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Register a new userNucler star formation in NGC 6240
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A. Pasquali
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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.
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WFPC2 images and STIS spectroscopic observations are presented of the double nucleus in the merger system NGC 6240. We find that: (a) the kinematics of the ionized gas is similar to that of the molecular gas, despite a different morphology; (b) the gaseous and stellar kinematics are quite different, suggesting an early merger stage; (c) neither the gaseous nor the stellar kinematics show an obvious sign of the supermassive black hole believed to be responsible for the X-ray emission of NGC 6240; and (d) the steep off-nuclear velocity gradient is not due to a 10E11 solar mass black hole, in contrast to earlier suggestions.
We present the first observations of H$^{13}$CN$(1-0)$, H$^{13}$CO$^+(1-0)$ and SiO$(2-1)$ in NGC,6240, obtained with the IRAM PdBI. Combining a Markov Chain Monte Carlo (MCMC) code with Large Velocity Gradient (LVG) modelling, and with additional data from the literature, we simultaneously fit three gas phases and six molecular species to constrain the physical condition of the molecular gas, including mass$-$luminosity conversion factors. We find $sim10^{10}M_odot$ of dense molecular gas in cold, dense clouds ($T_{rm k}sim10$,K, $n_{{rm H}_2}sim10^6$,cm$^{-3}$) with a volume filling factor $<0.002$, embedded in a shock heated molecular medium ($T_{rm k}sim2000$,K, $n_{{rm H}_2}sim10^{3.6}$,cm$^{-3}$), both surrounded by an extended diffuse phase ($T_{rm k}sim200$,K, $n_{{rm H}_2}sim10^{2.5}$,cm$^{-3}$). We derive a global $alpha_{rm CO}=1.5^{7.1}_{1.1}$ with gas masses $log_{10}left(M / [M_odot]right)=10.1_{10.0}^{10.8}$, dominated by the dense gas. We also find $alpha_{rm HCN} = 32^{89}_{13}$, which traces the cold, dense gas. The [$^{12}$C]/[$^{13}$C] ratio is only slightly elevated ($98^{230}_{65}$), contrary to the very high [CO]/[$^{13}$CO] ratio (300-500) reported in the literature. However, we find very high [HCN]/[H$^{13}$CN] and [HCO$^+$]/[H$^{13}$CO$^+$] abundance ratios $(300^{500}_{200})$ which we attribute to isotope fractionation in the cold, dense clouds.
We used the SPIRE/FTS instrument aboard the Herschel Space Observatory (HSO) to obtain the Spectral Line Energy Distributions (SLEDs) of CO from J=4-3 to J=13-12 of Arp 193 and NGC 6240, two classical merger/starbursts selected from our molecular line survey of local Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11} L_{sol}). The high-J CO SLEDs are then combined with ground-based low-J CO, {13}CO, HCN, HCO+, CS line data and used to probe the thermal and dynamical states of their large molecular gas reservoirs. We find the two CO SLEDs strongly diverging from J=4-3 onwards, with NGC6240 having a much higher CO line excitation than Arp193, despite their similar low-J CO SLEDs and L_{FIR}/L_{CO,1-0}, L_{HCN}/L_{CO} (J=1-0) ratios (proxies of star formation efficiency and dense gas mass fraction). In Arp193, one of the three most extreme starbursts in the local Universe, the molecular SLEDs indicate a small amount ~(5-15)% of dense gas (n>=10^{4}cm^{-3}) unlike NGC6240 where most of the molecular gas (~(60-70)%) is dense n~(10^4-10^5)cm^{-3}. Strong star-formation feedback can drive this disparity in their dense gas mass fractions, and also induce extreme thermal and dynamical states for the molecular gas.In NGC6240, and to a lesser degree in Arp193, we find large molecular gas masses whose thermal states cannot be maintained by FUV photons from Photon Dominated Regions (PDRs). We argue that this may happen often in metal-rich merger/starbursts, strongly altering the initial conditions of star formation. ALMA can now directly probe these conditions across cosmic epoch, and even probe their deeply dust-enshrouded outcome, the stellar IMF averaged over galactic evolution.
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.
We present UBVRI broad band, H$alpha$ narrow band photometry of the star forming complexes in the infra-red bright galaxy NGC 1084. Results of medium resolution spectroscopy of some of the brighter complexes are also discussed. Spectroscopic data is used to better estimate the internal reddening within the galaxy which is found to be highly variable and to calculate metallicity which is close to the solar value. Diagnostic diagram identifies the shocked regions within this galaxy. The narrow band H$alpha$ flux and its equivalent width are used to determine the star formation rates of the complexes and the distribution of ages. Star formation rates for a few of the complexes are found to be as high as 0.5 $M_{odot}$/year. The star forming complexes lie in the age range 3 Myr to 6.5 Myr. U-B vs V-I colour-colour mixed population model created using the Starburst99 model colours is used to estimate the ages of the stellar populations present within these regions. Using this technique, it is found that the star formation in NGC 1084 has taken place in a series of short bursts over the last 40 Myr or so. It is proposed that the likely trigger for enhanced star formation is merger with a gas rich dwarf galaxy.
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