The Star Cluster System of the NGC 7673 Starburst

We investigate the star cluster system in the starburst galaxy NGC 7673 using archival Hubble Space Telescope WFPC2 broad-band images. For the first time we are able to examine the internal structures of the prominent optical clumps in this galaxy. The clumps are composed of young stars, 16-33 % of which are in bright star clusters. We identify 268 star cluster candidates in both the F555W and F814W images, and 50 clusters with the F255W filter. These data allow us to estimate ages and masses using color-magnitude and two-color diagrams for our sample. We find a population of young, < 6 Myr clusters located throughout the galaxy with concentrations in the clumps. Star cluster mass estimates are 5-50 x 10^4 M_solar for the brightest objects. The starburst remains active in physically well-separated regions, indicating a widespread starburst trigger. We discuss clump lifetimes, their implications for the future evolution of NGC 7673, and possible relationships to high redshift starbursts.Comment: 18 pages, 15 figures. Accepted for publication in Astronomy & Astrophysic

3D Spectroscopy of Local Luminous Compact Blue Galaxies: Kinematics of NGC 7673

The kinematic properties of the ionized gas of local Luminous Compact Blue Galaxy (LCBG) NGC 7673 are presented using three dimensional data taken with the PPAK integral field unit at the 3.5-m telescope in the Centro Astron\'omico Hispano Alem\'an. Our data reveal an asymmetric rotating velocity field with a peak to peak difference of 60 km s$^{-1}$. The kinematic centre is found to be at the position of a central velocity width maximum ($\sigma=54\pm1$ km s$^{-1}$), which is consistent with the position of the luminosity-weighted centroid of the entire galaxy. The position angle of the minor rotation axis is 168$^{\circ}$ as measured from the orientation of the velocity field contours. At least two decoupled kinematic components are found. The first one is compact and coincides with the position of the second most active star formation region (clump B). The second one is extended and does not have a clear optical counterpart. No evidence of active galactic nuclei activity or supernovae galactic winds powering any of these two components has been found. Our data, however, show evidence in support of a previously proposed minor merger scenario in which a dwarf galaxy, tentatively identified with clump B, is falling into NGC 7673. and triggers the starburst. Finally, it is shown that the dynamical mass of this galaxy may be severely underestimated when using the derived rotation curve or the integrated velocity width, under the assumption of virialization.Comment: Accepted for publication by MNRAS. The paper contains 10 figures and 2 table

Dissecting the star-formation history of starburst galaxies: the case of NGC7673

We have collected archival data on NGC7673 to constrain the star-formation history that produced the young star clusters and the field stellar population in this galaxy during the last 2 Gyr. We have considered the sample of 50 star clusters detected by HST/WFPC2 in the UV, V and I bands and estimated their age, intrinsic reddening, and mass via comparison of their colours with STARBURST99 models. We have found two prominent epochs of cluster formation occurred about 20 Myr and 2 Myr ago, with somewhat minor events between 3 Myr and 6 Myr ago. The star clusters are characterised by an intrinsic reddening E(B-V) < 0.4 mag and a mass lower than 2e+06 solar masses. Out of the 50 star clusters, we have selected 31 located within the boundaries of the IUE large slit that was employed to obtain the spectrum of NGC7673 between 1150 Ang. and 3350 Ang. For each cluster, we have built a synthetic spectrum corresponding to the age, mass and intrinsic reddening derived from the cluster colours, properly redshifted to NGC7673. The spectra have then been added together in a final, clusters integrated spectrum. This and the IUE and FUSE spectra of NGC7673 have allowed us to describe the star-formation history of the unresolved stars in the field as either exponentially decaying or multi-burst. In the first case, we have derived an e-folding time of 700 (900) Myr and an initial star-formation rate of 16 (13) solar masses per year when the Fitzpatrick's (Calzetti's) extinction law is used. In the case of a multi-burst star-formation history, the field population turns out to be composed by a young (< 40 Myr) component 3 (2) times brighter than the star clusters, and a component as old as 850 (450) Myr, about 200 (100) times more massive than the star clusters together.Comment: 17 pages, 10 figures, accepted by MNRA

Clumpy galaxies at z~0.6: kinematics, stability, and comparison with analogs at other redshifts

Distant clumpy galaxies are thought to be Jeans-unstable disks, and an important channel for the formation of local galaxies, as suggested by recent spatially-resolved kinematic observations of z~2 galaxies. I study the kinematics of clumpy galaxies at z~0.6, and compare their properties with those of counterparts at higher and lower redshifts. I selected a sample of 11 clumpy galaxies at z~0.6 from the representative sample of emission line, intermediate-mass galaxies IMAGES. Selection was based on rest-frame UV morphology from HST/ACS images, mimicking the selection criteria commonly used at higher redshifts. Their spatially-resolved kinematics were derived in the frame of the IMAGES survey, using the VLT/FLAMES-GIRAFFE multi-integral field spectrograph. For those showing large-scale rotation, I derived the Toomre Q parameter, which characterizes the stability of their gaseous and stellar phases. I find that the fraction of UV-selected clumpy galaxies at z~0.6 is 20+/-12%. Roughly half of them (45+/-30%) have complex kinematics inconsistent with Jeans-unstable disks, while those in the remaining half (55+/-30%) show large-scale rotations. The latter reveal a stable gaseous phase, but the contribution of their stellar phase makes them globally unstable to clump formation. Clumpy galaxies appear to be less unstable at z~0.6 than at z~2, which could explain why the UV clumps tend to vanish in rest-frame optical images of z~0.6 clumpy galaxies, conversely to z~2 clumpy galaxies, in which the stellar phase can substantially fragment. This suggests that the former correspond to patchy star-formation regions superimposed on a smoother mass distribution. A possible and widespread scenario for driving clump formation relies on instabilities by cold streams penetrating the dark matter halos where clumpy galaxies inhabit. While such a gas accretion process is predicted to be significant in massive, z~2 haloes, it is also predicted to be strongly suppressed in similar, z~0.6 haloes, which could explain why lowest-z clumpy galaxies appear to be driven by a different mechanism. Instead, I found that interactions are probably the dominant driver leading to the formation of clumpy galaxies at z1 clumpy galaxies remains more uncertain. While cold flows could be an important driver at z~2, I also argue that the observed and cumulative merger fraction between z=2 and z=3 is large enough so that every z~2 galaxy might be the result of a merger that occurred within their past 1 Gyr. I conclude that it is premature to rule out mergers as a universal driver for galaxy evolution from z~2 down to z=0.Comment: Accepted for publication in MNRAS. 14 pages, 2 figures

Physical Properties of Young Stellar Populations in 24 Starburst Galaxies Observed with FUSE

We presents the main physical properties of very young stellar populations seen with FUSE in 24 individual starbursts. These characteristics have been obtained using the evolutionary spectral synthesis technique in the far-ultraviolet range with the LavalSB code. For each starburst, quantitative values for age, metallicity, initial mass function slope, stellar mass, and internal extinction have been obtained and discussed in details. Limits of the code have been tested. One main conclusion is that most starbursts (and probably all of them) cannot be represented by any continuous star formation burst in the far-ultraviolet. Also, quantitative values of various optical diagnostics related to these stellar populations have been predicted. Underlying stellar populations, dominated by B-type stars, have been detected in NGC 1140, NGC 4449, and possibly NGC 3991. We characterized the young stellar populations of less than 5 Myr in Seyfert 2 nuclei.Comment: 24 pages including figures. Accepted for publication in MNRA