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تسجيل مستخدم جديدExtra-planar HI in the starburst galaxy NGC 253
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Observations of the nearby starburst galaxy NGC 253 in the 21-cm line reveal the presence of neutral hydrogen in the halo, up to 12 kpc from the galactic plane. This extra-planar HI is found only in one half of the galaxy and is concentrated in a half-ring structure and plumes which are lagging in rotation with respect to the disk. The HI plumes are seen bordering the bright Halpha and X-ray halo emission. It is likely that, as proposed earlier for the Halpha and the X-rays, also the origin of the extra-planar HI is related to the central starburst and to the active star formation in the disk. A minor merger and gas accretion are also discussed as possible explanations. The HI disk is less extended than the stellar disk. This may be the result of ionization of its outer parts or, alternatively, of tidal or ram pressure stripping.
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Multi-wavelength observations of nearby spiral galaxies have shown that neutral and ionized gas are present up to a few kpc from the disk and that star formation and supernovae probably play an important role in bringing gas into the halo. We have ob
tained very sensitive HI observations of the face-on galaxy NGC 6946 and of the nearly edge-on starburst galaxy NGC 253. We find high velocity HI clouds in NGC 6946 and extra-planar gas with anomalous velocities in NGC 253. In both galaxies there seems to be a close connection between the star-forming disk and the halo HI. In the outer parts of NGC 6946 there is also evidence for recent gas accretion.
We present HI observations of the Sculptor Group starburst spiral galaxy NGC 253, obtained with the Karoo Array Telescope (KAT-7). KAT-7 is a pathfinder for the SKA precursor MeerKAT, under construction. The short baselines and low system temperature
of the telescope make it very sensitive to large scale, low surface brightness emission. The KAT-7 observations detected 33% more flux than previous VLA observations, mainly in the outer parts and in the halo for a total HI mass of $2.1 pm 0.1$ $times 10^{9}$ M$_{odot}$. HI can be found at large distances perpendicular to the plane out to projected distances of ~9-10 kpc away from the nucleus and ~13-14 kpc at the edge of the disk. A novel technique, based on interactive profile fitting, was used to separate the main disk gas from the anomalous (halo) gas. The rotation curve (RC) derived for the HI disk confirms that it is declining in the outer parts, as seen in previous optical Fabry-Perot measurements. As for the anomalous component, its RC has a very shallow gradient in the inner parts and turns over at the same radius as the disk, kinematically lagging by ~100 km/sec. The kinematics of the observed extra planar gas is compatible with an outflow due to the central starburst and galactic fountains in the outer parts. However, the gas kinematics shows no evidence for inflow. Analysis of the near-IR WISE data, shows clearly that the star formation rate (SFR) is compatible with the starburst nature of NGC 253.
Radio halos require the coexistence of extra-planar cosmic rays and magnetic fields. Because cosmic rays are injected and accelerated by processes related to star formation in the disk, they have to be transported from the disk into the halo. A verti
cal large-scale magnetic field can significantly enhance this transport. We observed NGC 253 using radio continuum polarimetry with the Effelsberg and VLA telescopes. The radio halo of NGC 253 has a dumbbell shape with the smallest vertical extension near the center. With an estimate for the electron lifetime, we measured the cosmic-ray bulk speed as 300+/-30 km/s which is constant over the extent of the disk. This shows the presence of a disk wind in NGC 253. We propose that the large-scale magnetic field is the superposition of a disk (r,phi) and a halo (r,z) component. The disk field is an inward-pointing spiral with even parity. The conical (even) halo field appears in projection as an X-shaped structure, as observed in other edge-on galaxies. Interaction by compression in the walls of the superbubbles may explain the observed alignment between the halo field and the lobes of hot Halpha- and soft X-ray emitting gas. The disk wind is a good candidate for the transport of small-scale helical fields, required for efficient dynamo action, and as a source for the neutral hydrogen observed in the halo.
(Abridged) Context. NGC 253 is one of only two starburst galaxies found to emit $gamma$-rays from hundreds of MeV to multi-TeV energies. Accurate measurements of the very-high-energy (VHE) (E $>$ 100 GeV) and high-energy (HE) (E $>$ 60 MeV) spectra a
re crucial to study the underlying particle accelerators and cosmic-ray interaction and transport. Aims. The measurement of the VHE $gamma$-ray emission of NGC 253 published in 2012 by H.E.S.S. was limited by large systematic uncertainties. Here, a measurement of the $gamma$-ray spectrum of NGC 253 is investigated in both HE and VHE $gamma$-rays. Methods. The data of H.E.S.S. observations are reanalysed using an updated calibration and analysis chain. The $Fermi$-LAT analysis employs more than 8 years of data processed using pass 8. The cosmic-ray particle population is evaluated from the combined HE--VHE $gamma$-ray spectrum using NAIMA. Results. The VHE $gamma$-ray energy spectrum is best fit by a power-law with a flux normalisation of $(1.34,pm,0.14^{mathrm{stat}},pm,0.27^{mathrm{sys}}) times 10^{-13} mathrm{cm^{-2} s^{-1} TeV^{-1}}$ at 1 TeV -- about 40 % above, but compatible with the value obtained in Abramowski et al. (2012). The spectral index $Gamma = 2.39 pm 0.14^{mathrm{stat}} pm 0.25^{mathrm{sys}}$ is slightly softer than but consistent with the previous measurement. At energies above $sim$3 GeV the HE spectrum is consistent with a power-law ranging into the VHE part of the spectrum measured by H.E.S.S. Conclusions. Two scenarios for the starburst nucleus are tested, in which the gas in the starburst nucleus acts as a target for hadronic cosmic rays. In these two models, the level to which NGC,253 acts as a calorimeter is estimated to a range of $f_{rm cal} = 0.1$ to $1$ while accounting for the measurement uncertainties.
We present a study of the young population in the starburst galaxy NGC 253. In particular, we focused our attention on searching young star groups, obtaining their main properties and studying their hierarchical organization. For this task, we used m
ultiband images and their corresponding photometric data obtained with the Advanced Camera for Surveys of the Hubble Space Telescope (ACS/HST). We have first derived the absorption affecting the different regions of the galaxy. Then, we applied an automatic and objective searching method over the corrected data in order to detect young star groups. We complemented this result with the construction of the stellar density map for the blue young population. A statistical procedure to decontaminate the photometric diagrams from field stars was applied over the detected groups and we estimated their fundamental parameters. As a result, we built a catalog of 875 new identified young groups with their main characteristics, including coordinates, sizes, estimated number of members, stellar densities, luminosity function (LF) slopes and galactocentric distances. We observed these groups delineate different structures of the galaxy, and they are the last step in the hierarchical way in which the young population is organized. From their size distribution, we found they have typical radius of $sim 40 - 50$ pc. These values are consistent with those ones found in others nearby galaxies. We estimated a mean value of the LF slope of 0.21 and an average density of 0.0006 stars/pc$^3$ for the identified young groups taking into account stars earlier than B6.
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