Kiloparsec-scale dust disks in high-redshift luminous submillimeter galaxies (1609.09649v1)

Abstract: We present high-resolution (0.16$"$) 870um Atacama Large Millimeter/submillimeter Array (ALMA) imaging of 16 luminous (L_IR ~ 4 x 10^12 L_sun) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. This dust imaging traces the dust-obscured star formation in these z~2.5 galaxies on ~1.3 kpc scales. The emission has a median effective radius of $R_e=0.24" \pm 0.02"$, corresponding to a typical physical size of $R_{e}=1.8\pm$0.2 kpc. We derive a median S\'ersic index of $n=0.9\pm0.2$, implying that the dust emission is remarkably disk-like at the current resolution and sensitivity. We use different weighting schemes with the visibilities to search for clumps on 0.12$"$ (~1.0 kpc) scales, but we find no significant evidence for clumping in the majority of cases. Indeed, we demonstrate using simulations that the observed morphologies are generally consistent with smooth exponential disks, suggesting that caution should be exercised when identifying candidate clumps in even moderate S/N interferometric data. We compare our maps to comparable-resolution HST H${160}$-band images, finding that the stellar morphologies appear significantly more extended and disturbed, and suggesting that major mergers may be responsible for driving the formation of the compact dust disks we observe. The stark contrast between the obscured and unobscured morphologies may also have implications for SED fitting routines that assume the dust is co-located with the optical/near-IR continuum emission. Finally, we discuss the potential of the current bursts of star formation to transform the observed galaxy sizes and light profiles, showing that the $z\sim0$ descendants of these SMGs are expected to have stellar masses, effective radii, and gas surface densities consistent with the most compact massive (M* ~ 1-2 x 10^11 M_sun) early-type galaxies observed locally.

Kiloparsec-Scale Dust Disks in High-Redshift Luminous Submillimeter Galaxies

This paper presents high-resolution (0.16") observations at 870 μm from the Atacama Large Millimeter/Submillimeter Array (ALMA) focused on 16 luminous submillimeter galaxies (SMGs) from the ALESS survey situated in the Extended Chandra Deep Field South. The study investigates dust-obscured star formation in galaxies at a redshift of approximately z = 2.5, attaining a spatial resolution on the scale of 1.3 kpc. The median effective radius for dust emission was found to be about R_e = 0.24", translating to a physical size of 1.8 kpc, with a median Sérsic index of n = 0.9, indicating a disk-like morphology.

A major finding of this study is the apparent absence of significant clumpiness that could challenge the notion of smooth star formation distributions. Although clumps on scales as small as 0.12" were sought using various visibility weighting schemes, the results aligned generally with smooth exponential disk models. Such findings suggest a lack of the kpc-scale star-forming clumps frequently posited in high-redshift galaxy formation scenarios. The study’s simulations caution against premature identification of clumps in moderate S/N interferometric data.

Comparison of these ALMA maps with analogous-resolution HST H_160-band images revealed a considerable discrepancy. The stellar structures in SMGs appear to be more extended and morphologically disturbed, hinting at a link to major mergers potentially responsible for compact dust disk formations observed. The contrast between optical and infrared morphologies underscores implications for SED fitting routines, which often presuppose co-location of dust with optical/near-IR emission.

Moreover, the study speculates about the evolutionary trajectory of such SMGs. It suggests that observed compact star formation could lead to z = 0 descendants exhibiting stellar masses and effective radii akin to the most compact, massive, early-type galaxies observable in the local universe.

The implications of these results are multifaceted. Practically, they emphasize caution in identifying structure within low S/N regions in galaxy studies. Theoretically, they question the prevalence of clumpy, unstable disks in high-redshift SMGs, advocating instead for smoother star formation processes potentially influenced by major mergers.

Looking forward, this research invites additional exploration on finer scales and with higher sensitivity to confirm these smooth disk observations and resolve finer structures. Such advancements could cement our understanding of high-redshift galaxy morphology and the processes guiding their evolution into massive elliptical galaxies observed today.