Universe Today
Description:
"A numerical method is described which is specifically designed to treat the dynamics of an infinitesimally this, differentially rotating, gaseous disk. The method is based on the Piecewise Parabolic Method (PPM), a higher-order extension of Godunov's method. Gravitational forces representing a linear spiral density wave in the stellar component of a galaxy are included. The calculation is Eulerian and is performed in a uniformly rotating frame of reference using plane polar coordinates. The equations are formulated in an exact perturbation form to explicitly eliminate all large, opposing terms representing force balance in the unperturbed, axis symmetric state, allowing the accurate computation of small perturbations. The method is ideally suited to the study of the gaseous response to a spiral density wave in a disk galaxy. A series two-dimensional hydrodynamical models is computed to test the gravitational response of a uniform, isothermal, massless gaseous disk to an imposed spiral gravitational perturbation. The parameters describing the mass distribution, rotation properties, and the spiral wave are based on the galaxy NGC 628. The solutions have shocks inside and outside co-rotation, depleting the region around co-rotation. The rate at which this region is depleted depends strongly on the strength of the imposed spiral perturbation. Potential perturbations of 10% of greater produce large radial inflows. The time needed for the gas to fall to the inner Linblad resonance in such models is only a small fraction of the Hubble time. The implied rapid evolution suggests that if galaxies exist with such large perturbations, either gas must be replenished from outside the galaxy or the perturbations must be transitory. Inside co-rotation with the spiral pattern, the loss of angular momentum by the gas increases the angular momentum of the stars, reducing the wave amplitude."
"The face-on spiral galaxy M74 (NGC 628) was observed by XMM-Newton on 2002 February 2. In total, 21 sources are found in the inner 5' from the nucleus (after rejection of a few sources associated to foreground stars). Hardness ratios suggest that about half of them belong to the galaxy. The higher luminosity end of the luminosity function is fitted by a power law of slope -0.8. This can be interpreted as evidence of ongoing star formation, in analogy with the distributions found in disks of other late-type galaxies. A comparison with previous Chandra observations reveals a new ultraluminous X-ray transient (LX~1.5×1039 ergs s-1 in the 0.3-8 keV band) about 4' north of the nucleus. We find another bright transient source (LX~5×1038 ergs s-1) about 5' northwest of the nucleus. The UV and X-ray counterparts of SN 2002ap are also found in this XMM-Newton observation; the hardness ratio of the X-ray counterpart suggests that the emission comes from the shocked circumstellar matter."
"The radial profile of the star-formation rate (SFR) in the galaxy NGC 628 is shown to be modulated by a spiral-density wave. The radial profile of the velocity of gas inflow into the spiral arm is similar to the radial distribution of the surface density of the SFR. The position of the corotation resonance is determined along with other parameters of the spiral-density wave via a Fourier analysis of the azimuthal distribution of the observed radial velocities in annular zones of the disk of NGC 628. The radial profile of the surface density of the SFR is determined using the empirical SFR—linear size relation for star-formation complexes (giant HII regions) and measurements of the coordinates, H alpha fluxes, and the sizes of HII regions in NGC 628."
"The formation of massive stars, usually in (super) star clusters, their rapid evolution and subsequent demise as supernovae has a major impact on their immediate surroundings. The combined effect of stellar winds and Supernovae, going off within rapid succession and within a small volume, creates expanding bubbles of coronal gas within the neutral Interstellar medium (ISM) In spiral and (dwarf) irregular galaxies. These expanding shells in turn sweep up and compress neutral gas which can lead to molecular cloud formation and the onset of secondary or induced star formation. Star forming areas disturb their surrounding ISM so a more "active", in terms of star formation, galaxy is expected to have a more inhomogeneous ISM. The star formation rate in NGC 628 is four times higher than in NGC 3184 and twice as high as in NGC 6946, which could explain the greater number of HI holes found in this galaxy. We find that the sizes of the HI holes range from 80 pc (close to the resolution limit) to 600 pc; the expansion velocities can reach 20 km s1; estimated ages are 2.5 to 35 Myr and the energies involved range from 1050 to 3.5 x 105Z ergs. The amount of neutral gas involved is of order 104 to 106 solar masses."
"Using images from the Hubble Space Telescope and the Gemini Telescope, we confirmed the disappearance of the progenitors of two type II supernovae (SNe) and evaluated the presence of other stars associated with them. We found that the progenitor of SN 2003gd, an M-supergiant star, is no longer observed at the SN location and determined its intrinsic brightness using image subtraction techniques. The progenitor of SN 1993J, a K-supergiant star, is also no longer present, but its B-supergiant binary companion is still observed. The disappearance of the progenitors confirms that these two supernovae were produced by red supergiants."
"Interpretation of observed properties of young stellar population in NGC 628 is carried out on the basis of the comparison of the high resolution UBVRI photometry data of 127 H-alpha regions in the galaxy with the detailed grid of the synthetic evolutionary models of stellar systems. The detailed grid of evolutionary models includes 2 regimes of star formation (instantaneous burst and a constant star formation), whole range of IMF (slope and an upper mass limit) and age (from 1 Myr up to 100 Myrs). The chemical abundance of the star forming regions was determined from the independent observations. The solution of the reverse problem of finding of age, regime of star formation, IMF parameters, and dust absorption in the star forming regions is produced with the aid of a special regularizing deviation functional. Reddening estimations are correlated with galactocentric distances of star forming regions, in conformity with a chemical abundance radial gradient derived from independent observations. Ages of star formation complexes also show a trend as a function of chemical composition."
"We have obtained ground-based I, J and K band images of the spiral galaxy, Messier 74 (NGC 628). This galaxy has been shown to possess a circumnuclear ring of star formation from both near-infrared spectroscopy of CO absorption and sub-millimetre imaging of CO emission. Circumnuclear rings of star formation are believed to exist only as a result of a bar potential. We show evidence for a weak oval distortion in the centre of M 74. We use the results of Combes & Gerin (1985) to suggest that this weak oval potential is responsible for the circumnuclear ring of star formation observed in M 74."
History of Observation:
"Nebula without stars, near the star Eta Piscium, seen by M. Mechain at the end of September 1780, and he reports: "This nebula doesn't contain any stars; it is fairly large, very obscure, and extremely difficult to observe; one can recognize it with more certainty in fine, frosty conditions". M. Messier looked for it and found it, as M. Mechain describes it: it has been compared directly with the star Eta Piscium."
"1799, December 28, 40 feet telescope. Very bright in the middle, but the brightness confined to a very small part, and is not round; about the bright middle is a very faint nebulosity to a considerable extent. The bright part seems to be of resolvable kind, but my mirror has been injured by condensed vapours."