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Register a new userROSAT observations of the dwarf starforming galaxy Holmerg II (UGC 4305)
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We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy Holmberg II (UGC4305). This is one of the most luminous dwarf galaxies (Lx~ 10^{40} erg s^{-1} cm^{-2}) detected in the ROSAT All-Sky Survey. The X-ray emission comes from a single unresolved point source, coincident with a large HII region which emits intense radio emission. The source is variable on both year and day timescales, clearly favouring accretion into a compact object rather than a supernova remnant or a superbubble interpretation for the origin of the X-ray emission. However, its X-ray spectrum is well-fit by a a Raymond-Smith spectrum with kT~0.8 keV, lower than the temperature of X-ray binaries in nearby spiral galaxies.
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We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT PSPC observations (total exposure time: 22 ksec). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium which is dominated by expanding structures such as HI holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant HI shells above our luminosity sensitivity limit of (L_limit(0.1 - 2.1 keV) ~ 10^{37}erg/s). This finding can probably be attributed to blow-out in the case of the largest holes and insufficient sensitivity (due to strong photoelectric absorption) in case of the smaller HI holes. However we find faint X-ray sources well beyond the stellar body but within the HI distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.
UGC 4483 is a nearby Blue Compact Dwarf (BCD) galaxy. HST observations have resolved the galaxy into single stars and this has led to the derivation of its star formation history and to a direct estimate of its stellar mass. We have analysed archival VLA observations of the 21 cm line and found that UGC 4483 has a steeply-rising rotation curve which flattens in the outer parts at a velocity of ~20 km/s. Radial motions of ~5 km/s may also be present. As far as we know, UGC 4483 is the lowest-mass galaxy with a differentially rotating HI disk. The steep rise of the rotation curve indicates that there is a strong central concentration of mass. We have built mass models using the HST information on the stellar mass to break the disk-halo degeneracy: old stars contribute ~50$%$ of the observed rotation velocity at 2.2 disk scale-lengths. Baryons (gas and stars) constitute an important fraction of the total dynamical mass. These are striking differences with respect to typical dwarf irregular galaxies (dIrrs), which usually have slowly-rising rotation curves and are thought to be entirely dominated by dark matter. BCDs appear to be different from non-starbursting dIrrs in terms of their HI and stellar distributions and their internal dynamics. To their high central surface brightnesses and high central HI densities correspond strong central rotation-velocity gradients. This implies that the starburst is closely related with the gravitational potential and the concentration of gas. We discuss the implications of our results on the properties of the progenitors/descendants of BCDs.
We want to get insight into the nature, i.e. the formation mechanism and the evolution, of UGC 7639, a dwarf galaxy in the Canes Venatici I Cloud (CVnIC). We used archival GALEX (FUV and NUV) and SDSS images, as well as Hyperleda and NED databases, to constrain its global properties. GALEX FUV/NUV images show that UGC 7639 inner regions are composed mostly by young stellar populations. In addition, we used smoothed particle hydrodynamics (SPH) simulations with chemo-photometric implementation to account for its formation and evolution. UGC 7639 is an example of blue dwarf galaxy whose global properties are well matched by our multi-wavelength and multi-technique approach, that is also a suitable approach to highlight the evolution of these galaxies as a class. We found that the global properties of UGC 7639, namely its total absolute B-band magnitude, its whole spectral energy distribution (SED), and its morphology are well-matched by an encounter with a system four times more massive than our target. Moreover, the current star formation rate (SFR) of the simulated dwarf, ~0.03 M_sun yr-1, is in good agreement with our UV-based estimate. For UGC 7639, we estimated a galaxy age of 8.6 Gyr. Following our simulation, the ongoing star formation will extinguish within 1.6 Gyr, thus leaving a red dwarf galaxy.
The nearly face-on SBc galaxy M83 (NGC 5236) was observed for 25 ksec with the ROSAT PSPC. We detected 13 point-like sources in this galaxy, 10 of which were previously unknown. We measured extended X-ray radiation from almost the whole optically visible galaxy. Comparing the diffuse soft and hard X-ray emission components, we observed a different asymmetric distribution and a slower radial decrease of the intensity profile of the soft X-ray emission. Both these results support the existence of a huge spherical gas halo of 10-15 kpc radius. On the other hand, the radial scale lengths of the hard X-ray radiation, that of the thermal radio emission and the profile of the optical surface brightness are similar, favouring the idea that all these emission processes are connected to star formation in the galaxys disk. M83 is the first face-on galaxy where the diffuse X-ray emission spectrum can be characterized by a two-temperature thermal plasma: a soft X-ray emitting warm `halo component and an internally absorbed hot `disk component which is dominating the emission in the hard (0.5-2.0 keV) ROSAT energy range. The combination of X-ray and radio polarization observations allows an estimate of the plasma parameter beta = U_therm/U_magn which is found to be 0.2+/-0.1. This result supports the hypothesis that magnetic fields play an important role for the evolution and structure of galactic gas haloes. The high energy input rate in the active star-forming disk of M83 seems to be responsible for the outflow of hot gas and the halo formation.
We present integrated Washington CT1 photometry of 18 bright blue objects discovered in the dwarf galaxy UGC 7636 which is located 5.5 southeast of the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. Several lines of evidence indicate that UGC 7636 is interacting violently with NGC 4472. These objects are very blue with colors of -0.4 < (C-T1) < 0.6, and their magnitudes are in the range of 20.6 < T1 < 22.9 mag which corresponds to absolute magnitudes of -10.6 < M(T1) < -8.3 mag for a distance modulus of (m-M)o = 31.2. These objects are grouped spatially in three regions: the central region of UGC 7636, the tidal tail region, and the HI cloud region. No such objects were found in the counter tail region. It is concluded that these objects are probably young star clusters which formed < 0.1Gyr ago during the interaction between UGC 7636 and NGC 4472. Surface photometry of UGC 7636 (r < 83) shows that there is a significant excess of blue light along the tidal tail region compared with other regions. The star clusters are bluer than the stellar light in the tidal tail region, indicating that these clusters might have formed later than most stars in the tidal tail region which were formed later than most stars in the main body of the galaxy.
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