Observation of new interstellar clouds in the Libra constellation inside the Local Bubble

¹ Department of Astronomy, ELTE Eötvös Loránd University, 1117 Budapest, Pázmány P. s. 1/A, Hungary
² University of Debrecen, Institute of Physics, Debrecen, Hungary
³ Astropolarimetry Research Group, Office of Supported Research Groups, Hungarian Research Network, 1052 Budapest, Piarista utca 4, Hungary
⁴ Environmental Optics Laboratory, Department of Biological Physics, ELTE Eötvös Loránd University, 1117 Budapest, Pázmány P. s. 1/A, Hungary
⁵ Fornax 2002 Ltd., 2119 Pécel, Ady Endre utca 1, Hungary

^★★ Corresponding author: sliz.judit@ttk.elte.hu

Received: 14 March 2025
Accepted: 22 July 2025

Abstract

Context. The structure of the interstellar medium of our immediate Galactic environment has typically been studied by extinction, line emission, and absorption measurements. Interstellar clouds with moderate extinction (0.3 < A_G < 1), however, may also appear as reflection nebulae in scattered light. Nearby translucent clouds may be detected by their cloud shine even inside the Local Bubble.

Aims. We explore a so far less studied area at high Galactic latitude in a search for translucent clouds inside the Local Bubble.

Methods. We mapped the sky in the visible spectral range on 21 July 2023 on the Namibian Khomas Highland with our telescope, which has a wide field of view. Optical imaging combined with multi-wavelength data helped us to localize clouds. We used Gaia DR3 data to estimate their distances and then derived their physical parameters.

Results. We detected a pattern of elongated reflection nebulae in the Libra constellation at high Galactic latitudes (l = 332°, b = 36°). The stripes of this new interstellar cloud are roughly parallel to the Galactic plane and are associated with similar structures seen in H I 21 cm maps. We identified four cloud layers: (i) The nearest component with an extinction of A_G ≤ 0.2 is closer than 50 pc. (ii) The second component with an extinction of A_G 0.5 has an estimated distance of 60 pc. (iii) The third component lies at 75 pc with a similar extinction. (iv) The fourth component with an extinction of A_G > 1.5 and an estimated distance of 135 pc may correspond to the wall of the Local Bubble (or Local Chimney), while the former components are inside of this wall. We named these clouds the Zebra nebula system because they have strong stripes. The cloud with the highest extinction in the centre of the region we studied is called the Zebra1 nebula. The size of this interstellar cloud is 6.5 pc × 1.6 pc, and it consists of mostly neutral atomic hydrogen, but small dense parts of the cloud may be molecular. Its total mass is 70M_⊙ × (d/[60] pc)². The Zebra1 nebula is surrounded by other less opaque clouds with similar distances. They are all located inside the Local Bubble.

Conclusions. Wide-field optical imaging is capable of locating nearby high-latitude interstellar clouds. Apparently, there are still clouds to discover inside the Local Bubble (or Local Chimney).