[Paper Review] The spiral galaxy M33 mapped in the FIR by ISOPHOT: A spatially resolved study of the warm and cold dust

H. Hippelein, M. Haas+5 more|ArXiv.org|Jun 26, 2003

Galaxies: Formation, Evolution, Phenomena29 references55 citations

TL;DR

This study uses spatially resolved far-infrared (FIR) maps of the spiral galaxy M33 from the ISO/ISOPHOT instrument to separate warm (45 K) and cold (17 K) dust components. It finds that the warm dust traces star-forming regions heated by OB stars, while the cold dust is more uniformly distributed and heated by diffuse interstellar radiation, with a local radio-FIR correlation driven by electron confinement in HII regions rather than global galactic processes.

ABSTRACT

The Sc galaxy M33 has been mapped with ISOPHOT in the far-infrared, at 60, 100, and 170mue. The spatial resolution of these FIR maps allows the separation of spiral arms and interarm regions and the isolation of a large number of star-forming regions. The spectral energy distribution in the FIR indicates a superposition of two components, a warm one originating from dust at ~45K, and a cold one, at ~16K. The warm component is concentrated towards the spiral arms and the star-forming regions, and is likely heated by the UV radiation from OB stars. The cold component is more smoothly distributed over the disk, and heated by the diffuse interstellar radiation. For the about 60 star-forming regions detected the H-alpha/FIR flux ratio increases significantly with the distance from the galaxy center, probably due to decreasing extinction. An anti-correlation of F_Ha/F_60 with F_170 suggests the intrinsic extinction to be related to the cold dust surface brightness according to A_V/S_170~0.03mag/MJy*sr. For the total galaxy the star formation rate (SFR) derived from the FIR is in agreement with that derived from the de-extincted H-alpha emission. For individual star-forming regions, a consistency between SFRs derived from the optical and from the FIR requires only a fraction of the UV radiation to be absorbed locally. The individual star-forming regions also show a local radio-FIR correlation. This local correlation is, however, due to quite different components than to those that lead to the well-known global radio-FIR correlation for entire galaxies.

Motivation & Objective

• To disentangle the spatial distributions of warm and cold dust components in a spiral galaxy using high-resolution FIR data.
• To investigate the heating mechanisms of dust components by comparing their spatial morphology with optical and radio emission.
• To assess the consistency between star formation rates derived from FIR and Hα emission, accounting for extinction effects.
• To examine the origin of the local radio-FIR correlation in star-forming regions and distinguish it from the global radio-FIR correlation in galaxies.

Proposed method

• Acquired deep FIR maps of M33 at 60, 100, and 170 µm using the ISOPHOT instrument on the Infrared Space Observatory (ISO) in raster scan mode.
• Combined partial northern and southern maps using sky subtraction and mosaicking techniques to cover the entire disk and background.
• Fitted spectral energy distributions (SEDs) of the galaxy and individual regions with two modified Planck functions (β ∝ λ⁻²) to separate warm and cold dust components.
• Used Hα emission and radio data to cross-calibrate star formation rates and assess extinction effects via the F_Hα/F_60 flux ratio.
• Analyzed the spatial correlation between FIR, Hα, and radio emission to investigate the origin of the local radio-FIR correlation.
• Applied extinction models based on the relation A_V/S_170 ∼ 0.03 mag MJy⁻¹ sr to quantify dust extinction in star-forming regions.

Experimental results

Research questions

• RQ1 What are the spatial distributions of warm and cold dust components in M33, and how do they correlate with optical and radio emission?
• RQ2 How do the heating mechanisms of warm and cold dust differ, and what role do OB stars and diffuse interstellar radiation play?
• RQ3 To what extent is UV radiation from massive stars absorbed locally in HII regions, and how does this affect the consistency between FIR and Hα-derived star formation rates?
• RQ4 What causes the local radio-FIR correlation in individual star-forming regions, and how does it differ from the global radio-FIR correlation in galaxies?
• RQ5 How does the observed F_Hα/F_60 ratio relate to the 170 µm surface brightness, and what does this imply about dust extinction?

Key findings

• The FIR emission in M33 is composed of a warm dust component at ~45 K, concentrated in spiral arms and HII regions, primarily heated by UV radiation from OB stars.
• A cold dust component at ~17 K is more smoothly distributed across the disk and is heated by the diffuse interstellar radiation field.
• The Hα/FIR flux ratio increases with galactocentric distance, indicating decreasing extinction in outer regions, consistent with a dust extinction relation A_V/S_170 ∼ 0.03 mag MJy⁻¹ sr.
• Star formation rates derived from FIR and de-reddened Hα emission are consistent for the entire galaxy, supporting the reliability of FIR-based SFR estimates.
• For individual HII regions, consistency between optical and FIR SFRs implies only a fraction of UV radiation is absorbed locally, suggesting significant radiation escape.
• A local radio-FIR correlation exists in star-forming regions, but it arises primarily from confinement of relativistic electrons in supernova remnants and local dust absorption, not from the same mechanisms as the global radio-FIR correlation.