Intense Star Formation within Resolved Compact Regions in a Galaxy at z=2.3 (1003.3674v1)

Abstract: Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimeter (sub-mm) wavelengths and which may be forming stars at rates upto 1,000Mo/yr. These intense bursts of star formation are believed to be driven by mergers between gas rich galaxies. However, probing the properties of individual star-forming regions within these galaxies is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here, we report observations of the sub-mm galaxy SMMJ2135-0102 at redshift z=2.3259 which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This cosmic magnification, when combined with high-resolution sub-mm imaging, resolves the star-forming regions at a linear scale of just ~100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are ~100x larger and 10^7 times more luminous. Although vigorously star-forming, the underlying physics of the star formation processes at z~2 appears to be similar to that seen in local galaxies even though the energetics are unlike anything found in the present-day Universe.

• The paper determines a star formation rate of about 210 solar masses per year by resolving ~100 pc regions using strong gravitational lensing.
• It employs sub-mm imaging and spectroscopy with a comprehensive lens model to accurately measure the size-luminosity relation similar to local giant molecular clouds.
• The findings imply that star formation physics remains consistent over cosmic time, offering key insights into the evolution of early galaxies.

Intense Star Formation in a Gravitationally Magnified z=2.3 Galaxy

The paper explores the intense star formation activity within spatially resolved regions of the sub-millimeter galaxy SMMJ2135-0102, observed at a redshift of z=2.3259. This galaxy has been gravitationally magnified by a factor of 32, facilitating a detailed examination of its star-forming regions at unprecedented scales (~100 parsecs). The primary focus is understanding the star formation processes distinctive to high-redshift galaxies and contrasting them with those in the local Universe.

Observational Technique and Data

Strong gravitational lensing offered the necessary magnification to analyze SMMJ2135-0102 using sub-mm imaging and spectroscopy. Observations were conducted using instruments such as the APEX/LABOCA and the Sub-Millimeter Array (SMA), providing detailed molecular and continuum emission data. The analysis benefitted from a comprehensive lens model of the interceding galaxy cluster, allowing for corrections in lensing distortions and accurate amplification factor derivation.

Primary Findings

1. Star Formation Rates and Energetics: The star formation rate within SMMJ2135-0102 was determined to be approximately 210 solar masses per year, derived from a bolometric luminosity corrected for lensing. Such high star formation rates, if continuous, suggest a significant build-up of stellar mass occurring over a relatively short cosmological timescale (~150 Myr).
2. Luminosity Density Comparison: Star-forming regions within SMMJ2135-0102 reveal luminosity densities akin to the dense cores of giant molecular clouds (GMCs) locally, although their size and luminosity are orders of magnitude greater. These regions are two orders of magnitude larger (~100 pc) compared to the microbial-sized star-forming regions in GMCs.
3. Consistency of Star Formation Physics Across Cosmic Time: Despite the vigorous star formation in a high-energy environment, the fundamental physics appears analogous to that in local galaxies. This conclusion is drawn from comparable size-luminosity relations observed.
4. Implications for Star Formation Efficiency: The calculated star formation efficiency matches those found in local ULIRGs and high-redshift SMGs, but is notably lower than in more extreme environments, termed 'hyper-starbursts' at higher redshifts (z~6).
5. Morphological Insights: The galaxy's structure is resolved into discrete components due to lensing, showing spatial resolution approaching that of GMCs in the Milky Way. Sub-mm emissions outline a morphology comprising multiple high-intensity star-forming regions, reflected in robust dynamical and baryonic mass estimations that align well with observed gas and stellar masses.

Implications and Future Work

The research offers profound insights into star formation within distant galaxies, providing a benchmark for comparability with GMCs in the local Universe. Notably, the scaling relation between size and luminosity, prevalent in nearby starburst galaxies, holds robustly yet with higher luminous outputs in the studied galaxy. This positions gravitational lensing as an invaluable tool for cosmological inquiries into early universe galactic evolution.

Looking forward, advancements in telescope technology, paired with serendipitous lensing alignments, are anticipated to refine the understanding of cosmic star formation processes. Observations of additional lensed high-redshift galaxies can further delineate the landscape of early star formation histories and potentially uncover deviations in star formation efficiency amid varying cosmic environments. This approach paves pathways for verifiable cosmological models integrating star formation recipes known from local Universe studies.

The paper adds a pivotal piece to the narrative of star formation in the early universe, outlining complex interplays of cosmic and galactic processes operative in bygone cosmic epochs.