[Paper Review] Revealing the cold dust in low-metallicity environments: I - Photometry analysis of the Dwarf Galaxy Survey with Herschel

A. Rémy-Ruyer, S. C. Madden+23 more|Ghent University Academic Bibliography (Ghent University)|Sep 5, 2013

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy138 references79 citations

TL;DR

This study presents Herschel photometry of 48 low-metallicity dwarf galaxies, analyzing their far-infrared to submillimeter dust properties via modified blackbody fitting. It reveals warmer dust (median T ≈ 32 K), lower dust-to-stellar mass ratios (0.02% vs. 0.1% in KINGFISH), and a significant submillimeter excess (up to 150%) in 41% of 500 µm-detected dwarfs—strongest in the most metal-poor systems—highlighting the need for longer-wavelength observations.

ABSTRACT

We present new photometric data from our Herschel Key Programme, the Dwarf Galaxy Survey (DGS), dedicated to the observation of the gas and dust in 48 low-metallicity environments. They were observed with PACS and SPIRE onboard Herschel at 70,100,160,250,350, and 500 microns. We focus on a systematic comparison of the derived FIR properties (FIR luminosity, dust mass, dust temperature and emissivity index) with more metal-rich galaxies and investigate the detection of a potential submm excess. The data reduction method is adapted for each galaxy to derive the most reliable photometry from the final maps. PACS flux densities are compared with the MIPS 70 and 160 microns bands. We use colour-colour diagrams and modified blackbody fitting procedures to determine the dust properties of the DGS galaxies. We also include galaxies from the Herschel KINGFISH sample, containing more metal-rich environments, totalling 109 galaxies. The location of the DGS galaxies on Herschel colour-colour diagrams highlights the differences in global environments of low-metallicity galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH galaxies (T_DGS~32 K, T_KINGFISH~23 K). The emissivity index, beta, is ~1.7 in the DGS, but metallicity does not make a strong effect on beta. The dust-to-stellar mass ratio is lower in low-metallicity galaxies: M_dust/M_star~0.02% for the DGS vs 0.1% for KINGFISH. Per unit dust mass, dwarf galaxies emit ~6 times more in the FIR than higher metallicity galaxies. Out of the 22 DGS galaxies detected at 500 micron, 41% present an excess in the submm not explained by our dust SED model. The excess mainly appears in lower metallicity galaxies (12+log(O/H) < 8.3), and the strongest excesses are detected in the most metal-poor galaxies. We stress the need for observations longwards of the Herschel wavelengths to detect any submm excess appearing beyond 500 micron.

Motivation & Objective

• To provide reliable far-infrared photometry for 48 low-metallicity dwarf galaxies observed with Herschel's PACS and SPIRE instruments.
• To compare the FIR/submillimeter dust properties of these dwarfs with more metal-rich galaxies from the KINGFISH sample (totaling 109 galaxies).
• To investigate the presence and origin of a potential submillimeter excess in low-metallicity environments.
• To examine the dependence of dust temperature, emissivity index, and dust mass on metallicity.
• To assess the impact of the radiation field on dust emission per unit dust mass in dwarf galaxies.

Proposed method

• Data reduction tailored to each galaxy to derive the most accurate flux densities from Herschel maps at 70, 100, 160, 250, 350, and 500 µm.
• Cross-calibration of Herschel PACS fluxes with Spitzer MIPS 70 and 160 µm bands to ensure photometric consistency.
• Use of Herschel colour-colour diagrams to analyze the FIR/submillimeter spectral energy distribution (SED) behaviour of DGS galaxies.
• Modified blackbody fitting to derive dust temperature (T), emissivity index (β), dust mass (M_dust), and FIR luminosity (L_FIR).
• Comparison of DGS results with the metal-rich KINGFISH sample to isolate metallicity-driven differences in dust properties.
• Statistical analysis of submillimeter excess by comparing observed 500 µm fluxes with predictions from modified blackbody models assuming β_theo = 2.0.

Experimental results

Research questions

• RQ1 How do the dust temperatures and emissivity indices of low-metallicity dwarf galaxies compare to those of more metal-rich galaxies?
• RQ2 What is the relationship between dust mass and stellar mass in low-metallicity dwarfs relative to metal-rich galaxies?
• RQ3 To what extent does the FIR/submillimeter emission in dwarf galaxies exceed predictions from standard modified blackbody models?
• RQ4 Does the submillimeter excess correlate with decreasing metallicity, particularly in the most metal-poor systems?
• RQ5 Are longer-wavelength observations (beyond 500 µm) necessary to fully resolve the submillimeter excess in low-metallicity galaxies?

Key findings

• The median dust temperature in the DGS sample is approximately 32 K, significantly warmer than the 23 K median in the KINGFISH sample, with a clear trend of increasing temperature at lower metallicities.
• The dust-to-stellar mass ratio in DGS galaxies is only 0.02%, substantially lower than the 0.1% observed in KINGFISH galaxies, indicating less dust per unit stellar mass in low-metallicity environments.
• Despite lower dust masses, dwarf galaxies emit about six times more FIR/submillimeter luminosity per unit dust mass than metal-rich galaxies, indicating a stronger radiation field impact on dust grains.
• Among the 22 DGS galaxies detected at 500 µm, 41% exhibit a submillimeter excess beyond the predictions of a modified blackbody model with β = 2.0, with the excess reaching up to 150% in the most metal-poor systems.
• The submillimeter excess is most prominent in galaxies with 12+log(O/H) ≤ 8.3, suggesting a strong link between low metallicity and excess emission at long submillimeter wavelengths.
• The study underscores the necessity of observations at wavelengths longer than 500 µm to fully characterize the submillimeter excess, as it may only become apparent beyond Herschel's longest bands.