proposing that cold mode accretion is responsible for Lyα emission in high-redshift galaxies. The study introduces the αRT radiative transfer code and utilizes hydrodynamic simulations to understand the transport of Lyα emission from cold accretion streams.

Key Findings

1. Lyα Emission from Cold Mode Accretion: The paper highlights that galaxies primarily accrue baryons via the cold mode, with accretion streams at temperatures (T\sim10^{4}-10^{5}) K emitting in the Lyα line. This emission is relevant for forming the extended Lyα blobs observed in high-redshift galaxies.

2. Radiative Transfer Effects: The αRT code is critical for studying Lyα transport, as radiative transfer influences the emitted luminosity's morphology and spectrum. Bulk velocity flows and resonant scatters significantly affect the emergent Lyα profiles, leading to double-peaked line spectra that are not indicative of galactic halo velocity dispersions.

3. Numerical and Physical Considerations: The study addresses key issues that could result in significant errors in cooling luminosity prediction: self-shielding, numerical resolution, and sub-resolution physics treatment. Accurate modeling of these factors is essential for robust simulation results.

4. Cooling Luminosity Dependency: The Lyα cooling luminosity is dependent on halo mass, ionizing background presence, and the treatment of dense, star-forming gas. Simulations indicate that pure cooling could produce (L_\alpha\sim10^{43}-10^{44}) erg s(^{-1}) luminosities, but these values necessitate considering the energy released from dense gas sufficient for star formation.

Implications and Future Prospects

• Astrophysical Relevance: Understanding Lyα emission from cold mode accretion can illuminate high-redshift galaxy formation mechanisms. While cooling radiation alone may not account for all luminous Lyα blobs, it could still account for less luminous sources or contribute significantly to the blobs' observed spatial extent and characteristics.

• Enhanced Radiative Transfer Models: Including feedback from star formation and AGN and considering self-shielding effects and sub-resolution physics can refine modeling of Lyα cooling. Improved simulations need to address how cooling radiation integrates with galactic winds and other astrophysical processes influencing Lyα signals.

• Self-Shielding Treatment: Self-shielding is pivotal in simulations for predicting cooling luminosities. On-the-fly simulation adjustments like disabling cosmic UV backgrounds in regions above certain densities are essential for modeling self-consistent thermal states.

Conclusions

The paper offers important contributions to understanding the association between galaxy formation and Lyα cooling radiation. It highlights the necessity of accurately capturing radiative transfer, ionization states, temperature distributions, and numerical fidelity to predict Lyα emissions correctly. As methodologies improve, incorporating more comprehensive physics will provide clearer insights into the nature of Lyα blobs and other related high-redshift phenomena.