Is the interstellar gas of starburst galaxies well mixed?

The extent to which the ISM in galaxies is well mixed is not yet settled. Measured metal abundances in the diffuse neutral gas of star--forming gas--rich dwarf galaxies are deficient with respect to that of the ionized gas. The reasons, if real, are not clear and need to be based on firm grounds. Far-UV spectroscopy of giant HII regions such as NGC604 in the spiral galaxy M33 using FUSE allows us to investigate possible systematic errors in the metallicity derivation. We still find underabundances of nitrogen, oxygen, argon, and iron in the neutral phase by a factor of~6. This could either be explained by the presence of less chemically evolved gas pockets in the sightlines or by dense clouds out of which HIIregions form. Those could be more metallic than the diffuse medium.Comment: 4 pages, 2 figures;contribution to Starbursts: from 30 Dor to Lyman Break Galaxies, 6 -10 September 2004, Institute of Astronomy, University of Cambridge, U

Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. II. Optical and Near Infrared Star Formation History of the Starburst Cluster NGC 2070 in 30 Doradus

We present a study of the recent star formation of 30 Doradus in the Large Magellanic Cloud (LMC) using the panchromatic imaging survey Hubble Tarantula Treasury Project (HTTP). In this paper we focus on the stars within 20 pc of the center of the massive ionizing cluster of 30 Doradus, NGC 2070. We recovered the star formation history by comparing deep optical and NIR color-magnitude diagrams (CMDs) with state-of-the-art synthetic CMDs generated with the latest PARSEC models, which include all stellar phases from pre-main sequence to post- main sequence. For the first time in this region we are able to measure the star formation using intermediate and low mass stars simultaneously. Our results suggest that NGC2070 experienced a prolonged activity. In particular, we find that the star formation in the region: i) exceeded the average LMC rate ~ 20 Myr ago; ii) accelerated dramatically ~ 7 Myr ago; and iii) reached a peak value 1-3 Myr ago. We did not find significant deviations from a Kroupa initial mass function down to 0.5 Msun. The average internal reddening E(B-V) is found to be between 0.3 and 0.4 mag.Comment: Submitted to Ap

The Star Formation History of IZw18

The star formation history in IZw18 has been inferred from HST/WFPC2 archival data. This is done by comparing the derived V, B-V and V, V-I color-magnitude diagrams and luminosity functions with synthetic ones, based on various sets of stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main body is not experiencing its first episode of star formation. Instead, it has been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body, the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to the shallower diagrams and the small number of resolved stars. The derived range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The IMF providing the best fit to the observed stellar populations in the main body has a slope 1.5, much flatter than in any similar galaxy analyzed with the same method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope (2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical Journa

LEGUS and Halpha-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age

We present a new catalog and results for the cluster system of the starburst galaxy NGC 4449 based on multi-band imaging observations taken as part of the LEGUS and Halpha-LEGUS surveys. We improve the spectral energy fitting method used to estimate cluster ages and find that the results, particularly for older clusters, are in better agreement with those from spectroscopy. The inclusion of Halpha measurements, the role of stochasticity for low mass clusters, the assumptions about reddening, and the choices of SSP model and metallicity all have important impacts on the age-dating of clusters. A comparison with ages derived from stellar color-magnitude diagrams for partially resolved clusters shows reasonable agreement, but large scatter in some cases. The fraction of light found in clusters relative to the total light (i.e., T_L) in the U, B, and V filters in 25 different ~kpc-size regions throughout NGC 4449 correlates with both the specific Region Luminosity, R_L, and the dominant age of the underlying stellar population in each region. The observed cluster age distribution is found to decline over time as dN/dt ~ t^g, with g=-0.85+/-0.15, independent of cluster mass, and is consistent with strong, early cluster disruption. The mass functions of the clusters can be described by a power law with dN/dM ~ M^b and b=-1.86+/-0.2, independent of cluster age. The mass and age distributions are quite resilient to differences in age-dating methods. There is tentative evidence for a factor of 2-3 enhancement in both the star and cluster formation rate ~100 - 300 Myr ago, indicating that cluster formation tracks star formation generally. The enhancement is probably associated with an earlier interaction event

Far-Ultraviolet & X-ray Observations of VV 114: Feedback in a Local Analog to Lyman Break Galaxies

We have analyzed FUSE, XMM, and Chandra observations of VV 114, a local galaxy merger with strong similarities to typical high-redshift Lyman Break Galaxies (LBGs). Diffuse thermal X-ray emission encompassing VV114 has been observed by Chandra and XMM. This region of hot (kT~0.59 keV) gas has an enhanced alpha to iron element ratio relative to solar abundances and follows the same relation as typical starbursts between its properties (luminosity, size, and temperature) and those of the starburst galaxy (star formation rate, dust temperature, galaxy mass). These results are consistent with the X-ray gas having been produced by shocks driven by a galactic superwind. The FUSE observations of VV 114 show strong, broad interstellar absorption lines with a pronounced blueshifted component(similar to what is seen in LBGs). This implies an outflow of material moving at 300-400 km/s relative to VV 114. The properties of the strong OVI absorption line are consistent with radiative cooling at the interface between the hot outrushing gas seen in X-rays and the cooler material seen in the other outflowing ions in the FUSE data. We show that the wind in VV114 has not created a ``tunnel'' that enables more than a small fraction (< few percent) of the ionizing photons from VV114 to escape into the IGM. Taken together, these data provide a more complete physical basis for understanding the outflows that seem to be generic in LBGs. This will lead to improved insight into the role that such outflows play in the evolution of galaxies and the inter-galactic medium.Comment: 33 pages, 13 figure

Etn@ref: a geodetic reference frame for Mt. Etna GPS networks

In volcanology, one of the most important instruments for scientific community interested in modelling the physical processes related to magma movements in the shallow crust is geodetic data. Since the end of the 1980s, GPS surveys and Continuous GPS stations (CGPS) have greatly improved the possibility to measure such movements with high time and space resolution. However, physical modelling requires that any external influence on the data, not directly related to the investigated quantity, must be filtered. One major tricky factor in determining a deformation field using GPS displacement vectors and velocities is the correct choice of a stable reference frame. In this work, using more than a decade of GPS measurements, we defined a local reference frame in order to refer the Mt. Etna ground deformation pattern to a rigid block. In particular, we estimated the Euler pole for the rigid block by minimizing, with a weighted least squares inversion, the adjustments to two horizontal components of GPS velocity at 13 “fiducial” sites located within 350 km around Mt. Etna. The inversion inferred an Euler pole located at 38.450° N and -107.702° E and a rotation rate of 0.263 deg/Myr

HST morphologies of local Lyman break galaxy analogs I: Evidence for starbursts triggered by merging

Heckman et al. (2005) used the Galaxy Evolution Explorer (GALEX) UV imaging survey to show that there exists a rare population of nearby compact UV-luminous galaxies (UVLGs) that closely resembles high redshift Lyman break galaxies (LBGs). We present HST images in the UV, optical, and Ha, and resimulate them at the depth and resolution of the GOODS/UDF fields to show that the morphologies of UVLGs are also similar to those of LBGs. Our sample of 8 LBG analogs thus provides detailed insight into the connection between star formation and LBG morphology. Faint tidal features or companions can be seen in all of the rest-frame optical images, suggesting that the starbursts are the result of a merger or interaction. The UV/optical light is dominated by unresolved (~100-300 pc) super starburst regions (SSBs). A detailed comparison with the galaxies Haro 11 and VV 114 at z=0.02 indicates that the SSBs themselves consist of diffuse stars and (super) star clusters. The structural features revealed by the new HST images occur on very small physical scales and are thus not detectable in images of high redshift LBGs, except in a few cases where they are magnified by gravitational lensing. We propose, therefore, that LBGs are mergers of gas-rich, relatively low-mass (~10^10 Msun) systems, and that the mergers trigger the formation of SSBs. If galaxies at high redshifts are dominated by SSBs, then the faint end slope of the luminosity function is predicted to have slope alpha~2. Our results are the most direct confirmation to date of models that predict that the main mode of star formation in the early universe was highly collisional.Comment: 32 pages, 15 figures. ApJ In pres

Interstellar abundances in the neutral and ionized gas of NGC604

We present FUSE spectra of the giant HII region NGC604 in the spiral galaxy M33. Chemical abundances are derived from far-UV absorption lines and are compared to those derived from optical emission lines. We derived the column densities of HI, NI, OI, SiII, PII, ArI, and FeII, fitting the line profiles with either a single component or several components. Our net results, assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of 10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only PII, ArI, and FeII lines should not be affected by the presence of hidden saturated components, while NI, OI, and SiII might be much more affected. If N, O, and Si are actually underabundant in the neutral gas of NGC604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.Comment: Accepted for publication in A&