Hubble Space Telescope images of submillimeter sources: large, irregular galaxies at high redshift

We present new Hubble Space Telescope STIS, high-resolution optical imaging of a sample of 13 submillimeter (submm) luminous galaxies, for which the optical emission has been pinpointed either through radio-1.4 GHz or millimeter interferometry. We find a predominance of irregular and complex morphologies in the sample, suggesting that mergers are likely common for submm galaxies. The component separation in these objects are on average a factor two larger than local galaxies with similarly high bolometric luminosities. The sizes and star formation rates of the submm galaxies are consistent with the maximal star formation rate densities of 20 Msun kpc^{-2} in local starburst galaxies (Lehnert & Heckman 1996). We derive quantitative morphological information for the optical galaxies hosting the submm emission; total and isophotal magnitudes, Petrosian radius, effective radius, concentration, aspect ratio, surface brightness, and asymmetry. We compare these morphological indices with those of other galaxies lying within the same STIS images. Most strikingly, we find ~70% of the submm galaxies to be extraordinarily large and elongated relative to the field population, regardless of optical magnitude. Comparison of the submm galaxy morphologies with those of optically selected galaxies at z~2-3 reveal the submm galaxies to be a morphologically distinct population, with generally larger sizes, higher concentrations and more prevalent major-merger configurations.Comment: 16 pages, 6 figures, scheduled for ApJ, v599, Dec10, 2003. Minor edits. For version with higher resolution figures, see http://www.submm.caltech.edu/~schapman/ms_v3.ps.g

Evidence for extended, obscured starbursts in submm galaxies

We compare high-resolution optical and radio imaging of 12 luminous submm galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable spatial resolution, 0.3" (2kpc). The radio emission traces the likely far-infrared morphology of these dusty, luminous galaxies. In ~30% of the sample the radio appears unresolved, suggesting that the emission is compact: either an obscured AGN or nuclear starburst. However, in the majority, ~70% (8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc). For these galaxies the radio morphologies are broadly similar to their restframe UV emission seen by HST. We discuss the probable mechanisms for the extended emission and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median SFRs are 1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of 45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance. We interpret this as resulting from highly structured internal obscuration, suggesting that the vigorous activity is producing wind-blown channels through the obscuration in these galaxies. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure

Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images

Medium-band imaging with HST/WFPC2 in the F410M filter has previously revealed a population of compact Lyman-alpha emission objects around the radio galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyman-alpha candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z~2.4 sub-galactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included). Contains 22 pages, including 5 figures and 2 tables. Accepted for the December issue of the Astronomical Journa

Further multiwavelength observations of the SSA22 Ly_alpha emitting `blob'

We present new follow-up observations of the sub-mm luminous Ly_alpha-emitting object in the SSA22 z=3.09 galaxy overdensity, referred to as `Blob 1' by Steidel et al.(2000). In particular we discuss high resolution Hubble Space Telescope optical imaging, Owens Valley Radio Observatory spectral imaging, Keck spectroscopy, VLA 20cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope sub-mm data. We detect several optical continuum components which may be associated with the core of the submillimeter emitting region. A radio source at the position of one of the HST components (22:17:25.94, +00:12:38.9) identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line, centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Ly_alpha cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot which may be an AGN or compact starburst. We obtain a Keck-LRIS spectrum of this object, despite its faintness (R=26.8). The spectrum reveals weak Ly_alpha emission, but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use non-detections in deep Chandra X-ray images to constrain the nature of the `Blob'. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Ly_alpha cloud.Comment: 8 pages, 9figs (low res), to appear in ApJ, for higher res figures, http://www.submm.caltech.edu/~schapman/sa22_sept4.ps.g

Free-form lens model and mass estimation of the high redshift galaxy cluster ACT-CL J0102-4915, "El Gordo"

We examine the massive colliding cluster El Gordo, one of the most massive clusters at high redshift. We use a free-form lensing reconstruction method that avoids making assumptions about the mass distribution. We use data from the RELICS program and identify new multiply lensed system candidates. The new set of constraints and free-form method provides a new independent mass estimate of this intriguing colliding cluster. Our results are found to be consistent with earlier parametric models, indirectly confirming the assumptions made in earlier work. By fitting a double gNFW profile to the lens model, and extrapolating to the virial radius, we infer a total mass for the cluster of $M_{200c}=(1.08^{+0.65}_{-0.12})\times10^{15}$M$_{\odot}$. We estimate the uncertainty in the mass due to errors in the photometric redshifts, and discuss the uncertainty in the inferred virial mass due to the extrapolation from the lens model. We also find in our lens map a mass overdensity corresponding to the large cometary tail of hot gas, reinforcing its interpretation as a large tidal feature predicted by hydrodynamical simulations that mimic El Gordo. Finally, we discuss the observed relation between the plasma and the mass map, finding that the peak in the projected mass map may be associated with a large concentration of colder gas, exhibiting possible star formation. El Gordo is one of the first clusters that will be observed with JWST, which is expected to unveil new high redshift lensed galaxies around this interesting cluster, and provide a more accurate estimation of its mass.Comment: 19 pages, 10 figures. Updated figure

The Axis Ratio Distribution of Local and Distant Galaxies

Surface photometry from 16 HST/WFPC2 fields in the I(F814W) filter is used to derive the distribution of apparent axis ratios for galaxies in progressively fainter magnitude intervals for I<25. We assess the systematic and accidental errors in ellipticity measurements as a function of image resolution and signal-to-noise ratio, and statistically correct for the effect of cosmological surface brightness dimming on our isophotal measurements. The axis ratio distribution for the local galaxy population was computed using logR measurements for 1569 RC3 galaxies with Bt<13 mag. Nonparametric tests are used to show that our distant samples, in the redshift range 0.1<z<1.5, are not statistically different from the local sample. We present image montages of galaxies selected randomly from different axis ratio and apparent magnitude ranges and discuss the evolutionary consequences of the lack of a strong difference between the ellipticity distributions in near and far data sets.Comment: LaTex, 35 pages, 8 figures, accepted for Dec97 A

Westphal-MMD11: An interacting, submillimeter luminous Lyman break galaxy

We present new Hubble Space Telescope, high-resolution optical imaging of the submm luminous Lyman-break galaxy, Westphal-MMD11, an interacting starburst at z=2.979. The new imaging data, in conjunction with re-analysis of Keck optical and near-IR spectra, demonstrate MMD11 to be an interacting system of at least three components: a luminous blue source, a fainter blue source, and an extremely red object (ERO) with R-K>6. The separations between components are \~8 kpc (Lambda=0.7, Omega_M=0.3, h=0.65), similar to some of the local ultra-luminous infrared galaxies (ULIGs). The lack of obvious AGN in MMD11, along with the fragmented, early stage merger morphology, suggest a young forming environment. While we cannot unambiguously identify the location of the far-IR emission within the system, analogy to similar ULIGs suggests the ERO as the likely far-IR source. The >10^{12} L_sun bolometric luminosity of MMD11 can be predicted reasonably from its rest frame UV properties once all components are taken into account, however this is not typically the case for local galaxies of similar luminosities. While LBGs as red in g-R and R-K as MMD11 are rare, they can only be found over the restricted 2.7 < z < 3.0 range. Therefore a substantial number of MMD11-like galaxies (~<0.62 arcmin^{-2}) may exist when integrated over the likely redshift range of SCUBA sources (z=1 -5), suggesting that SCUBA sources should not necessarily be seen as completely orthogonal to optically selected galaxies.Comment: 4 pages, 2 figures, updated to match proof