[Paper Review] The H alpha Galaxy Survey V. The star formation history of late-type galaxies

P. A. James, M. Prescott+1 more|ArXiv.org|Apr 14, 2008

Galaxies: Formation, Evolution, Phenomena42 references12 citations

TL;DR

This study analyzes the star formation history of 117 late-type Im and Sm galaxies using H𝛼 and R-band photometry, finding that their star formation has been predominantly continuous over a Hubble time, with no evidence of strong bursts. The current star formation rate, spatially co-located with the old stellar population, could produce their total stellar masses in ~13.8 Gyr, and gas reservoirs could sustain this rate for over another Hubble time.

ABSTRACT

This study of 117 low-redshift Im and Sm galaxies investigates the star formation rates of late-type galaxies, to determine whether they are quasi-continuous or dominated by bursts with quiescent interludes. We analyse the distribution of star formation timescales (stellar masses/star formation rates) for the entire sample, and of gas depletion timescales for those galaxies with gas mass measurements. We find that, on average, the late-type galaxies studied could have produced their total stellar masses by an extrapolation of their current star formation activity over a period of just under a Hubble time. This is not the case for a comparison sample of earlier-type galaxies, even those with disk-dominated morphologies and similar total stellar masses to the late-type galaxies. The earlier-type galaxies are on average forming their stars more slowly at present than the average rate over their past histories. No totally quiescent Im or Sm galaxies are found, and although some evidence of intrinsic variation in the star formation rate with time is found, this is typically less than a factor of 2 increase or decrease relative to the mean level. The Im and Sm galaxies have extensive gas reservoirs and can maintain star formation at the current rate for more than another Hubble time. The average spatial distribution of star formation in the Im galaxies, and to a lesser extent the Sm galaxies, is very similar to that of the older stellar population traced by the red light.

Motivation & Objective

• To determine whether late-type Im and Sm galaxies exhibit continuous star formation or burst-dominated histories.
• To assess whether the current star formation rate can account for the total stellar mass observed.
• To investigate the spatial correlation between current star formation (H𝛼) and the old stellar population (R-band).
• To evaluate the role of bulge presence in shaping star formation history across galaxy types.
• To examine gas depletion timescales and their implications for long-term star formation sustainability.

Proposed method

• Calculated star formation timescales as the ratio of total stellar mass to current star formation rate (M*/SFR).
• Used R-band photometry and stellar population synthesis models to estimate stellar masses and mass-to-light ratios.
• Compared observed R-K and J-K colours with model predictions to validate mass estimates and SF history assumptions.
• Analyzed mean radial profiles of H𝛼 and R-band fluxes to compare spatial distributions of current and old star formation.
• Measured gas depletion timescales using H I mass measurements from Westerbork observations.
• Applied statistical analysis to scatter in SFR and gas depletion timescales to infer intrinsic variability in star formation rates.

Experimental results

Research questions

• RQ1 Can the current star formation rate in late-type galaxies account for their total stellar mass over a Hubble time?
• RQ2 Is the star formation history of Im and Sm galaxies dominated by brief bursts or continuous activity?
• RQ3 How do the spatial distributions of current star formation (H𝛼) and old stellar populations (R-band) compare in these galaxies?
• RQ4 What is the relationship between gas reservoirs and the sustainability of current star formation rates?
• RQ5 How does the presence or absence of a bulge affect the star formation history of disk-dominated galaxies?

Key findings

• The average star formation timescale across the sample is just under a Hubble time (~13.8 Gyr), indicating continuous star formation.
• No completely quiescent Im or Sm galaxies were found; all show ongoing star formation at measurable levels.
• The spatial distribution of H𝛼 emission closely matches the R-band light profile, indicating co-spatiality between current and old star formation.
• Gas depletion timescales exceed a Hubble time, implying that current star formation can be sustained for more than 13.8 Gyr.
• Intrinsic variations in star formation rate are limited to a factor of ~1.8 above or below the mean, indicating low burstiness.
• Stellar population synthesis models assuming constant star formation accurately reproduce observed colours and mass-to-light ratios.