[Paper Review] The Herschel Virgo Cluster Survey. XX. Dust and gas in the foreground Galactic cirrus
This study analyzes far-infrared and submillimeter dust emission from high-latitude Galactic cirrus in the foreground of the Virgo Cluster using Herschel SPIRE, IRAS-IRIS, and Planck-HFI data, combined with ALFALFA H i gas maps. It derives dust emissivity and absorption cross-section for low- and intermediate-velocity gas components, finding consistent results with Planck at high Galactic latitudes and identifying localized deviations in the dust-gas correlation likely due to small-scale dust property variations or low-column molecular clouds.
We study the correlation between far-infared/submm dust emission and atomic gas column density in order to derive the properties of the high Galactic latitude, low density, Milky Way cirrus in the foreground of the Virgo cluster of galaxies. Dust emission maps from 60 to 850 um are obtained from SPIRE observations carried out within the Herschel Virgo Cluster Survey, complemented by IRAS-IRIS and Planck-HFI maps. Data from the Arecibo legacy Fast ALFA Survey is used to derive atomic gas column densities for two broad velocity components, low and intermediate velocity clouds. Dust emissivities are derived for each gas component and each far-infared/submm band. For the low velocity clouds, we measure an average emissivity 0.79 +/- 0.08 times 1E-20 MJy sr^-1 cm^2 at 250 um. After fitting a modified blackbody to the available bands, we estimated a dust absorption cross-section 0.49 +/- 0.13 times 1E-25 cm^2 H^-1 at 250 um (with dust temperature T = 20.4 +/- 1.5 K and spectral index beta = 1.53 +/- 0.17). The results are in excellent agreement with those obtained by Planck over a much larger coverage of the high Galactic latitude cirrus (50% of the sky vs 0.2% in our work). For dust associated with intermediate velocity gas, we confirm earlier Planck results and find a higher temperature and lower emissivity and cross-section. After subtracting the modelled components, we find regions at scales smaller than 20' where the residuals deviate significantly from the average, cosmic-infrared-background dominated, scatter. These large residuals are most likely due to local variations in the cirrus dust properties (and/or the dust/atomic-gas correlation) or to high-latitude molecular clouds with average N_H2
Motivation & Objective
- To characterize the physical properties of high-latitude, low-density Milky Way cirrus dust in the foreground of the Virgo Cluster.
- To investigate the correlation between far-infrared/submillimeter dust emission and atomic hydrogen (H i) column density at high Galactic latitudes.
- To determine dust emissivity and absorption cross-section for distinct H i velocity components—low-velocity (LVC) and intermediate-velocity (IVC) clouds.
- To assess the impact of cirrus contamination on the detection of intracluster dust in the Virgo Cluster.
- To identify and interpret spatial deviations from the average dust-gas correlation at sub-20 arcmin scales.
Proposed method
- Acquired dust emission maps from Herschel SPIRE at 60–850 μm, supplemented by IRAS-IRIS and Planck-HFI data.
- Used Arecibo ALFALFA Survey data to derive H i column densities for low- and intermediate-velocity gas components.
- Calculated dust emissivity εν as the ratio of dust surface brightness to H i column density across multiple FIR/submm bands.
- Fitted a modified blackbody (MBB) model to the SEDs of LVC and IVC components to derive dust temperature T, spectral index β, and absorption cross-section τν/N_H.
- Performed residual analysis after subtracting modeled LVC and IVC components to identify localized deviations from the average dust-gas correlation.
- Compared results with Planck and FIRAS data to validate emissivity and cross-section measurements and assess consistency with existing dust models.
Experimental results
Research questions
- RQ1What is the dust emissivity and absorption cross-section for low-velocity Galactic cirrus at high Galactic latitudes?
- RQ2How do the dust properties of intermediate-velocity clouds differ from those of low-velocity clouds in terms of temperature, emissivity, and spectral index?
- RQ3To what extent do localized deviations in the dust-gas correlation at scales <20 arcmin indicate variations in dust properties or untraced molecular gas?
- RQ4Can residual emission in the Virgo Cluster field be attributed to intracluster dust, or is it dominated by Galactic cirrus and cosmic infrared background fluctuations?
- RQ5How do the measured dust properties compare with those derived from Planck and FIRAS, and what do they imply for interstellar dust models?
Key findings
- The average dust emissivity for low-velocity clouds (LVC) at 250 μm is ε²⁵⁰ = (0.79 ± 0.08) × 10⁻²⁰ MJy sr⁻¹ cm².
- The LVC dust temperature is T = 20.4 ± 1.5 K, with a spectral index β = 1.53 ± 0.17, and the dust absorption cross-section is τ²⁵⁰/N_H = (0.49 ± 0.13) × 10⁻²⁵ cm² H⁻¹.
- For intermediate-velocity clouds (IVC), emissivity is lower: ε²⁵⁰ = (0.408 ± 0.054) × 10⁻²⁰ MJy sr⁻¹ cm², with a hotter dust temperature and lower cross-section, confirming Planck results.
- Residuals in the dust-gas correlation at scales <20 arcmin show deviations up to 80% above the average scatter, likely due to local variations in dust properties or small molecular clouds with N_H₂ ≲ 10²⁰ cm⁻².
- Negative residuals indicate H i features with no detectable dust emission, suggesting imperfect spatial correspondence between gas and dust tracers at small scales.
- No conclusive evidence for intracluster dust emission is found, as predicted Virgo ICM surface brightness is below the level of residual scatter dominated by cosmic infrared background fluctuations.
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This review was created by AI and reviewed by human editors.