A 0.6 Mpc H i structure associated with Stephan’s Quintet

Abstract

Stephan’s Quintet (SQ, co-moving radial distance = 85 ± 6 Mpc, taken from the NASA/IPAC Extragalactic Database (NED)1) is unique among compact groups of galaxies2–12. Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have probably generated tidal debris in the form of multiple gaseous and stellar filaments6,8,13, the formation of tidal dwarfs7,14,15 and intragroup-medium starbursts16, as well as widespread intergalactic shocked gas5,10,11,17. The details and timing of the interactions and collisions remain poorly understood because of their multiple nature18,19. Here we report atomic hydrogen (H i) observations in the vicinity of SQ with a smoothed sensitivity of 1σ = 4.2 × 1016 cm−2 per channel (velocity bin-width Δv = 20 km s−1; angular resolution = 4′), which are about two orders of magnitude deeper than previous observations8,13,20,21. The data show a large H i structure (with linear scale of around 0.6 Mpc) encompassing an extended source of size approximately 0.4 Mpc associated with the debris field and a curved diffuse feature of length around 0.5 Mpc attached to the south edge of the extended source. The diffuse feature was probably produced by tidal interactions in early stages of the formation of SQ (>1 Gyr ago), although it is not clear how the low-density H i gas (NH i ≲ 1018 cm−2) can survive the ionization by the intergalactic ultraviolet background on such a long time scale. Our observations require a rethinking of properties of gas in outer parts of galaxy groups and demand complex modelling of different phases of the intragroup medium in simulations of group formation.

Deep H i Mapping of Stephan’s Quintet and Its Neighborhood

“…Thus, the distribution of the diffuse H I and its velocity field can put new and crucial constraints to the interaction history models. The discovery of a large diffuse feature of the H I emission in the outskirts of intragroup medium of SQ was reported in a previous paper (Xu et al 2022). In this paper we present other results from the same observations, with the focus on the contribution of the diffuse gas to the total H I mass in the central region of the intragroup medium, and the detection of several neighboring galaxies.…”

“…The 6600 component, containing the majority of the H I emission associated with SQ (Figure 2), shows a strong central source plus low-level emissions in the outskirts. In particular, an arm/tail-like diffuse feature on the south of SQ has been discovered within this component (Xu et al 2022). The H I gas in the diffuse feature, which is outside the aperture within which the SQ spectrum is extracted (Figure 1), is confined in the velocity range of 6550-6750 km s −1 .…”

“…The FAST observations and the data reduction, including the sidelobe correction, are presented in detail in Xu et al (2022).…”

“…The central red circle ( = ¢ D 12 ) shows the aperture with which the total H I spectrum presented in Figure 2 is extracted. The orange line delineates the diffuse H I feature whose systemic velocity is written in brown labels (Xu et al 2022). Provided in the figure is also information for the member galaxies and the main tidal features in SQ (in red labels), for neighboring galaxies detected by FAST (in blue labels) whose positions are marked by blue circles, and for the neighboring galaxy KAZ 295 (in blue labels).…”

H i Absorption in Low-power Radio AGNs Detected by FAST

“…On the other hand, detailed observations with high spatial resolution can provide information on the origins and impacts of H I absorbing gas in low-power AGNs, which may help in investigating various underlying physical processes of H I absorptions and AGN activity between lowpower and high-power sources. Constructed as the most sensitive single-dish radio telescope, the Five-hundred-meter Aperture Spherical radio Telescope (FAST; Nan 2006;Nan et al 2011;Jiang et al 2020) has made significant progress in probing the faintest H I gas in our Milky Way (Hong et al 2022) and the nearby universe (Zhu et al 2021;Xu et al 2022). FAST has also successfully carried out a blind search and targeted observations of H I 21 cm absorption lines (Zhang et al 2021;Hu et al 2023;Jing et al 2023).…”