[Paper Review] Halpha3: an Halpha imaging survey of HI selected galaxies from ALFALFA. II. The star formation properties of galaxies in the Virgo cluster and surroundings

Giuseppe Gavazzi, Michele Fumagalli+6 more|arXiv (Cornell University)|Mar 12, 2013

Galaxies: Formation, Evolution, Phenomena49 citations

TL;DR

This study analyzes Hα imaging of 409 HI-selected galaxies from ALFALFA in the Virgo cluster and surroundings to investigate how environment affects star formation. Using Hα as a tracer of recent star formation and HI deficiency as a measure of environmental perturbation, it finds that ram pressure stripping progressively quenches star formation in late-type galaxies from the outside in, with Virgo removing gas and suppressing SFRs on a 2 Gyr timescale, consistent with the cluster being dynamically young.

ABSTRACT

We present the analysis of Halpha3, an Halpha imaging survey of 409 galaxies selected from the HI Arecibo ALFALFA Survey in the Local Supercluster, including the Virgo cluster. We explore the relations between the stellar mass, the HI mass and the current, massive SFR of nearby galaxies in the Virgo cluster and we compare them with those of isolated galaxies in the Local Supercluster, disentangling the role of the environment in shaping the star formation properties of galaxies at the present cosmological epoch. We investigate the relationships between atomic neutral gas and newly formed stars in different environments, across many morphological types, and over a wide range of stellar masses adopting an updated calibration of the HI deficiency parameter. Studying the mean properties of late-type galaxies in the Local Supercluster, we find that galaxies in increasing local galaxy density conditions (or decreasing projected angular separation from M87) show a significant decrease in the HI content and in the mean specific star formation rate, along with a progressive reddening of their stellar populations. The gradual quenching of the star formation occurs outside-in, consistently with the predictions of the ram pressure model. Once considered as a whole, the Virgo cluster is effective in removing neutral hydrogen from galaxies, and this perturbation is strong enough to appreciably reduce the SFR of its entire galaxy population. An estimate of the present infall rate of 300-400 galaxies per Gyr in the Virgo cluster is obtained from the number of existing HI-rich late-type systems, assuming 200-300 Myr as the time scale for HI ablation. If the infall process has been acting at constant rate this would imply that the Virgo cluster has formed approximately 2 Gyr ago, consistently with the idea that Virgo is in a young state of dynamical evolution.

Motivation & Objective

• To understand how the dense environment of the Virgo cluster affects the star formation and gas content of late-type galaxies.
• To quantify the role of environmental processes—particularly ram pressure—in quenching star formation in HI-rich galaxies.
• To estimate the current infall rate of galaxies into Virgo based on the number of surviving HI-rich systems and the timescale for gas removal.
• To compare the scaling relations between stellar mass, HI mass, and star formation rate in cluster versus field galaxies.

Proposed method

• Hα narrow-band imaging was used to measure the recent star formation rate (SFR) in 409 HI-selected galaxies from the ALFALFA survey.
• The HI deficiency parameter (Def_HI) was recalibrated and used to classify galaxies into three groups: unperturbed (Def_HI ≤ 0.3), perturbed (0.3 < Def_HI < 0.9), and highly perturbed (Def_HI ≥ 0.9).
• Optical data from the Sloan Digital Sky Survey (SDSS) were combined with Hα measurements to derive stellar masses and colors.
• Statistical analysis compared the SFR, HI mass, and stellar mass relations in Virgo versus isolated galaxies in the Local Supercluster.
• The timescale for HI ablation was estimated from the observed fraction of HI-rich galaxies, assuming a constant infall rate.
• The fundamental scaling relation between M_HI and M_* was analyzed to assess how environmental effects alter gas content relative to stellar mass.

Experimental results

Research questions

• RQ1 How does the star formation rate (SFR) of late-type galaxies in the Virgo cluster compare to that of isolated galaxies in the Local Supercluster, as traced by Hα emission?
• RQ2 To what extent does the HI deficiency parameter correlate with the specific star formation rate and stellar population age in Virgo cluster galaxies?
• RQ3 What is the timescale for gas removal in galaxies infalling into the Virgo cluster, and what does this imply about the cluster's dynamical age?
• RQ4 How do the scaling relations between stellar mass, HI mass, and SFR differ between cluster and field galaxies?
• RQ5 Is the observed quenching of star formation in Virgo consistent with ram pressure stripping as the dominant environmental mechanism?

Key findings

• Galaxies in increasingly dense regions of the Virgo cluster show a significant decrease in HI content and specific SFR, with stellar populations progressively reddening with increasing environmental density.
• The quenching of star formation occurs in a radial, outside-in manner, consistent with predictions of the ram pressure stripping model.
• The Virgo cluster as a whole is effective in removing neutral hydrogen, reducing the SFR of its entire galaxy population due to environmental effects.
• The estimated current infall rate of HI-rich galaxies into Virgo is 300–400 galaxies per gigayear, implying a dynamical age of approximately 2 Gyr if the infall has been constant.
• The scaling relation between HI mass and stellar mass in Virgo is significantly offset toward lower gas fractions compared to isolated galaxies, indicating strong environmental quenching.
• Environmental effects are ten times more effective at depleting gas than at directly quenching star formation, highlighting gas removal as the primary driver of quenching.