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We present the results from an optical study of the stellar & star formation properties of NGC 925 using the WIYN 3.5m telescope. Images in B,V,R, & H-alpha reveal a galaxy that is fraught with asymmetries. From isophote fits we discover that the bar center is not coincident with the center of the outer isophotes nor with the dynamical center (from Pisano et al. 1998). Cuts across the spiral arms reveal that the northern arms are distinctly different from the southern arm. The southern arm not only appears more coherent, but the peaks in stellar and H-alpha emission are found to be coincident with those of the HI distribution, while no such consistency is present in the northern disk. We also examine the gas surface density criterion for massive star formation in NGC 925, and find that its behavior is more consistent with that for irregular galaxies, than with late-type spirals. In particular, star formation persists beyond the radius at which the gas surface density falls below the predicted critical value for star formation for late-type spirals. Such properties are characteristic of Magellanic spirals, but are present at a less dramatic level in NGC 925, a late-type spiral.
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A number of theoretical and simulation results on star and structure formation in galaxy interactions and mergers is reviewed, and recent hydrodynamic simulations are presented. The role of gravity torques and ISM turbulence in galaxy interactions, in addition to the tidal field, is highlighted. Interactions can drive gas inflows towards the central kpc and trigger a central starburst, the intensity and statistical properties of which are discussed. A kinematically decoupled core and a supermassive central black hole can be fueled. Outside of the central kpc, many structures can form inside tidal tails, collisional ring, bridges, including super star clusters and tidal dwarf galaxies. The formation of super star clusters in galaxy mergers can now be directly resolved in hydrodynamic simulations. Their formation mechanisms and long-term evolution are reviewed, and the connection with present-day early-type galaxies is discussed.
We have made use of archival HST BVIJH photometry to constrain the nature of the three discrete sources, A1, A2 and B1, identified in the double nucleus of NGC 6240. STARBURST99 models have been fitted to the observed colours, under the assumption, first, that these sources can be treated as star clusters (i.e. single, instantaneous episodes of star formation), and subsequently as star-forming regions (i.e. characterised by continuous star formation). For both scenarios, we estimate ages as young as 4 million years, integrated masses ranging between 7x10^6 Msun (B1) and 10^9 Msun (A1) and a rate of 1 supernova per year, which, together with the stellar winds, sustains a galactic wind of 44 Msun/yr. In the case of continuous star formation, a star-formation rate has been derived for A1 as high as 270 Msun/yr, similar to what is observed for warm Ultraluminous Infrared Galaxies (ULIRGs) with a double nucleus. The A1 source is characterised by a mass density of about 1200 Msun/pc^3 which resembles the CO molecular mass density measured in cold ULIRGs and the stellar density determined in ``elliptical core galaxies. This, together with the recent discovery of a supermassive binary black hole in the double nucleus of NGC 6240, might indicate that the ongoing merger could shape the galaxy into a core elliptical.
We present a comprehensive multi-wavelength study of the star-forming region NGC 1893 to explore the effects of massive stars on low-mass star formation. Using near-infrared colours, slitless spectroscopy and narrow-band $Halpha$ photometry in the cluster region we have identified candidate young stellar objects (YSOs) distributed in a pattern from the cluster to one of the nearby nebulae Sim 129. The $V, (V-I)$ colour-magnitude diagram of the YSOs indicates that majority of these objects have ages between 1 to 5 Myr. The spread in the ages of the YSOs may indicate a non-coeval star formation in the cluster. The slope of the KLF for the cluster is estimated to be $0.34pm0.07$, which agrees well with the average value ($sim 0.4$) reported for young clusters. For the entire observed mass range $0.6 < M/M_odot le 17.7$ the value of the slope of the initial mass function, $`Gamma$, comes out to be $-1.27pm0.08$, which is in agreement with the Salpeter value of -1.35 in the solar neighborhood. However, the value of $`Gamma$ for PMS phase stars (mass range $0.6 < M/M_odot le 2.0$) is found to be $-0.88pm0.09$ which is shallower than the value ($-1.71pm0.20$) obtained for MS stars having mass range $2.5 < M/M_odot le 17.7$ indicating a break in the slope of the mass function at $sim 2 M_odot$. Estimated $`Gamma$ values indicate an effect of mass segregation for main-sequence stars, in the sense that massive stars are preferentially located towards the cluster center. The estimated dynamical evolution time is found to be greater than the age of the cluster, therefore the observed mass segregation in the cluster may be the imprint of the star formation process. There is evidence for triggered star formation in the region, which seems to govern initial morphology of the cluster.
NGC 925 ULX-1 and ULX-2 are two ultraluminous X-ray sources in the galaxy NGC 925, at a distance of 8.5 Mpc. For the first time, we analyzed high quality, simultaneous XMM-Newton and NuSTAR data of both sources. Although at a first glance ULX-1 resembles an intermediate mass black hole candidate (IMBH) because of its high X-ray luminosity ($(2$$-$$4)times10^{40}$ erg s$^{-1}$) and its spectral/temporal features, a closer inspection shows that its properties are more similar to those of a typical super-Eddington accreting stellar black hole and we classify it as a `broadened disc ultraluminous X-ray source. Based on the physical interpretation of this spectral state, we suggest that ULX-1 is seen at small inclination angles, possibly through the evacuated cone of a powerful wind originating in the accretion disc. The spectral classification of ULX-2 is less certain, but we disfavour an IMBH accreting at sub-Eddington rates as none of its spectral/temporal properties can be associated to either the soft or hard state of Galactic accreting black hole binaries.
We present an analysis of the optical properties of three Ultra Luminous X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from the George Mitchel spectrograph in the context of the Metal-THINGS survey. The optical properties for ULX-1 and ULX-3 are presented, while the spaxel associated with ULX-2 had a low S/N, which prevented its analysis. We also report the kinematics and dimensions of the optical nebula associated with each ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis demonstrates that most spaxels in NGC 925 are dominated by star-forming regions, including those associated with ULX-1 and ULX-3. Using the resolved gas-phase metallicities, a negative metallicity gradient is found, consistent with previous results for spiral galaxies, while the ionization parameter tends to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very low gas metallicity for its galactocentric distance, identified by two independent methods, while exhibiting a typical ionization. We find that such low gas metallicity is best explained in the context of the high-mass X-ray binary population, where the low-metallicity environment favours active Roche lobe overflows that can drive much higher accretion rates. An alternative scenario invoking accretion of a low-mass galaxy is not supported by the data in this region. Finally, ULX-3 shows both a high metallicity and ionization parameter, which is consistent with the progenitor being a highly-accreting neutron star within an evolved stellar population region.
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