Regions In M81 Research Articles

The radial metallicity gradient and the history of elemental enrichment in M 81 through emission-line probes

We present a new set of weak-line abundances of HII regions in M81, based on Gemini Multi-Object Spectrograph (GMOS) observations. The aim is to derive plasma and abundance analysis for a sizable set of emission-line targets to study the galactic chemical contents in the framework of galactic metallicity gradients. We used the weak-line abundance approach by deriving electron density and temperatures for several HII regions in M81. Gradient analysis is based on oxygen abundances.Together with a set of HII region abundances determined similarly by us with Multi-Mirror Telescope (MMT) spectra, the new data yield to a radial oxygen gradient of -0.088$\pm$0.013 dex kpc$^{-1}$, which is steeper than the metallicity gradient obtained for planetary nebulae (-0.044$\pm$0.007 dex kpc$^{-1}$). This result could be interpreted as gradient evolution with time: Models of galactic evolution with inside-out disk formation associated to pre-enriched gas infall would produce such difference of gradients, although stellar migration effects would also induce a difference in the metallicity gradients between the old and young populations. By comparing the M81 metallicity gradients with those of other spiral galaxies, all consistently derived from weak-line analysis, we can infer that similar gradient difference is common among spirals. The metallicity gradient slopes for HII regions and PNe seem to be steeper in M81 than in other galactic disks, which is likely due to the fact that M81 belongs to a galaxy group. We also found that M81 has experienced an average oxygen enrichment of 0.14$\pm$0.08 dex in the spatial domain defined by the observations. Our data are compatible with a break in the radial oxygen gradient slope around R$_{25}$ as inferred by other authors both in M81 and in other galaxies.

NGC 2207/IC 2163: A GRAZING ENCOUNTER WITH LARGE-SCALE SHOCKS

Radio continuum, Spitzer infrared, optical and XMM-Newton X-ray and UVM2 observations are used to study large-scale shock fronts, young star complexes, and the galactic nuclei in the interacting galaxies NGC 2207/IC 2163. There are two types of large-scale shock fronts in this galaxy pair. The shock front along the rim of the ocular oval in IC 2163 has produced vigorous star formation in a dusty environment. In the outer part of the companion side of NGC 2207, a large-scale front attributed to disk or halo scraping is particularly bright in the radio continuum but not in any tracers of recent star formation or in X-rays. This radio continuum front may be mainly in the halo on the back side of NGC 2207 between the two galaxies. Values of the flux density ratio S(8 um)/S(6 cm) of kpc-sized, Spitzer IRAC star-forming clumps in NGC 2207/IC 2163 are compared with those of giant H II regions in M81. We find evidence that in 2001 a radio supernova was present in the core of feature i, a mini-starburst on an outer arm of NGC 2207. X-ray emission is detected from the NGC 2207 nucleus and from nine discrete sources, one of which corresponds to SN 1999ec, and another may be a radio supernova or a background quasar. The X-ray luminosity and X-ray spectrum of the NGC 2207 nucleus suggests it is a highly absorbed, low luminosity AGN.

The JCMT Nearby Galaxies Legacy Survey

We present a first complete 12 CO J=3-2 map of M81, observed as part of the Nearby Galaxies Legacy Survey being carried out at the James Clerk Maxwell Telescope. We detect 9 regions of significant CO emission located at different positions within the spiral arms, and confirm that the global CO emission in the galaxy is low. We combine these data with a new H-alpha map obtained using the Isaac Newton Telescope and archival HI, 24 microns and FUV images to uncover a correlation between the molecular gas and star forming regions in M81. For the nine regions detected in CO J=3-2, we combine our CO J=3-2 data with existing CO J=1-0 data to calculate line ratios. We find that the ratio J=(3-2)/(1-0) is in agreement with the range of typical values found in the literature (0.2-0.8). Making reasonable assumptions, this allows us to constrain the hydrogen density to the range (10^3-10^4) cm^{-3}. We also estimated the amount of hydrogen produced in photo-dissociation regions near the locations where CO J=3-2 was detected.

10.1007/s11443-008-3003-4

Based on an axisymmetric galactic disk model, we estimate the equilibrium gas pressure P/k in the disk plane as a function of the galactocentric distance R for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For this purpose, we solve a self-consistent system of equations by taking into account the gas self-gravity and the presence of a dark pseudo-isothermal halo. We assume that the turbulent velocity dispersions of the atomic and molecular gases are fixed and that the velocity dispersion of the old stellar disk corresponds to its marginal stability (except for the Galaxy and the SMC). We also consider a model with a constant disk thickness. Of the listed galaxies, the SMC and M51 have the highest pressure at a given relative radius R/R 25, while M81 and the Galaxy has the lowest pressure. The pressure dependence of the relative molecular gas fraction confirms the existence of a positive correlation between these quantities, but it is not so distinct as that obtained previously when the pressure was estimated very roughly. This dependence breaks down for the inner regions of M81 and M106, probably because the gas pressure has been underestimated in the bulge region. We discuss the possible effects of factors other than the pressure affecting the relative content of the molecular gas in the galaxies under consideration.

ULTRALUMINOUS X-RAY SOURCE CORRELATIONS WITH STAR-FORMING REGIONS

Maps of low-inclination nearby galaxies in Sloan Digitized Sky Survey u-g, g-r and r-i colors are used to determine whether Ultraluminous X-ray sources (ULXs) are predominantly associated with star-forming regions of their host galaxies. An empirical selection criterion is derived from colors of HII regions in M81 and M101 that differentiates between the young, blue stellar component and the older disk and bulge population. This criterion is applied to a sample of 58 galaxies of Hubble type S0 and later and verified through an application of Fisher's linear discriminant analysis. It is found that 60% (49%) of ULXs in optically-bright environments are within regions blueward of their host galaxy's HII regions compared to only 27% (0%) of a control sample according to the empirical (Fisher) criterion. This is an excess of 3-sigma above the 32% (27%) expected if the ULXs were randomly distributed within their galactic hosts. This indicates a ULX preference for young, approximately <10 Myr, OB associations. However, none of the ULX environments have the morphology and optical brightness suggestive of a massive young super star cluster though several are in extended or crowded star-forming (blue) environments that may contain clusters unresolved by Sloan imaging. Ten of the 12 ULX candidates with estimated X-ray luminosities in excess of 3e39 erg/s are equally divided among the group of ULX environments redward of HII regions and the group of optically faint regions. This likely indicates that the brightest ULXs turn on at a time somewhat later than typical of HII regions; say 10-20 Myr after star formation has ended. This would be consistent with the onset of an accretion phase as the donor star ascends the giant branch if the donor is a <20 solar-mass star.

The pressure of an equilibrium interstellar medium in galactic disks

Based on an axisymmetric galactic disk model, we estimate the equilibrium gas pressure P/k in the disk plane as a function of the galactocentric distance R for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For this purpose, we solve a self-consistent system of equations by taking into account the gas self-gravity and the presence of a dark pseudo-isothermal halo. We assume that the turbulent velocity dispersions of the atomic and molecular gases are fixed and that the velocity dispersion of the old stellar disk corresponds to its marginal stability (except for the Galaxy and the SMC). We also consider a model with a constant disk thickness. Of the listed galaxies, the SMC and M51 have the highest pressure at a given relative radius R/R_25, while M81 has the lowest pressure. The pressure dependence of the relative molecular gas fraction confirms the existence of a positive correlation between these quantities, but it is not so distinct as that obtained previously when the pressure was estimated very roughly. This dependence breaks down for the inner regions of M81 and M106, probably because the gas pressure has been underestimated in the bulge region. We discuss the possible effects of factors other than the pressure affecting the relative content of molecular gas in the galaxies under consideration.

Ultraviolet through Far‐Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC 3031)

The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet to the far-infrared. We compare these data with models of the stellar, gas, and dust emission for subgalactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent star formation. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: (1) cold dust with a temperature = 18 ± 2 K, concentrated near the H II regions but also presenting a diffuse distribution; (2) warm dust with = 53 ± 7 K, directly linked with the H II regions; and (3) aromatic molecules, with diffuse morphology peaking around the H II regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for H II regions in M81 and M51: the UV, Hα, and three estimators based on Spitzer data. We find that the Hα luminosity absorbed by dust correlates tightly with the 24 μm emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 μm or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the Hα emission (or the UV) and an IR luminosity (especially the 24 μm emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.

Spatially Resolved Ultraviolet, Hα, Infrared, and Radio Star Formation in M81

We present Multiband Imaging Photometer for Spitzer (MIPS) observations of M81 at 24, 70, and 160 micron. The grand design nature of M81 is clearly seen, showing two well resolved spiral arms containing numerous bright star forming regions. The MIPS images reveal a significant amount of cold dust associated with the spiral arms. We investigate the variation of the ultraviolet (UV), H-alpha, and infrared (IR) luminosities and star formation rate (SFR) indicators across the face of M81 using the MIPS images and archival UV and H-alpha images. For regions in M81, we find that UV and H-alpha SFRs (uncorrected for dust attenuation) are always lower than the IR SFR. The cause of this behavior is dust attenuation and/or using SFR calibrations appropriate for entire galaxies, not regions in galaxies. The characteristics of the dust attenuation for the regions indicate the dust grains and/or geometry are different from those in starburst galaxies. The behavior of the infrared-radio correlation in M81 is seen to vary from the global average, with variations correlated with the morphology of M81.

Evolution of M81 with Exponentially Decreasing Star Formation Rate of PEGASE

Based on the large field multicolor observations of Beijing-Arizona-Taiwan-Connecticut (BATC) program, we obtain the spectral energy distribution (SED) for individual regions of M81. We study the structure and evolution of M81 with an evolutionary population synthesis (EPS) model, PEGASE. We find that the exponentially deceasing star formation rate (SFR) with star formation scale 3 Gyr (hereafter Exp, τ = 3 Gyr) gives the best agreement between the model predictions and the observed SEDs. We then obtain the structure, age distribution and evolutionary history of M81. There is a clear age gradient between the central and outer regions. The populations in the central regions are older than 7 Gyr, those in the outer regions are younger, at about 4.5 Gyr. The youngest components in the spiral arms have ages of about 2.5 Gyr or less.

H+[Nii] Observations of the HiiRegions in M81

In a first of a series of studies of the Hα + [N II] emission from nearby spiral galaxies, we present measurements of Hα + [N II] emission from H II regions in M81. Our method uses large-field CCD images and long-slit spectra and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey (the BATC survey). The CCD images are taken with the National Astronomical Observatories of China (NAOC) 0.6/0.9 m f/3 Schmidt telescope at the Xinglong Observing Station, using a multicolor filter set. Spectra of 10 of the brightest H II regions are obtained using the NAOC 2.16 m telescope with a Tek 1024 × 1024 CCD. The continua of the spectra are calibrated by flux-calibrated images taken from the Schmidt observations. We determine the continuum component of our Hα + [N II] image via interpolation from the more accurately measured backgrounds (M81 starlight) obtained from the two neighboring (in wavelength) BATC filter images. We use the calibrated fluxes of Hα + [N II] emission from the spectra to normalize this interpolated, continuum-subtracted Hα + [N II] image. We estimate the zero-point uncertainty of the measured Hα + [N II] emission flux to be ~8%. A catalog of Hα + [N II] fluxes for 456 H II regions is provided, with those fluxes being on a more consistent linear scale than previously available. The logarithmically binned Hα + [N II] luminosity function of H II regions is found to have slope α = -0.70, consistent with previous results (which allowed α = -0.5 to -0.8). From the overall Hα + [N II] luminosity of the H II regions, the star formation rate of M81 is found to be ~0.68 M⊙ yr-1, modulo uncertainty with extinction corrections.

Multiple Light Echoes from SN 1993J

Using the technique of point-spread function-matched image subtraction, we have analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 data to reveal details of at least two light echo structures, including some unknown before now, around SN 1993J in the galaxy M81. In particular, we see one partial sheet of material 81 pc in front of the supernova (SN) and tilted ~60° relative to the disk plane of M81 and another 220 pc in front of the SN, roughly parallel to the disk. The inferred echoing material is consistent with the H I surface density detected in this region of M81's disk; however, these data imply a fragmented covering factor for the echoing structures. We discuss prospects for future (roughly annual) visits by HST to image these and yet undiscovered echoes in the interstellar and circumstellar environment of SN 1993J.

Spatially Resolved Spectrophotometry of M81: Age, Metallicity, and Reddening Maps

In this paper we present a multicolor photometric study of the nearby spiral galaxy M81, using images obtained with the Beijing Astronomical Observatory 60/90 cm Schmidt telescope in 13 intermediate-band filters from 3800 to 10000 A. The observations cover the whole area of M81, with a total integration of 51 hr from 1995 to 1997 February. This provides a multicolor map of M81 in pixels of 17 × 17. Using theoretical stellar population synthesis models, we demonstrate that some BATC (Beijing-Arizona-Taiwan-Connecticut Multicolor Sky Survey) colors and color indices can be used to disentangle the age and metallicity effect. We compare in detail the observed properties of M81 with the predictions from population synthesis models and quantify the relative chemical abundance, age, and reddening distributions for different components of M81. We find that the metallicity of M81 is about Z = 0.03, with no significant difference over the whole galaxy. In contrast, an age gradient is found between stellar populations of the central regions and of the bulge and disk regions of M81: the stellar population in its central regions is older than 8 Gyr, while the disk stars are considerably younger (~2 Gyr). We also give the reddening distribution in M81. Some dust lanes are found in the galaxy bulge region, and the reddening in the outer disk is higher than that in the central regions.

ITAL]Hubble Space Telescope[/ITAL] Imaging of the Central 1 Kiloparsec of M81

M81 is the closest example of a galaxy that exhibits both LINER and Seyfert 1 characteristics. New, postrepair (cycle 5), Hubble Space Telescope imaging observations of the central kpc of M81 are presented, specifically, WFPC2 Hα and continuum images, a WFPC2 UV continuum (1500 A) image, and a V-band faint object camera (FOC) image. The continuum image shows a number of sinuous dust lanes that wind into the nucleus of M81. The Hα image reveals a small, ~7'' (120 pc), bright, elliptical region of emission that appears to be a nuclear disk. The center of the disklike structure is offset by less than 06 (10 pc) from the bright unresolved nucleus. The WFPC2 UV image shows that the disk is not ionized by massive stars. The highest angular resolution observations, obtained with the FOC, allow an upper limit of 004 (0.7 pc) to be placed on the size of the nucleus in the V band. The FOC image also shows that there are no stars, star clusters, or additional nuclei visible within the central 4'' × 4'' region of M81 that are brighter than the galaxy background light.

X-Ray Properties of the Nucleus of M81

Due to the appearance of SN 1993J, the region of M81 (NGC 3031) was observed with ASCA ten times between 1993 April and 1995 April. Data from eight of these observations have been used to study the 0.5–10 keV spectral and temporal properties of the source designated X-5, coincident with the M81 nucleus. The 2–10 keV luminosity of X-5 varied by a factor of 1.7 over a period of two years, around a mean of ∼ 2 × 1040 erg s−1 . Variations by ∼ 20% on a time scale of one day were also detected. The average spectrum of X-5 can be represented in the 2–10 keV range by a power-law continuum of the photon index, Γ = 1.85± 0.04, and a relatively low intrinsic absorption column of NH ∼ 1 × 1021 cm−2 . In the soft (&amp;lt; 2 keV) energy range, an additional thermal component with a temperature of 0.6–0.8 keV has been detected. An apparently broad or complex Fe-K emission line centered at 6.6–6.7 keV was also detected at an equivalent width of 170 ± 60 eV. These results indicate that the M81 nucleus is a low-luminosity active galactic nucleus, and suggest that similar objects are relatively numerous. A comparison is made between the nucleus of M81 and those of Seyfert galaxies.

Ultraviolet imaging telescope and optical emission-line observations of H II regions in M81

Images of the type Sab spiral galaxy M81 were obtained in far-UV and near-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission of 1990 December. Magnitudes in the two UV bands are determined for 52 H II regions from the catalog of Petit, Sivan, & Karachentsev (1988). Fluxes of the H-alpha and H-beta emission lines are determined from CCD images. Extinctions for the brightest H II regions are determined from observed Balmer decrements. Fainter H II regions are assigned the average of published radio-H-alpha extinctions for several bright H II regions. The radiative transfer models of Witt, Thronson, & Capuano (1992) are shown to predict a relationship between Balmer Decrement and H-alpha extinction consistent with observed line and radio fluxes for the brightest 7 H II regions and are used to estimate the UV extinction. Ratios of Lyman continuum with ratios predicted by model spectra computed for initial mass function (IMF) slope equal to -1.0 and stellar masses ranging from 5 to 120 solar mass. Ages and masses are estimated by comparing the H-alpha and far-UV fluxes and their ratio with the models. The total of the estimated stellar masses for the 52 H II regions is 1.4 x 10(exp 5) solar mass. The star-formation rate inferred for M81 from the observed UV and H-alpha fluxes is low for a spiral galaxy at approximately 0.13 solar mass/yr, but consistent with the low star-formation rates obtained by Kennicutt (1983) and Caldwell et al. (1991) for early-type spirals.