Region Of M82 Research Articles

Chemical evolution of the M82 B fossil starburst

M82 B is an old starburst site located in the eastern part of the M82 disc. We derive the distributions of age and metallicity of the star clusters located in this region of M82 by using theoretical evolutionary population synthesis models. Our analysis is based on the comparison of the $BVIJ$ photometry obtained by de Grijs et al. (2001) with the colours of single-generation stellar populations. We show that M82 B went through a chemical enrichment phase up to super-solar metallicities around the time of the last close encounter between M82 and its large neighbour galaxy M81. We date and confirm the event triggering the enhanced cluster formation at about 1 Gyr ago. At almost the same time an additional, distinct subpopulation of metal-poor clusters formed in the part of M82 B nearest to the galactic centre. The formation of these peculiar clusters may be related to infall of circumgalactic gas onto M82 B.

On the Temperature Structure of M87

We revisit the XMM-Newton observation of M87 focusing our attention on the temperature structure. We find that spectra for most regions of M87 can be adequately fit by single temperature models. Only in a few regions, which are cospatial with the E and SW radio arms, we find evidence of a second temperature. The cooler component (kT ~ 0.8-1 keV) fills a small volume compared to the hotter component (kT ~ 1.6-2.5 keV), it is confined to the radio arms rather than being associated with the potential well of the central cD and is probably structured in blobs with typical sizes smaller than a few 100 pc. Thermal conduction must be suppressed for the cool blobs to survive in the hotter ambient gas. Since the cool gas is observed only in those regions of M87 where we have evidence of radio halos our results favor models in which magnetic fields play a role in suppressing heat conduction. The entropy of the cool blobs is in general smaller than that of the hot phase gas thus cool blobs cannot originate from adiabatic evolution of hot phase gas entrained by buoyant radio bubbles, as suggested by Churazov et al. (2001). An exploration of alternative origins for the cool gas leads to unsatisfactory results.

Multicolor Photometry of 145 of the H [CSC]ii[/CSC] Regions in M33

This paper is the first in a series presenting CCD multicolor photometry for 145 H II regions, selected from 369 candidate regions from Boulesteix et al., in the nearby spiral galaxy M33. The observations, which covered the whole area of M33, were carried out with the Beijing Astronomical Observatory 60/90 cm Schmidt telescope, in 13 intermediate-band filters, covering a range of wavelengths from 3800 to 10000 A. This procedure provides a series of maps that can be converted into a multicolor map of M33, in pixels of 17 × 17. Using aperture photometry we obtain the spectral energy distributions (SEDs) for these H II regions. We also give their identification charts. Using the relationship between the Beijing-Arizona-Taiwan-Connecticut intermediate-band system used for the observations and the UBVRI broadband system, the magnitudes in the B and V bands are then derived. Histograms of the magnitudes in V and in B-V are plotted, and the color-magnitude diagram is also given. The distribution of magnitudes in the V band shows that the apparent magnitude of almost all the regions is brighter than 18, corresponding to an absolute magnitude of -6.62 for an assumed distance modulus of 24.62, which corresponds to a single main-sequence O5 star, while the distribution of color shows that the sample is blue, with a mode close to -0.05, as would be expected from a range of typical young clusters.

Star cluster formation and disruption time-scales - I. An empirical determination of the disruption time of star clusters in four galaxies

We present a new method to derive the cluster disruption time in selected regions of galaxies from the mass or age distribution of magnitude-limited cluster samples. If the disruption time of clusters in a region of a galaxy depends on their initial mass as t_4 x (M_cluster/10^4 M_sun)^gamma and if the cluster formation rate is constant, then the mass and age distributions of the observed clusters will each show two powerlaw relations. The values of t_4 and gamma can be derived from these relations. We used this method to derive the cluster disruption time in specific regions in four galaxies: the inner region of M51, a region of M33, the SMC and the solar neighbourhood. The values of gamma are the same in the four galaxies within the uncertainty and the mean value is gamma= 0.62 +- 0.06. However the disruption time t_4 of a cluster of 10^4 M_sun is very different in the different galaxies. The clusters in the SMC have the longest disruption time, t_4 = 8 Gyr, and the clusters at 1 to 3 kpc from the nucleus of M51 have the shortest disruption time of t_4 = 0.04 Gyr. The disruption time of clusters 1 to 5 kpc from the nucleus of M33 is t_4 = 0.13 Gyr and for clusters within 1 kpc from the Sun we find t_4 = 1.0 Gyr.

Neon Abundances in the HiiRegions of M33

We present neon abundances for 25 H II regions of M33, measured from line profiles of the mid-infrared transitions of [Ne II] and [Ne III] taken with the Infrared Space Observatory Short-Wavelength Spectrometer. The distribution of neon abundances as a function of galactocentric radius is best described as a step, -0.15 dex relative to the solar neon abundance from 0.7 to 4.0 kpc and -0.35 dex from 4.0 to 6.7 kpc, with estimated intrinsic scatter of 0.07 dex. The nearly flat neon abundance distribution differs from the steep oxygen abundance gradient found by previous investigators. Unless the oxygen abundance determinations are wrong, the chemical evolution of the galaxy has been radially dependent.

Intracluster Planetary Nebulae in Virgo: Photometric Selection, Spectroscopic Validation, and Cluster Depth

We have imaged an empty area of 34 × 34 arcmin2 15 north of the Virgo Cluster core to survey for intracluster planetary nebula (PN) candidates. We have implemented and tested a fully automatic procedure for the selection of emission-line objects in wide-field images, based on the on-off technique from Ciardullo and Jacoby. Freeman et al. have spectroscopically confirmed a sample of intracluster planetary nebulae in one Virgo field. We use the photometric and morphological properties of this sample to test our selection procedure. In our newly surveyed Virgo field, 75 objects were identified as best candidates for intracluster PNe. The luminosity function of the spectroscopically confirmed PNe shows a brighter cutoff than the planetary nebula luminosity function for the inner regions of M87. Such a brighter cutoff is also observed in the newly surveyed field and indicates a smaller distance modulus, implying that the front end of the Virgo Cluster is closer to us by a significant amount: 14% closer (2.1 Mpc) than M87 for the spectroscopic field, using the PN luminosity function distance of 14.9 Mpc to M87, and 18% closer (2.8 Mpc) than M87 for the newly surveyed field. Independent distance indicators (Tully-Fisher relation for Virgo spirals and surface brightness fluctuations for Virgo ellipticals) agree with these findings. From these two Virgo Cluster fields there is no evidence that the surface luminosity density for the diffuse stellar component in the cluster decreases with radius. The luminosity surface density of the diffuse stellar population is comparable to that of the galaxies.

Kinematics of Star Clusters in M33: Distinct Populations

We present velocity measurements for 107 star clusters in the nearby, low-luminosity spiral galaxy M33, from a homogeneous data set taken with the WIYN HYDRA spectrograph. The target clusters were chosen primarily from our previous catalogs based on multiband Hubble Space Telescope WFPC2 imaging and therefore have previous age estimates based on integrated colors. Our sample includes objects spanning the entire range of cluster ages found in M33, allowing us to study cluster kinematics as a function of age. We find that the cluster velocity dispersion with respect to the local disk motion increases dramatically with age. The young clusters have a maximum circular velocity of 87 ± 11 km s-1, similar to the previous rotation solution for H II regions in M33. When clusters are divided into age groups, the old clusters clearly have very different kinematics from the young population and from the gas disk. Monte Carlo simulations comparing synthetic disk plus halo systems with the observed velocity distribution for our old M33 clusters suggest that old clusters are likely to be composed of two distinct subpopulations, an 85% ± 5% halo component plus a 15% ± 5% disk component. This is the first evidence for distinct kinematic subpopulations among the old M33 clusters, and this galaxy appears to have a higher halo/bulge fraction of old stellar objects than found in the Galaxy. To further study the properties of halo clusters in M33, we use spectroscopic line index measurements for 17 clusters with the largest departure from disk motion, and thus the most pristine halo objects in our sample, to determine ages and approximate metal abundances. When compared with the Worthey models and with similar measurements for 12 Galactic globulars, our clusters have an age spread of ~5-7 Gyr, ~2 times as large as the spread among the Galactic sample. This small halo sample shows little evidence for a progression in metal abundance with age. There is also some support for different size (core radius) distributions when comparing "young" and "old" halo M33 clusters. We discuss these findings in the context of different galaxy formation models.

Star Formation in M51 Triggered by Galaxy Interaction

We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at 55'' spatial resolution using the Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is peaked at the nucleus, but is detectable over the entire region mapped, which covers much of the optical disk of the galaxy. There are also two strong secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to the north-east, and south-west of the nucleus. These secondary peaks are at the same distance from the nucleus as the corotation radius of the density wave pattern. The density wave also terminates at this location, and the outlying spiral structure is attributed to material clumping due to the interaction between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions, stimulating star formation, that we see as enhanced [CII] line emission. The [CII] emission at the peaks originates mainly from photodissociation regions (PDRs) formed on the surfaces of molecular clouds that are exposed to OB starlight, so that these [CII] peaks trace star formation peaks in M51. The total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or about 2% of the molecular gas as estimated from its CO(1-0) line emission. At the peak [CII] positions, the PDR gas mass to total gas mass fraction is somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the [CII] emitting photodissociated gas can be as high as 72% of the molecular mass.... (continued)

ASCA Observations of the Central Regions of M 31

Abstract Using ASCA, spatially integrated X-ray spectra of the central regions of M 31 were studied. Data were accumulated over three different circular regions, with radii of $3', 6'$, and $12'$, all centered on the nucleus. The spectra are relatively similar among the three regions. In the energy range above 1.5 keV, the spectra are reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of low-mass X-ray binaries. At energies below 1.5 keV, the spectra involve two additional softer components, expressed by thin-thermal plasma emission models of temperatures of $\sim 0.9 \,\mathrm{keV}$ and $\sim 0.3 \,\mathrm{keV}$. Over the central $12'$ (2.4 kpc) region and in the 0.5–10 keV energy band, the binary component has a luminosity of $2.6 \times 10^{39} \,\mathrm{erg} \,\mathrm{s}^{-1}$, while the two softer components both exhibit luminosities of $\sim 2 \times 10^{38} \,\mathrm{erg} \,\mathrm{s}^{-1}$. These results are compared with those from other missions, including Chandra and XMM-Newton in particular. A discussion is presented on the nature of the two softer spectral components besides the binary one.

HII[CLC]phot[/CLC]: Automated Photometry of H [CSC]ii[/CSC] Regions Applied to M51

We have developed a robust, automated method, hereafter designated HIIphot, which enables accurate photometric characterization of H II regions while permitting genuine adaptivity to irregular source morphology. HIIphot utilizes object recognition techniques to make a first guess at the shapes of all sources and then allows for departure from such idealized "seeds" through an iterative growing procedure. Photometric corrections for spatially coincident diffuse emission are derived from a low-order surface fit to the background after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 H II regions are detected above the 5 σ level. A simple, weighted power-law fit to the measured Hα luminosity function (H II LF) above log LHα = 37.6 gives α = -1.75 ± 0.06, despite a conspicuous break in the H II LF observed near LHα = 1038.9. Our best-fit slope is marginally steeper than measured by Rand, perhaps reflecting our increased sensitivity at low luminosities and to notably diffuse objects. H II regions located in interarm gaps are preferentially less luminous than counterparts which constitute M51's grand design spiral arms and are best fitted with a power-law slope of α = -1.96 ± 0.15. We assign arm/interarm status for H II regions based upon the varying surface brightness of diffuse emission as a function of position throughout the image. Using our measurement of the integrated flux contributed by resolved H II regions in M51, we estimate the diffuse fraction to be approximately 0.45—in agreement with the determination of Greenawalt et al. Automated processing of degraded narrowband data sets is undertaken in order to gauge (distance-related) systematic effects associated with limiting spatial resolution and sensitivity.

Star Formation in the Giant HiiRegions of M101

The molecular components of three giant H II regions (NGC 5461, NGC 5462, and NGC 5471) in the galaxy M101 are investigated with new observations from single-dish telescopes (James Clerk Maxwell Telescope and the NRAO 12 m) and from the Owens Valley Radio Observatory millimeter array. Of the three H II regions, only NGC 5461 had previously been detected in CO emission. We calculate preliminary values for the molecular mass of the GMCs in NGC 5461 by assuming a CO-to-H2 factor (X factor) and then comparing these values with the virial masses. We found that the appropriate X factor is 5 times smaller than the X factor in the Milky Way despite the lower metallicity of M101. We conclude that the data in this paper demonstrate for the first time that the value of X may decrease in regions with intense star formation. The molecular mass for the association of clouds in NGC 5461 is approximately 3 × 107 M☉ and is accompanied by 1-2 times as much atomic mass. The observed CO emission in NGC 5461 is an order of magnitude stronger than in NGC 5462, but it was not possible to detect molecular gas toward NGC 5471 with the James Clerk Maxwell Telescope. An even larger ratio of atomic to molecular gas in NGC 5471 was observed, which might be attributed to efficient conversion of molecular to atomic gas. The masses of the individual clouds in NGC 5461, which are gravitationally bound, cover a range of (2-8) × 105 M☉, comparable with the masses of Galactic giant molecular clouds. Higher star-forming efficiencies, and not massive clouds, appear to be the prerequisite for the formation of the large number of stars whose radiation is required to produce the giant H II regions in M101.

The Hii Regions of M33. II. A Photometric Catalog of 1272 Newly Identified Emission Regions

ABSTRACTWe have identified a total of 1272 newly recognized emission regions in M33. Combined with the previously cataloged total of 1066 H ii regions and supernova remnants, this brings M33’s total to 2338 emission regions. This paper provides photometry of the new objects in Hα, which is combined with data from previous catalogs to produce a global H ii region luminosity function (corrected for incompleteness) that reaches a faint luminosity limit of 2 × 1034 ergs s−1 and shows a broad maximum with a peak frequency at luminosities of 6 × 1035 ergs s−1. We also plot the H ii region size distribution and comment on unusual morphologies.

The Globular Cluster System in the Inner Region of M87

We have identified 1057 globular cluster candidates in a WFPC2 image of the inner region of M87. The globular cluster luminosity function (GCLF) can be well fitted by a Gaussian profile with a mean value of m0V=23.67±0.07 mag and σ=1.39±0.06 mag. The GCLF in five radial bins is found to be statistically the same at all points, showing no clear evidence of dynamical destruction processes based on the luminosity function (LF). Similarly, there is no obvious trend between the half-light radius of the clusters and the galactocentric distance. The core radius of the globular cluster density distribution is Rc=56'', considerably larger than the core of the stellar component (Rc=68). The mean color of the cluster candidates is V-I=1.09 mag, which corresponds to an average metallicity of Fe/H=-0.74 dex. The color distribution is bimodal everywhere, with a blue peak at V-I=0.95 mag and a red peak at V-I=1.20 mag. The red population is only 0.1 mag bluer than the underlying galaxy, indicating that these clusters formed late in the metal-enrichment history of the galaxy and were possibly created in a burst of star/cluster formation 3-6 Gyr after the blue population. We also find that both the red and the blue cluster distributions have a more elliptical shape (Hubble type E3.5) than the nearly spherical galaxy. The average half-light radius of the clusters is ≈2.5 pc, which is comparable to the 3 pc average effective radius of the Milky Way clusters, although the red clusters are ≈20% smaller than the blue ones.

Cold Massive Molecular Clouds in the Inner Disk of M31

We present new interferometric 12CO (1-0) and single-dish 12CO (3-2) observations of the central parts of D478, a large (>200 pc) dark dust cloud located in a quiescent region of the inner disk of M31, where single-dish 12CO (1-0) and 12CO (2-1) observations were previously obtained. Only a small fraction (<15%) of the 12CO (1-0) flux previously detected in this region with the single-dish telescope is recorded by the interferometer. Most of the 12CO (1-0) emission must therefore have the appearance of a smooth surface with very little structure on scales smaller than ≈ 25'' (85 pc). Together with the earlier 12CO (1-0) and 12CO (2-1) single-dish results, the new 12CO (3-2) data are in good agreement with LTE predictions for optically thick lines at Tex = Tkin = 3.5 K. These results rule out the conventional model for these clouds, consisting of warm clumps with a low filling factor (as would be the case if they resembled Galactic giant molecular clouds), and confirm that large, massive, cold molecular clouds exist in the inner disk of M31 with kinetic temperatures close to that of the cosmic microwave background. Such extremely low temperatures are likely to be a consequence of the low heating rate in these particular regions of M31, where very little massive star formation is occurring at present. From the 12CO line profile widths, we estimate the virial mass surface density of D478 to be 80-177 M☉ pc-2. This is a factor of 7-16 times larger than the value obtained by multiplying the 12CO profile integrals with the conventional X factor.

Kinematical Study of Old HII Regions and Optical Counterpart to the DRAO Canadian Galactic Plane Survey

AbstractThis project aims to tackle a few unresolved problems related to the interstellar medium (ISM) by acting as an optical counterpart to the Canadian Galactic Plane Survey (CGPS). We have three main objectives: (1) the study of large/old HII regions; (2) observations of targets-of-opportunity that may be found by the CGPS; (3) comparison of the kinematics of extragalactic (M33) and galactic HII regions. (1) Old HII regions having large spatial extents will be observed to establish their kinematical structure when almost no molecular material is left to produce photodissociated flows. Does turbulence play a role in these objects as for younger nebulae? (2) Ionised nebulae observed by the CGPS that are peculiar, either morphologically or by special association with neutral hydrogen, will also be observed. (3) The kinematical behaviour of old HII regions in M33 will be compared with that of galactic HII regions. With both data sets in hand we will check if HII regions, like supernovae, dump energy into the neutral ISM. The instrument used as well as some very preliminary data are presented.

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.

THE LUMINOSITY FUNCTION AND SIZE DISTRIBUTION OF THE H II REGIONS IN M33

ABSTRACT We present Hα fluxes and sizes for most of the H 2 regions in M33 listed in the catalog of Courtès et al. (1987). The observed bright end of the differential luminosity function is approximately a power law with exponent a = -2.40 ± 0.15, while the number counts turn over at an Hα luminosity of approximately 1036.4 erg s-1. After accounting for the effects of incompleteness in our sample, we find that the faint end of the corrected luminosity function remains almost constant instead of turning over. The cumulative size distribution approximately follows an exponential law with scale size D0 = 32 ± 1 pc. We briefly discuss the implications of our results for our understanding of recent high-mass star formation in M3.

DRIFT SCANNING USING INFRARED ARRAYS: A POWERFUL METHOD FOR BACKGROUND LIMITED IMAGING

ABSTRACT We present the first results using infrared arrays, devices which do not employ charge coupling, in a new "drift scanning" mode to significantly increase the area of the sky which can be surveyed, especially in the high background 3-5 \\mm bands. Even with fast electronics, wide field imaging in the thermal IR is difficult due to the high backgrounds. Often, only a sub-array of the frame can be readout before saturation. Drift scanning allows this sub-array to be a long narrow strip which is stepped perpendicular to the long side, by allowing the telescope to track at non-sidereal rates, and then a mosaic is constructed in the computer memory in real time. The method has been implemented with the UCLA Twin-Channel Infrared Camera system which employs two different classes of arrays. One is the NICMOS 3 HgCdTe array and the other is an SBRC InSb array; both detectors have 256\\xx256 pixels. Using a drift rate of 1 arcminute/minute on the Shane 3-m telescope at Lick Observatory, we are able to survey about 180 square arcminutes per hour at two wavelengths simultaneously (e.g. J,K or H,L). Limiting magnitudes are typically J=15, L= 12. Data reduction methods are described and observations of the globular cluster M3, the star forming region M17, and the Moon are used as illustration.

NEW MASSIVE STAR CLUSTER CANDIDATES IN M33

The central region of M33 is studied with aim new massive cluster candidates to be distinguished. This region has not been included in details in previous searches. From a collection of photographic plates, obtained under excellent seeing conditions at the Bulgarian National Observatory, using morphologic and photometric criteria, we have selected 26 new star cluster candidates, both young clusters and old globulars. Their coordinates and photographic UBV magnitudes are presented.

Electron temperatures in the Galactic H ii regions W43 and M17

Electron temperatures in the Galactic H ii regions W43 and M17