Central Region Of M31 Research Articles

A New Formation Model for M32: A Threshed Early-Type Spiral Galaxy?

The origin of M32, the closest compact elliptical galaxy (cE), is a long-standing puzzle of galaxy formation in the Local Group. Our N-body/smoothed particle hydrodynamics simulations suggest a new scenario in which the strong tidal field of M31 can transform a spiral galaxy into a compact elliptical galaxy. As a low-luminosity spiral galaxy plunges into the central region of M31, most of the outer stellar and gaseous components of its disk are dramatically stripped as a result of M31's tidal field. The central bulge component, on the other hand, is just weakly influenced by the tidal field, owing to its compact configuration, and retains its morphology. M31's strong tidal field also induces rapid gas transfer to the central region, triggers a nuclear starburst, and consequently forms the central high-density and more metal-rich stellar populations with relatively young ages. Thus, in this scenario, M32 was previously the bulge of a spiral galaxy tidally interacting with M31 several gigayears ago. Furthermore, we suggest that cE's like M32 are rare, the result of both the rather narrow parameter space for tidal interactions that morphologically transform spiral galaxies into cE's and the very short timescale (less than a few times 109 yr) for cE's to be swallowed by their giant host galaxies (via dynamical friction) after their formation.

Magnetic field surrounding the starburst nucleus of the galaxy M82 from polarized dust emission

Magnetic fields may play an important role in the star-formation process, especially in the central regions of 'starburst' galaxies where star formation is vigorous. But the field directions are very difficult to determine in the dense molecular gas out of which the stars form, so it has hitherto been impossible to test this hypothesis. Dust grains in interstellar clouds tend to be magnetically aligned, and it is possible to determine the alignment direction based on the polarization of optical light due to preferential extinction along the long axes of the aligned grains. This technique works, however, only for diffuse gas, not for the dense molecular gas. Here we report observations of polarized thermal emission from the aligned dust grains in the central region of M82, which directly traces the magnetic field structure (as projected onto the plane of the sky). Organized field lines are seen around the brightest star-forming regions, while in the dusty halo the field lines form a giant magnetic bubble possibly blown out by the galaxy's 'superwind'.

8.4. Properties of synchrotron emission and magnetic fields in the central region of M31

At the centre of M31 the nonthermal spectral index between λ20 cm and λ6 cm is −0.2. It slowly decreases along the southern arm and the northern filaments visible in Hα, but perpendicular to these features it increases much faster. The magnetic field runs along the arm and the filaments. These phenomena suggest the existence of a mono-energetic source of relativistic electrons in the nucleus.

Low-frequency observations of supernova remnants in M82

We have used MERLIN at 408 MHz to image the central starburst region ofM82 with a resolution of 0.5 arcsec. We detect ~20 supernova remnants at this frequency, which is a much smaller number than we detect at higher frequencies (~50). We attribute this difference to free-free absorption by ionized gas. We find that seven remnants show low-frequency turnovers in their spectra consistent with free-free absorption. These spectra are similarto the spectrum previously obtained for the brightest, youngest supernova remnant, 41.95+57.5. We find a mean optically thin spectral index of -0.6 and emission measures ~ las cm -6 pc for the remnants detected at 408 MHz. These emission measures are an order of magnitude smaller than other published values for the central region of M82 and we therefore suggest that the remnants we detect at 408 MHz are embedded within ionized gas towards the near side of the distribution. In addition to the supernova remnants, we detect the extended background in which several regions attributable to free-free absorption by ionized gas can be seen. The most striking is an extended absorption region (~1 00 pc across) approximately centred on 41.95+57.5 which may be an H II region photoionized by a cluster of early-type stars. The progenitor of 41.95+57.5 may well have been a member of this cluster. We find one source near the dynamical centre of M82, which we speculate could be a possible candidate for an active galactic nucleus.

Compact radio sources in the starburst galaxy M82 and the Sigma-D relation for supernova remnants

We have obtained an 8.4 GHz Very Large Array (VLA) A-array map of the starburst galaxy M82 with a resolution Full Width at Half Maximum (FWHM) approximately 0.182 sec. About 50 compact radio sources in the central region of M82 were detected with a peak surface brightness approximately greater than 10(exp -17) W/Hz/sq m/sr. Comparison with previous observations shows that most sources are declining in flux. Three previously visible sources have faded into the background of our map (approximately less than 0.2 mJy/beam), while a few sources, including the second and third brightest radio sources in M82, may have increased slightly in flux over the last decade. No new radio supernova was found. The birth rate of the compact radio sources is estimated to be 0.11 + or - 0.05/yr. We attribute the population of such bright, small supernova remnants (SNRs) in M82 to the high pressure in the central region that can truncate the mass loss during a red supergiant phase or allow dense ionized clouds to be present. The compact radio sources obey a Sigma(radio surface brightness) - D(diameter) relation which is remarkably similar to that followed by supernova remnants in the Galaxy and the Magellanic Clouds and by two of the strongest known extragalactic radio supernovae: SN 1986J and SN 1979C. A least-squares fit to the SNR data gives: Sigma(sub 8.4 GHz) (W/Hz/sq m/sr) = 4.4 x 10(exp -16) D(sub pc)(exp -3.5 +/- 0.1) covering seven orders of magnitude in Sigma. Possible selection effects are discussed and a theoretical discussion of the correlation is presented.

CO Emission from Molecular Clouds in the Central Region of M31

We detected 12 CO (J=1-0) line emission from a complex of dark cloud located at ∼250 pc from M31's center, which is the most conspicuous extinction feature within the central few hundred parsecs as observed with B, V, R, and I band CCD observations. The darkest cloud has a 30 pc size, and the virial mass is estimated to be ∼8.7×10 5 M ○. , which is the most massive molecular cloud in the bulge of M31. The conversion factor from CO intensity to H 2 column mass is found to be by an order of magnitude greater than that for galactic molecular clouds. The hydrogen mass-to-color excess ratio is also found to be anomalously large

Asymptotic giant branch stars in the central regions of M32

We discuss high-resolution (FWHM∼0″.4) near-infrared images of the central regions of the Local Group compact elliptical galaxy M32. The data are used to study the photometric properties of bright asymptotic giant branch stars in three annuli located within 30″ of the galaxy center. The brightest AGB stars in each annulus occur near I∼19, and hence are roughly a magnitude brighter than the brightest stars in a field 2' from the galaxy center.

CO (3 to 21) mapping and gas excitation in the core of M82

A 14-arcsec FWHM beam has been used to observe the CO (3 to 21) emission in the central region of M82; the map thus obtained clearly shows the double-peaked structure, seen at other wavelengths, which has been interpreted as a circumnuclear ring. The first (C-13)O (3 to 2) detections in M82 are also reported. Using a large velocity gradient model, the six lines constituting the three lowest transitions of (C-12)O and (C-13)O allow a direct determination to be made of the properties of the molecular gas in M82; the ratios of these lines are rather unusual, and cannot be reproduced with a standard one-component model. A one-component model with a lowered (C-12)O/(C-13)O abundance ratio with values below 25, similar to those at the center of the Galaxy, yields a significantly better fit to all of the CO lines. 24 refs.

Long-slit spectroscopy of near-ultraviolet NH absorption in the nuclei of M31

Using the Canada-France-Hawaii Telescope long-slit near-ultraviolet spectra of the central regions of M31 and M32 have been obtained. The Delta v = 0 NH absorption bands near 3360 A are detected out to distances of about 32 arcsec in M31 and about 15 arcsec in M32, and the spatial behavior of these features in each galaxy has been studied. NH absorption in M31 appears to vary in a complicated manner with respect to radial distance from the galaxy center, and there is no evidence for a systematic gradient. Moreover, the variations in NH strength appear to be more or less symmetric on a north/south axis about the central nucleus of M31. As for M32, NH absoption becomes progressively stronger with increasing distance from the center of this galaxy. Various explanations for the radial behavior of NH absorption in each galaxy are considered; these results may reflect changes in the relative contributions of early-type stars to the near-ultraviolet light output, or a radial variation in N/Fe. 43 refs.

A minimization procedure applied to population synthesis in galaxy nuclei using a star cluster library: M31, M32

We apply the MINOS optimization system to the population synthesis of galactic nuclei, using a grid of star cluster equivalent widths as a function of age and metallicity. For some classes of red galaxy nuclei, this observational approach to population synthesis produces results similar to those predicted by the theoretical approach of Arimoto and Yoshii (1987). The synthesis results for blue nuclei tend to scatter more in the age vs metallicity plane, probably because we use only visible and near-infrared spectral data. Additional information at shorter wavelengths will possibly produce better-focused solutions in the plane. However, strong bursts of star formation can easily be identified. We provide in this contribution population synthesis for the central regions of M31 and M32. The bulge and the semi-stellar nucleus of M31 are dominated by the old metal-rich population: the semi-stellar nucleus has reached up [Z/Z⊙]≃0.6 and the bulge [Z/Z⊙]≃0.3. In the central region of M32, a metallicity up to the solar value has been reached and the synthesis indicates that the intermediate age component is not dominant.

Shell-like Structure in 41.9+58, A Powerful SNR in M82

We have studied the brightest of the many compact radio components, designated 41.9+58, in the central region of M82 (Unger et al. 1984; Kronberg et al. 1985) with the EVN at frequencies of 1.6 and 5 GHz (Wilkinson and de Bruyn, 1984; 1987).

Emission line velocity field in the central region of M82

Emission line velocity field in the central region of M82

The distribution of the CO J = 2-1 emission from M82

The CO J = 2-1 emission has been mapped for the first time at high spatial resolution in the central region of M82. The region of strong CO emission has an extent of 750 x 550 pc and has a radial velocity distribution which is centered about a mean systemic velocity of v/sub LSR/ = 220 km s/sup -1/ and which is in agreement with the (Ne II) rotation curve. anomalous emission is found at v/sub LSR/ = 170 km s/sup -1/ which is displaced by 250 pc (0.25) to the SW of the center of the CO emission and has a significant minor axis extension. This emission may be related to the compact radio source 41.9+58, the optical filaments, or possibly to the intergalactic gas cloud with which M82 is thought to be colliding.

Velocity dispersions in M31 and M32

view Abstract Citations (41) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Velocity dispersions in M31 and M32 Whitmore, B. C. Abstract Velocity dispersions have been obtained in the central regions of M31 and M32 using spectra obtained with the 1.3-m telescope at McGraw-Hill Observatory. Using a Fourier quotient method, it is found that velocity dispersion in the nucleus of M31 is 181 + or - 12 km/s; and in the bulge it is 151 + or - 16 km/s. Numerical experiments have been performed which indicate that each of these values must be corrected for mixing by light from the other component and for rotational contamination. The ratio of the intrinsic dispersions has been estimated to be sigma(bulge)/sigma(nuc) = 0.83 + or - 0.12. The velocity dispersion in the nucleus of M32 was found to be 75 + or - 7 km/s. Publication: The Astrophysical Journal Pub Date: November 1980 DOI: 10.1086/158442 Bibcode: 1980ApJ...242...53W Keywords: Andromeda Galaxy; Galactic Nuclei; Stellar Motions; Velocity Distribution; Astronomical Models; Astronomical Spectroscopy; Data Reduction; Elliptical Galaxies; Optical Correction Procedure; Astrophysics full text sources ADS | data products NED (2)

Detection of the CO J = 2-1 line in M82 and IC 342

We have searched for the CO(2..-->..1) line at 230 GHz in four galaxies and have detected it in two, IC 342 and M82. Comparison with data for the CO(1..-->..0) line in these galaxies shows that the molecular gas in IC 342 appears to be optically thick; that in M82, however, is of low optical depth. A comparison of the gas and stellar content of the central region of M82 suggests that much of the current star formation is taking place as high-mass stars.

Color Changes and Absorption-Line Variations Over the Inner Disk of M31 and the Central Regions of M32 and NGC 4472

Color changes and absorption line variations in M31, M32 and NGC 4472, attempting synthesis of M31 disk population