NICMOS Imaging of the Nuclei of Arp 220

Abstract

We report high-resolution imaging of the ultraluminous infrared galaxy Arp 220 at 1.1, 1.6, and 2.22 mm with the Near-Infrared Camera and Multiobject Spectrometer on the Hubble Space Telescope. The diffraction-limited images at 0Љ .1-0Љ .2 resolution clearly resolve both nuclei of the merging galaxy system and reveal for the first time a number of luminous star clusters in the circumnuclear envelope. The morphologies of both nuclei are strongly affected by dust obscuration, even at 2.2 mm: the primary nucleus (west) presents a crescent shape, concave to the south, and the secondary (eastern) nucleus is bifurcated by a dust lane with the southern component being very reddened. In the western nucleus, the morphology of the 2.2 mm emission is most likely the result of obscuration by an opaque disk embedded in the nuclear star cluster. The morphology of the central starburst cluster in the western nucleus is consistent with either a circumnuclear ring of star formation or a spherical cluster with the bottom half obscured by the embedded dust disk. Comparison of centimeter-wave radio continuum maps with the near-infrared images suggests that the radio nuclei lie in the dust disk on the west and near the highly reddened southern component of the eastern complex. The radio nuclei are separated by 0Љ .98 (corresponding to 364 pc at 77 Mpc), and the half-widths of the infrared nuclei are ∼0Љ .2-0Љ .5. At least eight unresolved infrared sources-probably globular clusters-are also seen in the circumnuclear envelope at radii of 2Љ-7Љ. Their nearinfrared colors do not significantly constrain their ages.

Two Populations of Young Massive Star Clusters in Arp 220

“…Since the absolute pointing of HST can be off by $1 00 or more (e.g., Whitmore & Zhang 2002), we compared our coordinates for seven clusters that we were able to cross-identify in common with the near-infrared HST Near-Infrared Camera and Multi-Object Spectrometer ( NICMOS) study of Scoville et al (1998), in which the coordinates were established to well within 1 00 absolute accuracy from the position of the central radio source (see their discussion). We found that it was necessary to shift the raw ACS coordinates by 3B5 in right ascension and 0B3 in a Aperture correction from a radius of 3 pixels to a large radius, interpolated from Sirianni et al (2005).…”

“…Near-infrared observations by Scoville et al (1998) identified eight bright star cluster candidates in Arp 220. Shioya et al (2001) combined these two sets of data to estimate ages for three of these star clusters in the range of 10-100 Myr.…”

The Submillimeter Array 1.3 mm line survey of Arp 220

“…This galaxy is an advanced merger system as evidenced by the large tidal tails observed in the optical (Joseph & Wright 1985;Kim et al 2002;Koda & Subaru Cosmos Team 2009) and the double nuclei (separation ∼0.98 ) observed in radio (Norris 1988), mm (Scoville et al 1997;Downes & Solomon 1998;Sakamoto et al 1999), submm (Sakamoto et al 2008) and near-IR wavelenghts (Graham et al 1990;Scoville et al 1998). These nuclei are surrounded by two counterrotating gas disks as well as a larger outer disk encompassing both (Sakamoto et al 1999;Mundell et al 2001).…”

“…The nuclei in Arp 220 are affected by a severe obscuration at 2.2 μm (Scoville et al 1998). Even at 1 mm, the dust towards the more massive western nucleus is found to be significantly optically thick (τ ∼ 1, Downes & Eckart 2007).…”

NGST: Seeing the First Stars and Galaxies Form

“…(STScI) nation views of these disks, neither NGST nor the IRAC camera on SIRTF will be capable of completely penetrating these regions but will be able to detect the outer skins of star formation in these tori. The recent NIC-MOS images of the ultraluminous IR galaxy Arp 220 easily resolves the surface of the inner torus or two separate star forming regions (Scoville et al 1998). NGST, with 0.3 arcsec resolution at 10 uui, would be capable of resolving and penetrating 300 parsec dust tori at redshifts of z I".J 1.…”