[Paper Review] The Ages and Masses of Lyman Alpha Galaxies at Redshift $z\sim 4.5$

Steven L. Finkelstein, James E. Rhoads+3 more|arXiv (Cornell University)|Dec 18, 2006

Astronomy and Astrophysical Research66 citations

TL;DR

This study uses MMT broadband photometry to determine the ages and stellar masses of 98 Lyman alpha galaxies at z ~ 4.5, finding that galaxies with the highest Lyα equivalent widths are among the youngest known in the early universe (average age 4 Myr), while those with lower EWs are older (40–200 Myr). Stellar masses range from 2×10⁷ to 2×10⁹ M⊙, with dust effects potentially enhancing Lyα EWs in older, dusty systems.

ABSTRACT

We examine the stellar populations of a sample of 98 redshift 4.5 Lyman alpha emitting galaxies using their broadband colors derived from deep photometry at the MMT. These galaxies were selected by narrowband excess from the Large Area Lyman Alpha survey. Twenty-two galaxies are detected in two or more of our MMT filters (g', r', i' and z'). By comparing broad and narrowband colors of these galaxies to synthetic colors from stellar population models, we determine their ages and stellar masses. The highest equivalent width objects have an average age of 4 Myr, consistent with ongoing star formation. The lowest EW objects show an age of 40 - 200 Myr, consistent with the expectation that larger numbers of stars are causing low EWs. We found masses ranging from 2e7 solar masses for the youngest objects in the sample to 2e9 solar masses for the oldest. It is possible that dust effects could produce large equivalent widths even in older populations by allowing the Lyman alpha photons to escape, even while the continuum is extinguished, and we present models for this scenario also.

Motivation & Objective

• To determine the stellar population properties—specifically age and mass—of high-redshift Lyman alpha galaxies at z ~ 4.5.
• To assess the role of dust in enhancing Lyα equivalent widths by comparing observed broadband colors to synthetic models.
• To distinguish between young star-forming populations and older, dust-affected systems in Lyα-emitting galaxies.
• To evaluate whether dust quenching of the continuum, while leaving Lyα unaffected, can explain high observed Lyα EWs.
• To compare derived physical properties with previous studies and assess the significance of sample selection and sensitivity.

Proposed method

• Acquired deep broadband photometry (g’, r’, i’, z’) using the MMT/Megacam for 98 Lyα galaxies selected via narrowband excess from the Large Area Lyman Alpha survey.
• Stacked photometric data into six bins based on Lyα equivalent width (EW) to improve signal-to-noise and derive average colors.
• Compared observed broadband colors (r′−nb, r′−i′) to synthetic colors from stellar population models with varying ages, star formation histories (constant and exponentially declining), and dust extinction.
• Used color-color diagrams to identify regions corresponding to zero-dust, clumpy-dust, and uniform-dust scenarios, assessing likelihood of dust-enhanced Lyα emission.
• Quantified the statistical significance of object positions relative to the zero-dust line using 1σ distance metrics.
• Evaluated the impact of dust on continuum and Lyα emission by modeling clumpy versus uniform dust distributions, particularly in the context of resonant scattering.

Experimental results

Research questions

• RQ1 What are the typical ages and stellar masses of Lyα galaxies at z ~ 4.5, as derived from broadband photometry?
• RQ2 Can dust in a clumpy distribution enhance the observed Lyα equivalent width while suppressing the continuum, explaining high EWs in older systems?
• RQ3 How do the observed broadband colors of these galaxies compare to synthetic models with varying star formation histories and dust content?
• RQ4 To what extent can dust quenching of the continuum, without affecting Lyα photons, account for the observed high Lyα EWs in the sample?
• RQ5 Are the youngest Lyα galaxies in the sample consistent with ongoing star formation, and how do their masses compare to other high-redshift galaxy surveys?

Key findings

• The youngest Lyα galaxies in the sample, with the highest Lyα equivalent widths, have an average age of 4 Myr, consistent with ongoing star formation.
• Galaxies with lower Lyα equivalent widths have older ages, ranging from 40 to 200 Myr, indicating a population with a range of evolutionary stages.
• Stellar masses range from 2×10⁷ M⊙ for the youngest, continuum-undetected objects to 2×10⁹ M⊙ for the oldest, lowest-EW systems.
• The observed color-color distribution shows that most objects lie within 1.1σ of the zero-dust line, suggesting no strong evidence for clumpy dust enhancement in individual objects.
• Stacked data show a mean distance of 1.17σ from the zero-dust line, with smaller error bars allowing more confident exclusion of clumpy dust enhancement in low- and mid-EW stacks.
• The study concludes that while dust may enhance Lyα EWs in some systems, the majority of high-EW objects are best explained by young, low-mass, actively star-forming galaxies rather than dust-reddened older systems.