Searching for Galactic hidden gas through interstellar scintillation: results from a test with the NTT-SOFI detector

Abstract

Aims. Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at a longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude these scintillation effects to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder toward an observational strategy to search for scintillation, probing the sensitivity of future surveys and estimating the background level. Methods. We searched for scintillation induced by molecular gas in visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen (H 2 −He) during two nights. We took long series of 10 s infrared exposures with the ESO-NTT telescope toward stellar populations located behind visible nebulae and toward the Small Magellanic Cloud (SMC). We therefore searched for stars exhibiting stochastic flux variations similar to what is expected from the scintillation effect. According to our simulations of the scintillation process, this search should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of ∼3 × 10 −5 g/cm 2 /10 000 km. Results. We found one light-curve that is compatible with a strong scintillation effect through a turbulent structure characterized by a diffusion radius R diff < 100 km in the B68 nebula. Complementary observations are needed to clarify the status of this candidate, and no firm conclusion can be established from this single observation. We can also infer limits on the existence of turbulent dense cores (of number density n > 10 9 cm −3 ) within the dark nebulae. Because no candidate is found toward the SMC, we are also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions. The limits set by this test do not seriously constrain the known models, but we show that the short time-scale monitoring for a few 10 6 star × hour in the visible band with a >4 m telescope and a fast readout camera should allow one to quantify the contribution of turbulent molecular gas to the Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suited for this search.

Photonic orbital angular momentum in starlight

“…where L z is the scale size of fractal structures within the cloud and σ 3n is the dispersion of the volume number density which is limited by the number density of the medium (Habibi et al 2011). His analysis lead to estimated values of R diff = 17 Km for L z = 30 AU size "clumpuscules" within a much larger interstellar cloud having a mean density of 10 10 cm −3 at λ = 1.25 μm which demonstrates sufficient strength to cause scintillation.…”

“…3. Habibi et al (2011) scaled the turbulence strength as the diffusion radius, R diff , which is proportional to the coherence length, r 0 , for atmospheric turbulence; r 0 = 3.18R diff (Narayan 1992). For an astronomical cloud then, …”

Simulation of optical interstellar scintillation

“…This paper is a companion paper to the observational results published in Habibi et al (2011), and it focusses on the simulation of the scintillation effects that were searched for. Cold transparent molecular clouds are one of the last possible candidates for the missing baryons of cosmic structures on different scales (Pfenniger & Combes 1994;Revaz 2005 andMcGaugh et al 2010).…”

“…Since these hypothesised clouds do not emit or absorb light, they are invisible for the terrestrial observer, so we have to investigate indirect detection techniques. Our proposal for detecting such transparent clouds is to search for the scintillation of the stars located behind the transparent medium, caused by the turbulence of the cloud (Moniez 2003 andHabibi et al 2011). The objective of this technical paper is to describe the way we can connect observations to scintillation parameters through a realistic simulation.…”

“…The objective of this technical paper is to describe the way we can connect observations to scintillation parameters through a realistic simulation. We used these connections in the companion paper (Habibi et al 2011) to establish constraints both from null results (towards SMC) and from observations pointing to a possible scintillation effect (towards nebula B68). Similar studies of propagation through a stochastic medium followed by Fresnel diffraction have been made by Coles et al (1995) and for use in radio-astronomy by Hamidouche & Lestrade (2007).…”