[Paper Review] Galactic outflows and the kinematics of damped Lyman alpha absorbers

Sungryong Hong, Neal Katz+4 more|arXiv (Cornell University)|Aug 25, 2010

Astrophysics and Cosmic Phenomena4 references9 citations

TL;DR

This study uses high-resolution cosmological hydrodynamical simulations with heuristic galactic outflows to show that momentum-driven wind models successfully reproduce the observed wide-velocity DLA kinematics at z=3, which previous models failed to match. The key result is that outflows enhance the cross-section of massive halos by ejecting gas from small satellite galaxies, thereby increasing the fraction of wide DLAs.

ABSTRACT

The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to reproduce in hierarchical galaxy formation models, particularly the preponderance of wide systems. We investigate DLA kinematics at z=3 using high-resolution cosmological hydrodynamical simulations that include a heuristic model for galactic outflows. Without outflows, our simulations fail to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA kinematics are in much better agreement with observations. Comparing two outflow models, we find that a model based on momentum-driven wind scalings provides the best match to the observed DLA kinematic statistics of Prochaska & Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that would be identified in emission. Narrow DLAs can arise from any halo and galaxy mass, but wide ones only arise in halos with mass >10^11 Mo, from either large central or small satellite galaxies. This implies that the success of this outflow model originates from being most efficient at pushing gas out from small satellite galaxies living in larger halos. This increases the cross-section for large halos relative to smaller ones, thereby yielding wider kinematics. Our simulations do not include radiative transfer effects or detailed metal tracking, and outflows are modeled heuristically, but they strongly suggest that galactic outflows are central to understanding DLA kinematics. An interesting consequence is that DLA kinematics may place constraints on the nature and efficiency of gas ejection from high-z galaxies.

Motivation & Objective

• To resolve the persistent failure of hierarchical galaxy formation models to reproduce the observed wide-velocity extent in damped Lyman-alpha absorbers (DLAs).
• To investigate how galactic outflows influence the kinematics of DLAs in high-redshift cosmological simulations.
• To determine whether outflow models can naturally produce the observed statistical distribution of DLA velocity widths, particularly the prominent tail of wide systems.
• To assess the role of halo mass and galaxy type (central vs. satellite) in producing wide DLAs under different outflow prescriptions.

Proposed method

• Conducting high-resolution cosmological hydrodynamical simulations of galaxy formation at z=3 with a heuristic model for galactic outflows.
• Implementing two outflow models: one based on energy-driven scaling and another on momentum-driven wind scaling.
• Measuring DLA kinematics by identifying H I absorption systems with N_HI > 2×10^20 cm⁻² and analyzing their velocity widths relative to halo virial velocities.
• Comparing simulated DLA velocity width distributions to observational data from Prochaska & Wolfe (1997), particularly the observed tail of wide systems.
• Using the halo mass function and scaling the DLA cross-section as v_vir^3 to assess the impact of outflows on the kinematic distribution.
• Analyzing the spatial and dynamical origin of DLAs, focusing on their offset from central galaxies and their dependence on halo mass and galaxy type.

Experimental results

Research questions

• RQ1 Why do standard hierarchical models fail to reproduce the observed wide-velocity extent in DLA kinematics?
• RQ2 Can galactic outflows—particularly momentum-driven winds—resolve the discrepancy between simulated and observed DLA velocity distributions?
• RQ3 What is the role of halo mass and galaxy type (central vs. satellite) in producing wide DLAs in the presence of outflows?
• RQ4 How do outflows alter the effective cross-section of halos for DLA absorption, especially in low-mass systems?
• RQ5 To what extent do outflows explain the observed kinematic statistics without requiring ad hoc modifications to halo structure or mass function?

Key findings

• Without galactic outflows, simulations fail to produce enough wide DLA systems, replicating the long-standing mismatch with observations.
• The momentum-driven wind model produces DLA kinematics that are in excellent agreement with the observed velocity width distribution from Prochaska & Wolfe (1997).
• Wide DLAs predominantly arise in halos with mass ≥ 10^11 M☉, especially from small satellite galaxies in larger halos, due to efficient gas ejection from these systems.
• Outflows increase the effective cross-section of massive halos by pushing gas out from small satellites, thereby enhancing the probability of wide DLA formation.
• Narrow DLAs can form in halos of any mass, but wide DLAs are strongly correlated with massive halos and are only produced when outflows are active.
• The success of the momentum-driven model suggests that outflows are not just a byproduct but a fundamental mechanism shaping DLA kinematics and potentially constraining the efficiency of high-redshift galactic feedback.