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Register a new userThe peculiar filamentary HI structure of NGC 6145
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In this paper, we report the peculiar HI morphology of the cluster spiral galaxy NGC 6145, which has a 150 kpc HI filament on one side that is nearly parallel to its major axis. This filament is made up of several HI clouds and the diffuse HI gas between them, with no optical counterparts. We compare its HI distribution with other one-sided HI distributions in the literature, and find that the overall HI distribution is very different from the typical tidal and ram-pressure stripped HI shape, and its morphology is inconsistent with being a pure accretion event. Only about 30% of the total HI gas is anchored on the stellar disk, while most of HI gas forms the filament in the west. At a projected distance of 122 kpc, we find a massive elliptical companion (NGC 6146) with extended radio emission, whose axis points to an HI gap in NGC 6145. The velocity of the HI filament shows an overall light-of- sight motion of 80 to 180 km/s with respect to NGC 6145. Using the long-slit spectra of NGC 6145 along its major stellar axis, we find that some outer regions show enhanced star formation, while in contrast, almost no star formation activities are found in its center (less than 2 kpc). Pure accretion, tidal or ram-pressure stripping is not likely to produce the observed HI filament. An alternative explanation is the jet-stripping from NGC 6146, although direct evidence for a jet-cold gas interaction has not been found.
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The recent data collected by {it Herschel} have confirmed that interstellar structures with filamentary shape are ubiquitously present in the Milky Way. Filaments are thought to be formed by several physical mechanisms acting from the large Galactic scales down to the sub-pc fractions of molecular clouds, and they might represent a possible link between star formation and the large-scale structure of the Galaxy. In order to study this potential link, a statistically significant sample of filaments spread throughout the Galaxy is required. In this work we present the first catalogue of $32,059$ candidate filaments automatically identified in the Hi-GAL survey of the entire Galactic Plane. For these objects we determined morphological (length, $l^{a}$, and geometrical shape) and physical (average column density, $N_{rm H_{2}}$, and average temperature, $T$) properties. We identified filaments with a wide range of properties: 2$$,$leq l^{a}leq$, 100$$, $10^{20} leq N_{rm H_{2}} leq 10^{23}$,cm$^{-2}$ and $10 leq Tleq$ 35,K. We discuss their association with the Hi-GAL compact sources, finding that the most tenuous (and stable) structures do not host any major condensation and we also assign a distance to $sim 18,400$ filaments for which we determine mass, physical size, stability conditions and Galactic distribution. When compared to the spiral arms structure, we find no significant difference between the physical properties of on-arm and inter-arm filaments. We compared our sample with previous studies, finding that our Hi-GAL filament catalogue represents a significant extension in terms of Galactic coverage and sensitivity. This catalogue represents an unique and important tool for future studies devoted to understanding the filament life-cycle.
We have produced for the first time a detailed velocity map of the giant filamentary nebula surrounding NGC 1275, the Perseus clusters brightest galaxy, and revealed a previously unknown rich velocity structure across the entire nebula. We present new observations of the low-velocity component of this nebula with the optical imaging Fourier transform spectrometer SITELLE at CFHT. With its wide field of view ($sim$11$times$11), SITELLE is the only integral field unit spectroscopy instrument able to cover the 80 kpc$times$55 kpc (3.8$times$2.6) large nebula in NGC 1275. Our analysis of these observations shows a smooth radial gradient of the [N II]$lambda$6583/$text{H} alpha$ line ratio, suggesting a change in the ionization mechanism and source across the nebula, while the dispersion profile shows a general decrease with increasing distance from the AGN at up to $sim 10$ kpc. The velocity map shows no visible general trend or rotation, indicating that filaments are not falling uniformly onto the galaxy, nor being pulled out from it. Comparison between the physical properties of the filaments and Hitomi measurements of the X-ray gas dynamics in Perseus are also explored.
We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40 and 1.5-km/s resolution position-position-velocity cube resulting from the combination of the single-dish and interferometric observations in The HI/OH/Recombination (THOR) line survey. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm, $l approx 28^{circ}$ and $v_{rm LSR}approx 100$ km/s, (ii) toward $l approx 45^{circ}$ and $v_{rm LSR}approx 50$ km/s, (iii) around the Riegel-Crutcher cloud, and (iv) toward the terminal velocities. Comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of the large-scale dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behaviour of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe.
NGC 2419 is a peculiar Galactic globular cluster in terms of size/luminosity, and chemical abundance anomalies. Here, we present Stromgren $uvby$ photometry of the cluster. Using the gravity- and metallicity-sensitive $c_1$ and $m_1$ indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius with an estimated contamination by non-members of only 1%. We derive photometric [Fe/H] of red giants, and depending on which literature metallicity relation we use, find reasonable to excellent agreement with spectroscopic [Fe/H]. We demonstrate explicitly that the photometric errors are not Gaussian, and using a realistic model for the photometric uncertainties, find a formal internal [Fe/H] spread of $sigma=0.11^{+0.02}_{-0.01}$ dex. This is an upper limit to the clusters true [Fe/H] spread and may partially/entirely reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive $delta_4$ anti-correlates strongly with Mg abundance, indicating that the 2nd generation stars are N-enriched. Absence of similar correlations in some other CN-sensitive indices supports the second generation being He-rich, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed $delta_4$ distribution is slightly better fit by two discrete populations, with the N-enhanced stars accounting for 53$pm$5%. NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly N-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as {omega}Cen. (abridged)
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