[Paper Review] Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties

A. Boselli, J. Lequeux+8 more|arXiv (Cornell University)|Apr 2, 1998

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study analyzes mid-infrared (Mid-IR) emission from a complete sample of Virgo cluster galaxies using ISO/ISOCAM data, revealing that early-type galaxies emit primarily via the Rayleigh-Jeans tail of cold stars, while late-type galaxies show strong thermal dust emission. Key findings include a non-linear relationship between Mid-IR and UV radiation fields, with hot dust emission dropping under strong UV fields, and evidence that unidentified infrared bands—dominant at 6.75 μm—are suppressed in low-mass, dwarf galaxies due to UV destruction of carriers.

ABSTRACT

We analyse the integrated properties of the Mid-IR emission of a complete, optically selected sample of galaxies in the Virgo cluster observed with the ISOCAM instrument on board the ISO satellite. The analysis shows that the Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies (E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar component. The Mid-IR emission of late-type galaxies is instead dominated by the thermal emission from dust. The small dust grains emitting in the Mid-IR have an excess of emission if compared to big grains emitting in the Far-IR. While the Far-IR emission increases with the intensity of the interstellar radiation field, their Mid-IR emission is non--linearly related to the UV radiation field. The spectral energy distributions of the target galaxies indicate that there is a linear relationship between the UV radiation field and the Mid-IR emission of galaxies for low or intermediate activities of star formation, while the emission from the hot dust seems to drop for strong UV fields. The Mid-IR colour of late-type galaxies is not related to their activity of star formation. The properties of the dust emission in the Mid-IR seem more related to the mass than to the morphological type of the target galaxy. Since the activity of star formation is anticorrelated to the mass of galaxies, this reflects a relationship between the emission of dust in the Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for a given UV field are low mass, dwarf galaxies. These observational evidences are easily explained if the carriers of the Unidentified Infrared Bands that dominate the 6.75 mic emission are destroyed by the intense UV radiation field of dwarf galaxies, although abundance effects can also play a role.

Motivation & Objective

• To characterize the integrated mid-infrared (Mid-IR) emission properties of a complete, optically selected sample of galaxies in the Virgo cluster.
• To determine the dominant emission mechanisms—stellar vs. dust—across different galaxy types (early-type vs. late-type).
• To investigate the relationship between Mid-IR emission, UV radiation field intensity, and galaxy mass.
• To explore the origin of the 6.75 μm emission feature, particularly its suppression in low-mass, dwarf galaxies.

Proposed method

• Analysis of integrated spectral energy distributions (SEDs) from ISO/ISOCAM observations at wavelengths up to 15 μm.
• Classification of galaxies into early-type (E, S0, S0a) and late-type (spirals, irregulars) based on optical morphology.
• Comparison of Mid-IR emission with UV radiation field intensity derived from optical and UV data.
• Use of mid-IR color indices to assess dust emission properties and their relation to star formation activity.
• Evaluation of the role of dust grain size distribution by comparing small grain (Mid-IR) and large grain (Far-IR) emission features.
• Assessment of the impact of UV radiation on the stability of unidentified infrared bands (UIBs) at 6.75 μm through correlation with galaxy mass and UV field strength.

Experimental results

Research questions

• RQ1 What is the dominant source of mid-infrared emission in early-type galaxies in the Virgo cluster?
• RQ2 How does mid-infrared emission from late-type galaxies relate to the intensity of the interstellar radiation field (ISRF)?
• RQ3 Why is the 6.75 μm emission feature suppressed in low-mass, dwarf galaxies despite their star formation activity?
• RQ4 Is the mid-infrared emission from dust non-linearly related to the UV radiation field, and if so, how does this affect the interpretation of dust heating?
• RQ5 To what extent do galaxy mass and morphological type correlate with mid-infrared dust emission properties?

Key findings

• The mid-infrared emission of early-type galaxies (E, S0, S0a) is dominated by the Rayleigh-Jeans tail of cold stellar emission, not dust.
• Late-type galaxies exhibit strong thermal emission from dust in the mid-infrared, with small grains showing excess emission relative to large grains.
• Mid-infrared emission does not scale linearly with UV radiation field intensity; instead, hot dust emission decreases under strong UV fields.
• A linear relationship exists between UV radiation field and mid-infrared emission for low to intermediate star formation activities, but this breaks down at high UV intensities.
• The 6.75 μm emission feature is suppressed in low-mass, dwarf galaxies, likely due to destruction of the carriers of the unidentified infrared bands (UIBs) by intense UV radiation.
• Dust emission in the mid-infrared is more strongly correlated with galaxy mass than with morphological type, suggesting mass-dependent dust properties or UV field effects.