The Dust in M31 (1908.03458v1)

Abstract: We have analysed Herschel observations of M31, using the PPMAP procedure. The resolution of PPMAP images is sufficient (31 pc on M31) that we can analyse far-IR dust emission on the scale of Giant Molecular Clouds. By comparing PPMAP estimates of the far-IR emission optical depth at 300 microns (tau_300), and the near-IR extinction optical depth at 1.1 microns (tau_1.1) obtained from the reddening of RGB stars, we show that the ratio R_OBS.tau = tau_1.1/tau_300 falls in the range 500 to 1500. Such low values are incompatible with many commonly used theoretical dust models, which predict values of R_MODEL.kappa = kappa_1.1/kappa_300 (where kappa is the dust opacity coefficient) in the range 2500 to 4000. That is, unless a large fraction, at least 60%, of the dust emitting at 300 microns is in such compact sources that they are unlikely to intercept the lines of sight to a distributed population like RGB stars. This is not a new result: variants obtained using different observations and/or different wavelengths have already been reported by other studies. We present two analytic arguments for why it is unlikely that at least 60% of the emitting dust is in sufficiently compact sources. Therefore it may be necessary to explore the possibility that the discrepancy between observed values of R_OBS.tau and theoretical values of R_MODEL.kappa is due to limitations in existing dust models. PPMAP also allows us to derive optical-depth weighted mean values for the emissivity index, beta = - dln(kappa_lambda)/dln(lambda), and the dust temperature, T, denoted betabar and Tbar. We show that, in M31, R_OBS.tau is anti-correlated with betabar according to R_OBS.tau = 2042(+/-24)-557(+/-10)betabar. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31.

• The paper presents a detailed analysis of dust emission in M31, revealing significant discrepancies between observed and predicted optical depths.
• The study uses high-resolution Herschel observations at 31 parsecs and the PPMAP technique to resolve dust within giant molecular clouds.
• Results uncover an anti-correlation between the emissivity index and dust temperature, prompting a reassessment of current dust mass opacity coefficients.

Analysis of Dust in M31 Using Herschel Observations

The paper titled "The dust in M31" by A. P. Whitworth et al. presents an in-depth analysis of dust properties within the Andromeda galaxy (M31) by leveraging observations from the Herschel Space Observatory. The focal point of the research is on acquiring a high-resolution understanding of dust characteristics at the scale of Giant Molecular Clouds (GMCs), elucidating discrepancies between observed dust emission and theoretical models, and presenting data that challenge conventional dust models.

The team employs Herschel's far-infrared imaging capabilities to achieve spatial resolutions sufficient to assess dust emission over small scales, approximately 31 parsecs, within M31. This scale enables the examination of dust within significant astrophysical structures such as GMCs. The paper analyzes the optical depth of dust emission at 300 µm, contrasting it with near-infrared extinction optical depths at 1.1 µm obtained through the reddening of red giant branch (RGB) stars. The resulting ratio of these optical depths, termed as ${\cal R}^{\mbox{\tiny obs.}_\tau}$, falls within 500 to 1500, systematically lower than the predictions from standard theoretical dust models, which estimate ${\cal R}^{\mbox{\tiny model}_\kappa}$ in the range of 2500 to 4000.

One of the paper's key contributions is its challenge to standard dust models, suggesting these models fail to capture the true nature of dust within M31. There might be an underlying assumption in these models that a substantial fraction, potentially exceeding 60%, of the dust is in compact sources that escape detection. However, the authors provide arguments against the likelihood of such compact contributions sufficiently explaining the observed discrepancies.

The authors explore an analytic framework via which discrepancies in ${\cal R}^{\mbox{\tiny obs.}_\tau}$ might emerge, including the prospect of unrecognized high concentrations of emitting dust confined within minimal spatial regions or the existence of inadequacies within existing dust models. The paper provides two analytic arguments questioning the contribution from compact sources and thus leans towards implicating flaws within current dust models.

Furthermore, the research produces maps of the emissivity index ${\bar\beta}$ and the dust temperature ${\bar T}$, revealing a significant anti-correlation between ${\cal R}^{\mbox{\tiny obs.}_\tau}$ and ${\bar\beta}$, which could impose a stringent constraint on theories of interstellar dust.

The implications of this research are multifaceted:

1. Theoretical Implications: The findings invite a reevaluation of dust models, suggesting the need for revisions that accommodate lower theoretical values of ${\cal R}^{\mbox{\tiny model}_\kappa}$ compatible with the observed values.
2. Practical Implications: If the dust mass opacity coefficient at 300 µm ($\kappa_{_{300}}$) is indeed underestimated, as suggested by these findings, the recalibration could influence the inferred dust masses of external galaxies, possibly impacting our understanding of rapid dust formation and processing in high-redshift galaxies.
3. Methodological Advances: The application of the PPMAP technique exemplifies a significant advancement in resolving the fine structure of interstellar dust distributions, enabling astronomers to conduct empirical investigations into dust characteristics that were previously averaged over larger, coarser scales.

In conclusion, this paper serves not only as a critical assessment of existing dust models but also as a promotion of novel techniques in dust observation and analysis. It underscores the complexity of interstellar dust, urging continued refinement in both observational strategies and theoretical models. Future work in this arena will likely focus on integrating these observations to develop comprehensive, multi-component dust models that better encapsulate the data presented in this paper, potentially revolutionizing our understanding of dust in galaxies like M31.