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We present a catalog of extended objects in the vicinity of M81 based a set of 24 Hubble Space Telescope Advanced Camera for Surveys (ACS) Wide Field Camera (WFC) F814W (I-band) images. We have found 233 good globular cluster candidates; 92 candidate HII regions, OB associations, or diffuse open clusters; 489 probable background galaxies; and 1719 unclassified objects. We have color data from ground-based g- and r-band MMT Megacam images for 79 galaxies, 125 globular cluster candidates, 7 HII regions, and 184 unclassified objects. The color-color diagram of globular cluster candidates shows that most fall into the range 0.25 < g-r < 1.25 and 0.5 < r-I < 1.25, similar to the color range of Milky Way globular clusters. Unclassified objects are often blue, suggesting that many of them are likely to be HII regions and open clusters, although a few galaxies and globular clusters may be among them.
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We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-rich ([Fe/H]=-0.60+/-0.10), alpha-enhanced ([Alpha/Fe]=0.20+/0.05), core-collapsed (core radius r_c=1.2 pc, tidal radius r_t = 76r_c), old (>13 Gyr) cluster. It has an ultraviolet excess equivalent of ~2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 10 million solar masses, the cluster is comparable in mass to M31-G1 and is four times more massive than Omega Cen. The values of r_c, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.
We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analog in size and structure to the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.
The sample of known star clusters, the fundamental building blocks of galaxies, in the Milky Way is still extremely incomplete for objects beyond a distance of 1-2kpc. Many of the more distant and young clusters are compact and hidden behind large amounts of extinction. We thus utilised the deep high resolution near infrared surveys UGPS and VVV to uncover so far unknown compact clusters and to analyse their properties. Images of all objects in the area covered by these two surveys, and which are listed as Galaxy in SIMBAD have been inspected and 125 so far unknown stellar clusters and candidate clusters have been identified. Based on the frequent associations with star formation indicators (nebulosities, IRAS sources, Hii regions, masers) we find that the typical cluster in our sample is young, at distances between 1-10kpc and has a typical apparent radius of 25arcsec. We suggest more systematic searches e.g. at all positions of 2MASS extended sources to increase the completeness of the known cluster sample beyond distances of 2kpc.
In this paper, we presented metal abundance properties of 144 M81 globular clusters. These globulars consist of the largest globular cluster sample in M81 till now. Our main results are: the distribution of metallicities are bimodal, with metallicity peaks at [Fe/H]sim-1.51 and -0.58, and the metal-poor globular clusters tend to be less spatially concentrated than the metal-rich ones; the metal-rich globular clusters in M81 do not demonstrate a centrally concentrated spatial distribution as the metal-rich ones in M31 do; like our Galaxy and M31, the globular clusters in M81 have a small radial metallicity gradient. These results are consistent with those obtained based on a small sample of M81 globular clusters. In addition, this paper showed that there is evidence that a strong rotation of the M81 globular cluster system around the minor axis exists, and that rotation is present in the metal-rich globular cluster subsample, while the metal-poor globular cluster subsample shows no evidence for rotation. The most significant difference between the rotation of the metal-rich and metal-poor globular clusters occurs at intermediate projected galactocentric radii. The results of this paper confirm the conclusion of Schroder et al. that M81s metal-rich globular clusters at intermediate projected radii were associated with a thick disk of M81.
We present Swift UV/Optical Telescope (UVOT) imaging of the galaxies M81 and Holmberg IX. We combine UVOT imaging in three near ultraviolet (NUV) filters (uvw2: 1928 {AA}, uvm2: 2246 {AA}, and uvw1: 2600 {AA}) with ground based optical imaging from the Sloan Digital Sky Survey to constrain the stellar populations of both galaxies. Our analysis consists of three different methods. First we use the NUV imaging to identify UV star forming knots and then perform SED modeling on the UV/optical photometry of these sources. Second, we measure surface brightness profiles of the disk of M81 in the NUV and optical. Last we use SED fitting of individual pixels to map the properties of the two galaxies. In agreement with earlier studies we find evidence for a burst in star formation in both galaxies starting ~200 Myr ago coincident with the suggested time of an M81-M82 interaction. In line with theories of its origin as a tidal dwarf we find that the luminosity weighted age of Holmberg IX is a few hundred million years. Both galaxies are best fit by a Milky Way dust extinction law with a prominent 2175 {AA} bump. In addition, we describe a stacked median filter technique for modeling the diffuse background light within a galaxy, and a Markov chain method for cleaning segment maps generated by SExtractor.
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