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تسجيل مستخدم جديدDetection of the 13CO(J=6-5) Transition in the Starburst Galaxy NGC 253
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We report the detection of 13CO(J=6-5) emission from the nucleus of the starburst galaxy NGC 253 with the redshift (z) and Early Universe Spectrometer (ZEUS), a new submillimeter grating spectrometer. This is the first extragalactic detection of the 13CO(J=6-5) transition, which traces warm, dense molecular gas. We employ a multi-line LVG analysis and find ~ 35% - 60% of the molecular ISM is both warm (T ~ 110 K) and dense (n(H2) ~ 10^4 cm^-3). We analyze the potential heat sources, and conclude that UV and X-ray photons are unlikely to be energetically important. Instead, the molecular gas is most likely heated by an elevated density of cosmic rays or by the decay of supersonic turbulence through shocks. If the cosmic rays and turbulence are created by stellar feedback within the starburst, then our analysis suggests the starburst may be self-limiting.
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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 hal
f-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.
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.
The CANGAROO-II telescope observed sub-TeV gamma-ray emission from the nearby starburst galaxy NGC 253. The emission region was extended with a radial size of 0.3-0.6 degree. On the contrary, H.E.S.S could not confirm this emission and gave upper lim
its at the level of the CANGAROO-II flux. In order to resolve this discrepancy, we analyzed new observational results for NGC 253 by CANGAROO-III and also assessed the results by CANGAROO-II. Observation was made with three telescopes of the CANGAROO-III in October 2004. We analyzed three-fold coincidence data by the robust Fisher Discriminant method to discriminate gamma ray events from hadron events. The result by the CANGAROO-III was negative. The upper limit of gamma ray flux was 5.8% Crab at 0.58 TeV for point-source assumption. In addition, the significance of the excess flux of gamma-rays by the CANGAROO-II was lowered to less than 4 sigma after assessing treatment of malfunction of photomultiplier tubes.
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.
(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.
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