How the power spectrum of dust continuum images may hide the presence of a characteristic filament width

Context. Herschel observations of interstellar clouds support a paradigm for star formation in which molecular filaments play a central role. One of the foundations of this paradigm is the finding, based on detailed studies of the transverse column density profiles observed with Herschel, that nearby molecular filaments share a common inner width of ∼0.1 pc. The existence of a characteristic filament width has been recently questioned, however, on the grounds that it seems inconsistent with the scale-free nature of the power spectrum of interstellar cloud images. Aims. In an effort to clarify the origin of this apparent discrepancy, we examined the power spectra of the Herschel/SPIRE 250 μm images of the Polaris, Aquila, and Taurus–L1495 clouds in detail and performed a number of simple numerical experiments by injecting synthetic filaments in both the Herschel images and synthetic background images. Methods. We constructed several populations of synthetic filaments of 0.1 pc width with realistic area filling factors (Afil) and distributions of column density contrasts (δc). After adding synthetic filaments to the original Herschel images, we recomputed the image power spectra and compared the results with the original, essentially scale-free power spectra. We used the χ2variance of the residuals between the best power-law fit and the output power spectrum in each simulation as a diagnostic of the presence (or absence) of a significant departure from a scale-free power spectrum. Results. We find that χ2variance depends primarily on the combined parameter δ22 Afil. According to our numerical experiments, a significant departure from a scale-free behavior and thus the presence of a characteristic filament width become detectable in the power spectrum when δ22 Afil ⪆ 0.1 for synthetic filaments with Gaussian profiles and δ22 Afil ⪆ 0.4 for synthetic filaments with Plummer-like density profiles. Analysis of the real Herschel 250 μm data suggests that δ22 Afil is ∼0.01 in the case of the Polaris cloud and ∼0.016 in the Aquila cloud, significantly below the fiducial detection limit of δ22 Afil ∼ 0.1 in both cases. In both clouds, the observed filament contrasts and area filling factors are such that the filamentary structure contributes only ∼1/5 of the power in the image power spectrum at angular frequencies where an effect of the characteristic filament width is expected. Conclusions. We conclude that the essentially scale-free power spectra of Herschel images remain consistent with the existence of a characteristic filament width ∼0.1 pc and do not invalidate the conclusions drawn from studies of the filament profiles.

[1]  P. Andre',et al.  Characterizing the properties of nearby molecular filaments observed with Herschel , 2018, Astronomy & Astrophysics.

[2]  K. Tassis,et al.  Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension , 2018, Science.

[3]  S. Bontemps,et al.  Testing the universality of the star formation efficiency in dense molecular gas , 2017, 1705.00213.

[4]  G. Panopoulou,et al.  A closer look at the “characteristic” width of molecular cloud filaments , 2016, 1611.07532.

[5]  S. Basu,et al.  A MAGNETIC RIBBON MODEL FOR STAR-FORMING FILAMENTS , 2016, 1609.02989.

[6]  N. Peretto,et al.  A census of dense cores in the Taurus L1495 cloud from the Herschel Gould Belt Survey , 2016, 1602.03143.

[7]  C. Federrath On the universality of interstellar filaments: theory meets simulations and observations , 2015, 1510.05654.

[8]  N. Peretto,et al.  Possible link between the power spectrum of interstellar filaments and the origin of the prestellar core mass function , 2015, 1509.01819.

[9]  N. Peretto,et al.  A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey , 2015, 1507.05926.

[10]  E. Rosolowsky,et al.  Filament Identification through Mathematical Morphology , 2015, 1507.02289.

[11]  P. Hennebelle,et al.  Ion-neutral friction and accretion-driven turbulence in self-gravitating filaments , 2013, 1310.3330.

[12]  A. Men'shchikov,et al.  A multi-scale filament extraction method: getfilaments , 2013, 1309.2170.

[13]  Canada.,et al.  WHAT DETERMINES THE DENSITY STRUCTURE OF MOLECULAR CLOUDS? A CASE STUDY OF ORION B WITH HERSCHEL , 2013, 1304.0327.

[14]  N. Peretto,et al.  Herschel view of the Taurus B211/3 filament and striations: evidence of filamentary growth? , 2012, 1211.6360.

[15]  J. Fischera,et al.  Physical properties of interstellar filaments , 2012, 1204.3608.

[16]  N. Peretto,et al.  Astronomy Astrophysics Letter to the Editor Characterizing interstellar filaments with Herschel in IC 5146 ⋆,⋆⋆ , 2022 .

[17]  M. Lombardi,et al.  ON THE STAR FORMATION RATES IN MOLECULAR CLOUDS , 2010, 1009.2985.

[18]  N. Evans,et al.  THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES , 2010, 1009.1621.

[19]  L. Calzoletti,et al.  Herschel : the first science highlights Special feature L etter to the E ditor Direct estimate of cirrus noise in Herschel Hi-GAL images , 2010 .

[20]  R. Emery,et al.  Herschel -SPIRE observations of the Polaris flare: Structure of the diffuse interstellar medium at the sub-parsec scale , 2010, 1005.2746.

[21]  H. Roussel,et al.  From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt survey , 2010, 1005.2618.

[22]  M. Sauvage,et al.  A Herschel study of the properties of starless cores in the Polaris Flare dark cloud region using PACS and SPIRE , 2010, 1005.2519.

[23]  P. Solomon,et al.  The Star Formation Rate and Dense Molecular Gas in Galaxies , 2003, astro-ph/0310339.

[24]  E. Falgarone,et al.  On the Use of Fractional Brownian Motion Simulations to Determine the Three-dimensional Statistical Properties of Interstellar Gas , 2003, astro-ph/0304539.

[25]  S. Miyama,et al.  A Production Mechanism for Clusters of Dense Cores , 1997 .