* Astronomy

Blobrana · L

RE: NGC 253

Blobrana · L

Title: HI observations of the nearest starburst galaxy NGC 253 with the SKA precursor KAT-7
Author: D. M. Lucero, C. Carignan, E. C. Elson, T. H. Randriamampandry, T. H. Jarrett, T. A. Oosterloo, G. H. Heald

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±0.1 x 10⁹ solar masses. 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.

Read more (9856kb, PDF)

Blobrana · L

Title: ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
Author: David S. Meier (1,2,3), Fabian Walter (4,2), Alberto D. Bolatto (5), Adam K. Leroy (4), Jürgen Ott (2), Erik Rosolowsky (7), Sylvain Veilleux (8,5), Steven R. Warren (5), Axel Weiss (9), Martin A. Zwaan (10), Laura K. Zschaechner (4) ((1) New Mexico Institute of Mining and Technology, (2) National Radio Astronomy Observatory, Socorro, NM, (3) Adjunct Astronomer, National Radio Astronomy Observatory, (4) Max-Planck Institut für Astronomie, (5) University of Maryland, College Park, Department of Astronomy and Joint Space-Science Institute, (6) National Radio Astronomy Observatory, Charlottesville, VA, (7) University of Alberta, (8) University of Maryland, College Park, (9) Max Planck Institut für Radioastronomie, (10) European Southern Observatory)

We present spatially resolved (~50 pc) imaging of molecular gas species in the central kiloparsec of the nearby starburst galaxy NGC 253, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C17O isotopic line ratio (~=350), we show that 12CO(1-0) has moderate optical depths. A comparison of the HCN and HCO+ with their 13C-substituted isotopologues shows that the HCN(1-0) and HCO+(1-0) lines have optical depths at least comparable to CO(1-0). H13CN/H13CO+ (and H13CN/HN13C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C2H and CN). High density molecular gas tracers (including HCN, HCO+, and CN) are detected at the base of the molecular outflow. We also detect hydrogen recombination lines that, like their counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C2S, H2CS and CH3SH) and have morphologies similar to SiO.

Read more (1099kb, PDF)

Blobrana · L

Sculptor Galaxy

Blobrana · L

RE: NGC 253

Blobrana · L

Title: NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear Emission in NGC 253
Authors: B. D. Lehmer, D. R. Wik, A. E. Hornschemeier, A. Ptak, V. Antoniou, M. K. Argo, K. Bechtol, S. Boggs, F. E. Christensen, W. W. Craig, C. J. Hailey, F. A. Harrison, R. Krivonos, J.-C. Leyder, T.J. Maccarone, D. Stern, T. Venters, A. Zezas, W. W. Zhang

We present results from three nearly simultaneous NuSTAR and Chandra monitoring observations between 2012 Sep 2 and 2012 Nov 16 of local star-forming galaxy NGC 253. The 3-40 keV NuSTAR intensity of the inner 20 arcsec (~400 pc) nuclear region varied by a factor of ~2 across the three monitoring observations. The Chandra data reveal that the nuclear region contains three bright X-ray sources, including a luminous (L2-10 keV ~ few x 10^39 erg/s) point source ~1 arcsec from the dynamical center of the galaxy (within the 3sigma positional uncertainty of the dynamical center); this source drives the overall variability of the nuclear region at energies >3 keV. We make use of the variability to measure the spectra of this single hard X-ray source when it was in bright states. The spectra are well described by an absorbed (NH ~ 1.6 x 10^23 cm^-2) broken power-law model with spectral slopes and break energies that are typical of ultraluminous X-ray sources (ULXs), but not AGN. A previous Chandra observation in 2003 showed a hard X-ray point source of similar luminosity to the 2012 source that was also near the dynamical center (~0.4 arcsec); however, this source was offset from the 2012 source position by ~1 arcsec. We show that the probability of the 2003 and 2012 hard X-ray sources being unrelated is >>99.99% based on the Chandra spatial localisations. Interestingly, the Chandra spectrum of the 2003 source (3-8 keV) is shallower in slope than that of the 2012 hard X-ray source. Its proximity to the dynamical center and harder Chandra spectrum indicate that the 2003 source is a better AGN candidate than any of the sources detected in our 2012 campaign; however, we were unable to rule out a ULX nature for this source. Future NuSTAR and Chandra monitoring would be well equipped to break the degeneracy between the AGN and ULX nature of the 2003 source, if again caught in a high state.

Read more (292kb, PDF)

Blobrana · L

Black Hole Naps Amidst Stellar Chaos

Nearly a decade ago, NASA's Chandra X-ray Observatory caught signs of what appeared to be a black hole snacking on gas at the middle of the nearby Sculptor galaxy. Now, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), which sees higher-energy X-ray light, has taken a peek and found the black hole asleep.
The slumbering black hole is about 5 million times the mass of our sun. It lies at the center of the Sculptor galaxy, also known as NGC 253, a so-called starburst galaxy actively giving birth to new stars. At 13 million light-years away, this is one of the closest starbursts to our own galaxy, the Milky Way.
The Milky Way is all around more quiet than the Sculptor galaxy. It makes far fewer new stars, and its behemoth black hole, about 4 million times the mass of our sun, is also snoozing.
Read more

Blobrana · L

Title: The excitation of near-infrared H2 emission in NGC 253
Authors: M. J. F. Rosenberg, P. P. van der Werf, F. P. Israel

Because of its large angular size and proximity to the Milky Way, NGC 253, an archetypal starburst galaxy, provides an excellent laboratory to study the intricacies of this intense episode of star formation. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and UV excitation as the dominant driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line tracers. Using SINFONI observations, we create linemaps of Br\gamma, [FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of the gas and stellar gas velocity field, we find a kinematic center in agreement with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H_2 line ratios and the morphological comparison between H_2 and Br\gamma and [FeII], we find that excitation from UV photons is the dominant excitation mechanisms throughout NGC 253. We employ a diagnostic energy level diagram to quantitatively differentiate between mechanisms. We compare the observed energy level diagrams to PDR and shock models and find that in most regions and over the galaxy as a whole, fluorescent excitation is the dominant mechanism exciting the H_2 gas. We also place an upper limit of the percentage of shock excited H_2 at 29%. We find that UV radiation is the dominant excitation mechanism for the H_2 emission. The H_2 emission does not correlate well with Br\gamma but closely traces the PAH emission, showing that not only is H_2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Br\gamma, such as B stars.

Read more (385kb, PDF)

Blobrana · L

Title: An X-ray study of the galactic-scale starburst-driven outflow in NGC 253
Authors: Ikuyuki Mitsuishi, Noriko Y. Yamasaki, Yoh Takei

X-ray properties of hot interstellar gas in a starburst galaxy NGC 253 were investigated to gain a further understanding of starburst-driven outflow activity by XMM-Newton and Suzaku. Spectroscopic analysis for three regions of the galaxy characterized by multiwavelength observations was conducted. The hot gas was represented by two thin thermal plasmas with temperatures of kT ~0.2 and ~0.6 keV. Abundance ratios i.e., O/Fe, Ne/Fe, Mg/Fe and Si/Fe, are consistent between three regions, which suggests the common origin of the hot gas. The abundance patterns are consistent with those of type II supernova ejecta, indicating that the starburst activity in the central region provides metals toward the halo through a galactic-scale starburst-driven outflow. The energetics also can support this indication on condition that 0.01-50 {\eta}^0.5 % of the total emission in the nuclear region has flowed to the halo region. To constrain the dynamics of hot interstellar gas, surface brightness and hardness ratio profiles which trace the density and temperature were extracted. Assuming a simple polytropic equation of state of gas, T{rho}^(1-{\gamma}) = const, we constrained the physical condition. {\gamma} is consistent with 5/3 at the hot disk and T is constant (\gamma = 1) in the halo. It is suggested that the hot gas expands adiabatically from the central region towards the halo region while it moves as free expansion from the inner part of the halo towards the outer part of the halo as the outflow. We constrained the outflow velocity to be >100 km s^-1 from the observed temperature gradient in the halo. In comparison with the escape velocity of ~220 km s^-1 for NGC 253, it is indicated that the hot interstellar gas can escape from the gravitational potential of NGC 253 by combining the outflow velocity and the thermal velocity.

Read more (5316kb, PDF)

Blobrana · L

Title: A possible influence of magnetic fields on the rotation of gas in NGC 253
Authors: Joanna Jaocha, ukasz Bratek, Jan Pkala, Marek Kutschera

Magnetic fields present in galaxy NGC 253 are exceptionally strong so that they can influence the rotation of matter and hence the mass-to-light ratio. In this context the issue of the presence of non-baryonic dark matter halo in this galaxy is addressed.

Read more (108kb, PDF)