Extended mid-infrared emission from VV 114: probing the birth of a ULIRG

We present our 5-16 micron spectro-imaging observations of VV114, an infrared luminous early-stage merger, taken with the ISOCAM camera on-board ISO. We find that only 40% of the mid-infrared (MIR) flux is associated with a compact nuclear region, while the rest of the emission originates from a rather diffuse component extended over several kpc. This is in stark contrast with the very compact MIR starbursts usually seen in luminous infrared galaxies. A secondary peak of MIR emission is associated with an extra-nuclear star forming region which displays the largest Halpha equivalent width in the whole system. Comparing our data with the distribution of the molecular gas and cold dust, as well as with radio observations, it becomes evident that the conversion of molecular gas into stars can be triggered over large areas at the very first stages of an interaction. The presence of a very strong continuum at 5 microns in one of the sources indicates that an enshrouded active galactic nucleus may contribute to 40% of its MIR flux. We finally note that the relative variations in the UV to radio spectral properties between the merging galaxies provide evidence that the extinction-corrected star formation rate of similar objects at high z, such as those detected in optical deep surveys, can not be accurately derived from their rest-frame UV properties.Comment: 14 pages, 5 figures, accepted for publication in A&

Comparing Chandra and SIRTF Observations for Obscured Starbursts and AGN at High Redshift

Tracking the star formation rate to high redshifts requires knowledge of the contribution from both optically visible and obscured sources. The dusty, optically-obscured galaxies can be located by X-ray and infrared surveys. To establish criteria for selecting such sources based only on X-ray and infrared surveys, we determine the ratio of infrared to X-ray brightness that would be observed by SIRTF and Chandra for objects with the same spectral shapes as nearby starbursts if seen at high redshift. The parameter IR/X is defined as IR/X = (flux density observed in SIRTF MIPS 24 $\mu$m filter in mJy)/(total flux observed within 0.5-2.0 keV in units of 10^-16 ergs\s\cm^2). Based on observations of NGC 4038/39 (``The Antennae''), NGC 3690+IC 694 (Arp 299 or Mkn 171), M 82, and Arp 220, nine starburst regions are compared using mid-infrared spectra taken by the Infrared Space Observatory (ISO) and X-ray spectra obtained with Chandra . The IR/X are determined as they would appear for 1<z<3. The mean IR/X over this redshift range is 1.3 and is not a significant function of redshift or luminosity, indicating that SIRTF surveys reaching 0.4 mJy at 24 $\mu$m should detect the same starbursts as deep CXO surveys detect at a flux of 0.3x10^-16 ergs/s/cm^2. The lower bound of IR/X for starbursts is about 0.2, suggesting that objects with IR/X smaller than this have an AGN X-ray component in addition to the starburst. Values of IR/X for the obscured AGN within NGC 1068, the Circinus galaxy, and NGC 6240 are also determined for comparison although interpretation is complicated by the circumnuclear starbursts in these galaxies. Any sources found in surveys having IR/X>4 would not match any of the objects considered.Comment: accepted for publication in Ap

Mid-infrared observations of the ultraluminous galaxies IRAS14348-1447, IRAS19254-7245, and IRAS23128-5919

We present a study of the three ultraluminous infrared galaxies IRAS14348-1447, IRAS19254-7245, and IRAS23128-5919, based on mid-infrared (MIR) spectro-imaging (5-18microns) observations performed with ISOCAM. We find that the MIR emission from each system, which consists of a pair of interacting late type galaxies, is principally confined to the nuclear regions with diameters of 1-2kpc and can account for more than 95% of their IRAS 12micron flux. In each interacting system, the galaxy hosting an active galactic nucleus (AGN) dominates the total spectrum and shows stronger dust continuum (12-16microns) relative to the Unidentified Infrared Band (UIB) emission (6-9microns), suggestive of its enhanced radiation field. The MIR dominant galaxy also exhibits elevated 15micron/Halpha and 15micron/K ratios which trace the high extinction due to the large quantities of molecular gas and dust present in its central regions. Using only diagnostics based on our mid-infrared spectra, we can establish that the Seyfert galaxy IRAS19254-7245 exhibits MIR spectral features of an AGN while the MIR spectrum of the Seyfert (or LINER) member of IRAS23128-5919 is characteristic of dust emission principally heated by star forming regions.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 9 figure

K-band Spectroscopy of Clusters in NGC 4038/4039

Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae''). Two of the clusters are located in the overlap region of the two galaxies, and together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission from this pair of merging galaxies. The other two clusters, each of them spatially resolved into two components, are located in the northern galaxy, one in the western and one in the eastern loop of blue clusters. Comparing our analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST data), and V-K colors with stellar population synthesis models indicates that the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a significant age spread (4-13 Myrs). The starbursts in the nuclei are much older (65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star formation northward of its K-band peak. Using our derived age estimates and assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100 M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters have masses between 0.5 and 5 * 10^6M_sun.Comment: 10 pages, 3 figures, ApJ accepte

Starburst in the Intragroup Medium of Stephan's Quintet

Based on new ISO mid-infrared observations and ground based $H_\alpha$ and near-infrared observations, we report the detection of a bright starburst in the intragroup medium (IGM) of the famous compact group of galaxies Stephan's Quintet (Source A in Fig.1). We demonstrate that this starburst is caused by a collision between a high velocity ($\delta$V$\sim$ 1000 km/sec) intruder galaxy (NGC7318b) and the IGM of the group. While this is the only starburst known today that is induced by a galaxy/cold-intergalactic-medium collision, it provides new constraints to the theory for interaction-induced starbursts, and may hint at a new mechanism for the star formation excess seen in more distant clusters.Comment: 17 pages, 2 PS figures. Accepted by Ap

The Mass Function of Super Giant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/39)

We have used previously published observations of the CO emission from the Antennae (NGC 4038/39) to study the detailed properties of the super giant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5E6 to 9E8 solar masses, the molecular complexes follow a power-law mass function with a slope of -1.4 +/- 0.1, which is very similar to the slope seen at lower masses in molecular clouds and cloud cores in the Galaxy. Compared to the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Antennae contain clouds that are an order of magnitude more massive. Many of the youngest star clusters lie in the gas-rich overlap region, where extinctions as high as Av~100 imply that the clusters must lie in front of the gas. Combining data on the young clusters, thermal and nonthermal radio sources, and the molecular gas suggests that young massive clusters could have formed at a constant rate in the Antennae over the last 160 Myr and that sufficient gas exists to sustain this cluster formation rate well into the future. However, this conclusion requires that a very high fraction of the massive clusters that form initially in the Antennae do not survive as long as 100 Myr. Finally, we compare our data with two models for massive star cluster formation and conclude that the model where young massive star clusters form from dense cores within the observed super giant molecular complexes is most consistent with our current understanding of this merging system. (abbreviated)Comment: 40 pages, four figures; accepted for publication in Ap

Dust enshrouded star-forming activity in Arp 299

We present mid-infrared spectro-imaging (5 - 16 microns) observations of the infrared luminous interacting system Arp 299 (=Mrk171 =IC694+NGC3690) obtained with the ISOCAM instrument aboard ISO. Our observations show that nearly 40% of the total emission at 7 and 15 microns is diffuse, originating from the interacting disks of the galaxies. Moreover, they indicate the presence of large amounts of hot dust in the main infrared sources of the system and large extinctions toward the nuclei. While the observed spectra have an overall similar shape, mainly composed of Unidentified Infrared Bands (UIB) in the short wavelength domain, a strong continuum at ~ 13 microns and a deep silicate absorption band at 10 microns, their differences reveal the varying physical conditions of each component. For each source, the spectral energy distribution (SED) can be reproduced by a linear combination of a UIB "canonical" spectral template and a hot dust continuum due to a 230-300 K black body, after independently applying an extinction correction to both of them. We find that the UIB extinction does not vary much throughout the system (A_V ~ 5 mag) suggesting that most UIBs originate from less enshrouded regions. IC694 appears to dominate the infrared emission of the system and our observations support the interpretation of a deeply embedded nuclear starburst located behind an absorption of about 40 mag. The central region of NGC3690 displays a hard radiation field characterized by a [NeIII]/[NeII] ratio > 1.8. It also hosts a strong continuum from 5 to 16 microns which can be explained as thermal emission from a deeply embedded (A_V ~ 60 mag) compact source, consistent with the mid-infrared signature of an active galactic nucleus (AGN), and in agreement with recent X-ray findings.Comment: to be published in Astronomy and Astrophysics - 12 page

High-Resolution Imaging of Molecular Gas and Dust in the Antennae (NGC 4038/39): Super Giant Molecular Complexes

We present new aperture synthesis CO maps of the Antennae (NGC 4038/39) obtained with the Caltech Millimeter Array. These sensitive images show molecular emission associated with the two nuclei and a partial ring of star formation to the west of NGC 4038, as well as revealing the large extent of the extra-nuclear region of star formation (the ``overlap region''), which dominates the CO emission from this system. The largest molecular complexes have masses of 3-6x10^8 M_sun, typically an order of magnitude larger than the largest structures seen to date in more quiescent galaxy disks. The extremely red luminous star clusters identified previously with HST are well-correlated with the CO emission, which supports the conclusion that they are highly embedded young objects rather than old globular clusters. There is an excellent correlation between the CO emission and the 15 micron emission seen with ISO, particularly for the brightest regions. The most massive complexes in the overlap region have similar [NeIII]/[NeII] ratios, which implies that all these regions are forming many massive stars. However, only the brightest mid-infrared peak shows strong, rising continuum emission longward of 10 microns, indicative of very small dust grains heated to high temperatures by their proximity to nearby luminous stars. Since these grains are expected to be removed rapidly from the immediate environment of the massive stars, it is possible that this region contains very young (< 1 Myr) sites of star formation. Alternatively, fresh dust grains could be driven into the sphere of influence of the massive stars, perhaps by the bulk motions of two giant molecular complexes. The kinematics and morphology of the CO emission in this region provide some support for this second scenario.Comment: Accepted for publication in The Astrophysical Journal, 13 pages, 5 figures, higher quality color images available at http://www.astro.cornell.edu/staff/vassilis/papers/ngc4038_co.ps.g

The K luminosity-metallicity relation for dwarf galaxies and the tidal dwarf galaxies in the tails of HCG 31

We determine a K-band luminosity-metallicity (K-Z) relation for dwarf irregular galaxies, over a large range of magnitudes, -20.5 < M_K < -13.5, using a combination of K photometry from either the 2-micron all sky survey (2MASS) or the recent study of Vadivescu er al. (2005), and metallicities derived mainly with the T_e method, from several different studies. We then use this newly-derived relation, together with published K_s photometry and our new spectra of objects in the field of HCG 31 to discuss the nature of the possible tidal dwarf galaxies of this group. We catalogue a new member of HCG 31, namely "R", situated ~40 kpc north of the group center, composed by a ring of H alpha knots which coincides with a peak in HI. This object is a deviant point in the K-Z relation (it has too high metallicity for its luminosity) and its projected distance to the parent galaxy and large gas reservoir makes it one of the most promising tidal dwarf galaxy candidates of HCG 31, together with object F. The subsystems A1, E, F, H and R all have metallicities similar to that of the galaxies A+C and B, result that is expected in a scenario where those were formed from material expelled from the central galaxies of HCG 31. While objects A1, E and H will most probably fall back onto their progenitors, F and R may survive as tidal dwarf galaxies. We find that two galaxies of HCG 31, G and Q, have A+em spectral signatures, and are probably evolving toward a post-starburst phase.Comment: 32 pages, 4 figures - Submitted to AJ - A version of this paper with full resolution figures can be found at http://www.astro.iag.usp.br/~eduardo/HCG31-KZrelation.pd

EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation

We report the discovery of eleven beat Cepheids in the Small Magellanic Cloud, using data obtained by the EROS microlensing survey. Four stars are beating in the fundamental and first overtone mode (F/1OT), seven are beating in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the SMC Cepheids is the same as the LMC one.Comment: 4 pages, Latex file with 4 .ps figures. accepted for publication in A A Letter