Hubble space telescope observations of young star clusters in NGC-4038/4039, 'the antennae' galaxies

Abstract

New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented. NGC 4038/4039, nicknamed "The Antennae," is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger. Down to the limiting magnitude of V~23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks. The mean absolute magnitude of these objects is M_v_ = -11 mag, with the brightest objects reaching M_V_ ~ -15. Their mean apparent color indices are U-V= -0.7 mag and V-I = 0.8 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (U-V)_0_= -1.0 mag and (V-I)_0_ = 0.5. Their mean effective radius, determined from slightly resolved images, is 18 pc (for H_0_ = 50 km s^-1^ Mpc^-1^). Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters. The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galaxy mergers. Based on their U-V and V-I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old. Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations. The cluster luminosity function (LF) is approximately a power law, φ(L)dL is proportional to L^-1.78+/-0.05^dL, with no hint of a turnover at fainter magnitudes. This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a FWHM of ~3 mag. Possible explanations for this apparent difference include: (1) We have not observed faint enough to see the turnover, (2) the initial LF of star clusters is a power law but the fainter objects dissolve with time, (3) conditions at the present epoch favor the formation of a wide range of cluster masses while conditions at earlier epochs favored the formation of massive clusters, and (4) the NGC 4038/4039 clusters may not evolve into normal globular clusters. Besides the blue clusters, we also find about a dozen extremely red objects with V-I > 3.0. The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent. We propose that these objects may be very young star clusters still embedded in their placental dust cocoons.