[Paper Review] Pinning down the ram-pressure-induced halt of star formation in the Virgo cluster spiral galaxy NGC 4388. A joint inversion of spectroscopic and photometric data

C. Pappalardo, A. Lançon+4 more|arXiv (Cornell University)|Feb 18, 2010

Galaxies: Formation, Evolution, Phenomena38 references20 citations

TL;DR

This study combines VLT FORS2 spectroscopy and multi-wavelength photometry to reconstruct the star formation history of NGC 4388 in the Virgo cluster, revealing a rapid halt in star formation approximately 190 ± 30 Myr ago due to ram pressure stripping. The joint inversion of spectroscopic and photometric data using non-parametric and parametric methods pinpoints the stripping age with high precision, consistent with revised dynamical models and extended HI tail observations.

ABSTRACT

In a galaxy cluster, the evolution of spiral galaxies depends on their cluster environment. Ram pressure due to the rapid motion of a spiral galaxy within the hot intracluster medium removes the galaxy's interstellar medium from the outer disk. Once the gas has left the disk, star formation stops. The passive evolution of the stellar populations should be detectable in optical spectroscopy and multi-wavelength photometry. The goal of our study is to recover the stripping age of the Virgo spiral galaxy NGC 4388, i.e. the time elapsed since the halt of star formation in the outer galactic disk using a combined analysis of optical spectra and photometry. We performed VLT FORS2 long-slit spectroscopy of the inner star-forming and outer gas-free disk of NGC 4388. We developed a non-parametric inversion tool that allows us to reconstruct the star formation history of a galaxy from spectroscopy and photometry. The tool was tested on a series of mock data using Monte Carlo simulations. The results from the non-parametric inversion were refined by applying a parametric inversion method. The star formation history of the unperturbed galactic disk is flat. The non-parametric method yields a rapid decline of star formation < 200 Myr ago in the outer disk. The parametric method is not able to distinguish between an instantaneous and a long-lasting star formation truncation. The time since the star formation has dropped by a factor of two from its pre-stripping value is 190 +- 30 Myr. We are able to give a precise stripping age that is consistent with revised dynamical models.

Motivation & Objective

• To determine the time since star formation ceased in the outer disk of NGC 4388, a spiral galaxy in the Virgo cluster, due to ram pressure stripping.
• To develop and validate a joint inversion method that combines spectroscopic and photometric data to recover star formation histories with improved accuracy.
• To test whether the star formation truncation was instantaneous or gradual using both non-parametric and parametric modeling approaches.
• To reconcile the derived stripping age with extended HI tail observations and revised dynamical models of the galaxy's motion.

Proposed method

• A non-parametric inversion tool was developed to reconstruct star formation histories from combined optical spectroscopy and multi-wavelength photometry, minimizing χ² while applying penalization to avoid unphysical oscillations.
• The method was validated using Monte Carlo simulations on mock datasets to assess stability and robustness under varying signal-to-noise ratios and initial guess configurations.
• A parametric method was applied to refine the stripping age by modeling star formation as flat before a sudden drop at a lookback time t, with metallicity fixed at solar.
• The parametric model compared synthetic spectra from different truncation times to the observed outer disk spectrum of NGC 4388, selecting the best-fit stripping age.
• The joint analysis accounted for uncertainties in stellar population models, particularly for young stellar populations (<10 Myr).
• The penalization scheme in the non-parametric method suppressed sharp age variations, necessitating the use of parametric modeling to assess instantaneous vs. gradual truncation.

Experimental results

Research questions

• RQ1 What is the precise time elapsed since star formation was halted in the outer disk of NGC 4388 due to ram pressure stripping?
• RQ2 Can the joint analysis of spectroscopy and photometry resolve whether the star formation decline was instantaneous or gradual?
• RQ3 How does the derived stripping age compare with the observed extent of the extended HI tail in NGC 4388?
• RQ4 Is the non-parametric inversion method stable and reliable under realistic observational noise and model uncertainties?
• RQ5 Does the derived stripping age agree with revised dynamical models of the galaxy’s orbit in the Virgo cluster?

Key findings

• The star formation history of the inner disk of NGC 4388 is flat, indicating no prior truncation or environmental disturbance.
• The non-parametric inversion reveals a rapid decline in star formation activity in the outer disk, with the most significant drop occurring at a lookback time of ≤200 Myr.
• The parametric method yields a best-fit stripping age of 190 ± 30 Myr, corresponding to the time since star formation dropped by a factor of two from its pre-stripping level.
• The derived stripping age is consistent with the observed extent of the extended HI tail (110 × 25 kpc) and the radial velocity difference between the galaxy and the tail.
• The results support a revised dynamical model where the ram pressure peak occurred ~170 Myr ago, in excellent agreement with the derived age.
• The method successfully distinguishes between flat and truncated star formation histories, with the parametric model confirming that the truncation is best described as rapid rather than gradual.