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Register a new userHigh-resolution UKIRT observations of circumnuclear star formation in M100
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Stuart D. Ryder
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We present high-resolution, near-infrared imaging of the circumnuclear region of the barred spiral galaxy M100 (=NGC 4321), accompanied by near-infrared spectroscopy. We identify a total of 43 distinct regions in the K-band image, and determine magnitudes and colours for 41 of them. By comparison with other near-infrared maps we also derive colour excesses and K-band extinctions for the knots. Combining the imaging and spectroscopic results, we conclude that the knots are the result of bursts of star formation within the last 15-25 Myr. We discuss the implications of these new results for our dynamical and evolutionary understanding of this galaxy.
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We present low resolution (R~450) K-band spectroscopy for 16 of the 43 circumnuclear star-forming knots in M100 identified by Ryder & Knapen (1999). We compare our measurements of equivalent widths for the Br-gamma emission line and CO 2.29 micron absorption band in each knot with the predictions of starburst models from the literature, and derive ages and burst parameters for the knots. The majority of these knots are best explained by the result of short, localised bursts of star formation between 8 and 10 Myr ago. By examining both radial and azimuthal trends in the age distribution, we present a case for sequential triggering of star formation, most likely due to the action of a large-scale shock. In an appendix, we draw attention to the fact that the growth in the CO spectroscopic index with decreasing temperature in supergiant stars is not as regular as is commonly assumed.
We have obtained integral-field spectroscopic data, using the SAURON instrument, of the bar and starbursting circumnuclear region in the barred spiral galaxy M100. From our data we have derived kinematic maps of the mean velocity and velocity dispersion of the stars and the gas, which we present here. We have also produced maps of the total, [OIII], and Hbeta intensity. The gas velocity field shows significant kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across the miniature spiral arms in the nuclear pseudo-ring. The stellar velocity field shows similar non-circular motions. The gas velocity dispersion is notably smaller where the star formation occurs in the nuclear zone and HII regions.
High-resolution radio observations of nearby starburst galaxies have shown that the distribution of their radio emission consists of a compact (<150 pc), high surface brightness, central radio source immersed in a low surface brightness circumnuclear halo. This radio structure is similar to that detected in bright Seyferts galaxies like NGC 7469 or Mrk 331, which display clear circumnuclear rings. While the compact, centrally located radio emission in these starbursts might be generated by a point-like source (AGN), or by the combined effect of multiple radio supernovae and supernova remnants (e.g., the evolved nuclear starburst in Arp~220), it seems well established that the circumnuclear regions of those objects host an ongoing burst of star-formation (e.g., NGC 7469; Colina et al. 2001, Alberdi et al. 2006). Therefore, high-resolution radio observations of Luminous Infra-Red Galaxies (LIRGs) in our local universe are a powerful tool to probe the dominant dust heating mechanism in their nuclear and circumnuclear regions. In this contribution, we show results obtained from VLA-A, MERLIN, and EVN (VLBI) radio observations of the galaxies NGC 7469 (D~70 Mpc) and IRAS 18293-3413 (D ~ 79 Mpc), where two extremely bright radio supernovae have been found. High-resolution studies of these and other LIRGs would allow us to determine the core-collapse supernova rate in them, as well as their star-formation rate.
Increasing evidence suggests that, similar to their low-mass counterparts, high-mass stars form through a disk-mediated accretion process. At the same time, formation of high-mass stars still necessitates high accretion rates, and hence, high gas densities, which in turn can cause disks to become unstable against gravitational fragmentation. We study the kinematics and fragmentation of the disk around the high-mass star forming region AFGL 2591-VLA 3 which was hypothesized to be fragmenting based on the observations that show multiple outflow directions. We use a new set of high-resolution (0.19 arcsec) IRAM/NOEMA observations at 843 micron towards VLA 3 which allow us to resolve its disk, characterize the fragmentation, and study its kinematics. In addition to the 843 micron continuum emission, our spectral setup targets warm dense gas and outflow tracers such as HCN, HC$_3$N and SO$_2$, as well as vibrationally excited HCN lines. The high resolution continuum and line emission maps reveal multiple fragments with subsolar masses within the inner 1000 AU of VLA 3. Furthermore, the velocity field of the inner disk observed at 843 micron shows a similar behavior to that of the larger scale velocity field studied in the CORE project at 1.37 mm. We present the first observational evidence for disk fragmentation towards AFGL 2591-VLA 3, a source that was thought to be a single high-mass core. While the fragments themselves are low-mass, the rotation of the disk is dominated by the protostar with a mass of 10.3$pm 1.8~M_{odot}$. These data also show that NOEMA Band 4 can obtain the highest currently achievable spatial resolution at (sub-)mm wavelengths in observations of strong northern sources.
Circumnuclear star forming regions, also called hotspots, are often found in the inner regions of some spiral galaxies where intense processes of star formation are taking place. In the UV, massive stars dominate the observed circumnuclear emission even in the presence of an active nucleus, contributing between 30 and 50% to the H$beta$ total emission of the nuclear zone. Spectrophotometric data of moderate resolution (3000 < R < 11000) are presented from which the physical properties of the ionized gas: electron density, oxygen abundances, ionization structure etc. have been derived.
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