[Paper Review] Did most present-day spirals form during the last 8 Gyrs? A formation history with violent episodes revealed by panchromatic observations

F. Hammer, H. Flores+4 more|CERN Bulletin|Oct 21, 2004

Astronomy and Astrophysical Research158 citations

TL;DR

This paper proposes that most present-day intermediate-mass spirals formed their stars and underwent major structural changes during the last 8 billion years through violent, episodic star formation driven by major mergers, minor mergers, and gas infall. It demonstrates via multi-wavelength observations that 38% of stellar mass in these galaxies formed since z=1, primarily in luminous infrared galaxies (LIRGs), with 75±25% of spirals having recently experienced a major merger, reshaping their bulges and disks in line with hierarchical galaxy formation.

ABSTRACT

(abridged) Studies of distant galaxies have shown that ellipticals and large spirals were already in place 8 Gyr ago, leading to a very modest recent star formation in intermediate mass galaxies. This is challenged by a recent analysis (Heavens et al. 2004) of the fossil record of the stellar populations of ~10^5 nearby galaxies, which show that intermediate mass galaxies have formed or assembled the bulk of their stars 4 to 8 Gyr ago. Here we present direct observational evidence supporting the Heavens et al's findings from a long term, multi-wavelength study of 195 z>0.4 intermediate mass galaxies, mostly selected from the CFRS survey. We show that a recent and efficient star formation is revealed at IR wavelengths since ~15% of intermediate mass galaxies at z>0.4 are indeed luminous IR galaxies (LIRGs), a phenomenon far more common than in the local Universe. The star formation in LIRGs is sufficient in itself to produce 38% of the total stellar mass of intermediate mass galaxies and then to account for most of the reported stellar mass formation since z=1. The high occurrence of LIRGs is easily understood only if they correspond to episodic peaks of star formation, during which galaxies are reddened through short IREs (infrared episodes). We examine how galaxy IREs can be related to the emergence at high redshift, of the abundant population of galaxies with small size, blue core and many irregularities. We show that recent merging and gas infall naturally explain both morphological changes and episodic star formation history in a hierarchical galaxy formation frame. We propose a simple scenario in which 75+-25% of intermediate mass spirals have experienced recently their last major merger event, leading to a drastic reshaping of their bulges and disks during the last 8 Gyrs.

Motivation & Objective

• To resolve the discrepancy between low observed UV star formation rates and high inferred stellar mass formation since z=1.
• To investigate the role of dust-enshrouded star formation (via IR observations) in accounting for the missing stellar mass in intermediate-mass galaxies.
• To test whether violent, episodic star formation linked to mergers and gas infall can explain morphological evolution and mass assembly in spirals.
• To quantify the contribution of major mergers, minor mergers, and gas infall to recent star formation using IR and optical data.
• To reconcile the observed decline in LIRG and compact galaxy densities with the stability of large spiral and elliptical number densities over cosmic time.

Proposed method

• Conducted a long-term, multi-wavelength survey of 195 z > 0.4 intermediate-mass galaxies from the Canada-France Redshift Survey (CFRS), combining UV, optical, and infrared data.
• Used infrared luminosity (LIR > 10^11 L☉) to identify luminous infrared galaxies (LIRGs), which are strong indicators of high, dust-enshrouded star formation.
• Calculated the contribution of LIRGs to total stellar mass formation by integrating their star formation rates and lifetimes, estimating 38% of total stellar mass formed since z=1.
• Modeled the occurrence of infrared episodes (IREs) as short-lived, dust-reddened starburst phases, estimating 4–5 × (τIRE / 0.1 Gyr)^−1 IREs per galaxy from z=1 to z=0.4.
• Analyzed the luminosity-metallicity relation of z≈0.7 emission-line galaxies, finding them on average metal-poor by a factor of ~2 compared to local spirals.
• Evaluated galaxy formation scenarios by testing their consistency with eight observed evolutionary trends, including declining UV and IR luminosity densities, merger rates, and LIRG/compact galaxy number densities.

Experimental results

Research questions

• RQ1 To what extent do dust-enshrouded starbursts (LIRGs) account for the missing stellar mass formation in intermediate-mass galaxies since z=1?
• RQ2 How frequently do intermediate-mass spirals undergo violent, merger-driven star formation episodes (IREs) over the last 8 Gyr?
• RQ3 Can the observed morphological evolution of spirals (e.g., bulge growth, disk formation) be explained by recent major mergers and gas infall within a hierarchical formation framework?
• RQ4 What is the relative contribution of major mergers, minor mergers, and gas infall to the recent peak in cosmic star formation history?
• RQ5 Why do LIRGs and compact, irregular galaxies decline in number density from z≈1 to today, while large spirals and ellipticals remain stable in number density?

Key findings

• Luminous infrared galaxies (LIRGs) at z > 0.4 account for 38% of the total stellar mass in intermediate-mass galaxies, indicating that most recent star formation is hidden by dust.
• The high occurrence of LIRGs at z > 0.4 implies that galaxies experienced episodic, violent star formation, with each galaxy undergoing 4 to 5 × (τIRE / 0.1 Gyr)^−1 infrared episodes (IREs) from z=1 to z=0.4.
• The average metallicity of z≈0.7 emission-line galaxies is about a factor of 2 lower than that of local spirals, supporting the idea of recent, metal-poor star formation episodes.
• 75±25% of intermediate-mass spirals have recently undergone a major merger, which explains the reshaping of their bulges and disks and links distant and local galaxies in a coherent formation scenario.
• The scenario explains the simultaneous decline in UV and IR luminosity densities, merger rates, and number densities of LIRGs and compact galaxies from z=1 to today, while large spirals and ellipticals remain stable.
• The model predicts that 42%, 22%, and 36% of the recent IR star formation density are due to major mergers, minor mergers, and gas infall, respectively, consistent with observational constraints.