[Paper Review] Star formation time-scales in the nearby, prototype starburst galaxy M82

Richard de Grijs|arXiv (Cornell University)|Jun 29, 2001

Galaxies: Formation, Evolution, Phenomena2 references3 citations

TL;DR

This paper investigates star formation time-scales in M82, the nearest prototype starburst galaxy, using optical and near-infrared data to analyze young stellar populations and compact clusters. It finds that the current starburst is ~10 Myr old, with a previous burst ~100 Myr ago, and that massive star clusters in M82 B are consistent with young globular cluster analogues, suggesting ongoing globular cluster formation in local starbursts.

ABSTRACT

The last tidal encounter between M82 and M81, some 500 Myr ago, had a major impact on what was probably an otherwise normal, quiescent disc galaxy. It caused a concentrated burst of star formation in the form of massive star clusters, which decreased rapidly, within a few 100 Myr. The current starburst in the centre of the galaxy is likely either due to large-scale propagating star formation or possibly related to late infall of tidally disrupted debris from M82 itself. It may, in fact, be a combination of these two mechanisms, in the sense that the star formation in the active core is actually propagating, while the overall evolution of the starburst is due to tidal debris raining back onto the disc of the galaxy, causing the present-day starburst.

Motivation & Objective

• To determine the star formation time-scales in M82, the nearest and best-studied starburst galaxy.
• To investigate whether the current starburst is driven by large-scale propagating star formation or late infall of tidally disrupted debris.
• To assess the nature and evolution of young massive star clusters in M82, particularly in the fossil starburst region M82 B.
• To evaluate whether these clusters are viable analogues to Galactic globular clusters and what their survival prospects are.
• To constrain the initial mass function and mass-to-light ratios of young clusters using integrated spectral analysis.

Proposed method

• Analyzes ground-based and Hubble Space Telescope optical and near-infrared imaging and spectroscopy to study surface brightness and extinction effects.
• Uses integrated spectrophotometry (20–40 Å resolution) to derive mean stellar population ages from Balmer line strengths and discontinuities.
• Applies theoretical single-burst stellar population models with Salpeter initial mass function to estimate cluster masses from age-dependent mass-to-light ratios.
• Compares observed cluster luminosities and sizes with those of known young compact clusters and Galactic globular clusters.
• Evaluates dynamical mass estimates from high-dispersion spectroscopy for the most luminous clusters, such as M82 F.
• Assesses the possibility of a top-heavy initial mass function in specific clusters using spectral evidence and dynamical constraints.

Experimental results

Research questions

• RQ1 What are the time-scales of past and current star formation episodes in M82’s central starburst region?
• RQ2 Is the current starburst in M82 driven by propagating star formation or by late infall of tidally disrupted material?
• RQ3 Are the young massive clusters in M82 B consistent with being analogues of Galactic globular clusters in terms of mass, size, and evolutionary potential?
• RQ4 What is the initial mass function of the young clusters in M82, and does it deviate from the standard Salpeter form?
• RQ5 What are the survival prospects of these young clusters to evolve into long-lived globular cluster systems?

Key findings

• The current starburst in M82 is approximately 10 Myr old, based on Brγ equivalent width and spectral fitting of the active core.
• A fossil starburst region, M82 B, is consistent with a burst that occurred ~100 Myr ago, as indicated by its A-star dominated spectrum and Balmer discontinuity.
• Young star clusters in M82 B have masses in the range 10^4–10^6 M☉, with a median of 10^5 M☉, and are consistent with being young globular cluster analogues.
• Cluster sizes and morphologies are consistent with those of young compact clusters in other starburst galaxies and with globular cluster progenitors.
• Dynamical mass estimates for the most luminous clusters (e.g., M82 F) are ~10^6 M☉, supporting their massive nature.
• Evidence suggests M82 F may have a top-heavy present-day mass function, but this may be due to local mass segregation rather than a universal initial mass function variation.