The ALPINE-ALMA [CII] Survey: Size of Individual Star-Forming Galaxies at z=4-6 and their Extended Halo Structure

Abstract

We present the physical extent of [C ii] 158µm line-emitting gas from 46 star-forming galaxies at z = 4−6 from the ALMA Large Program to INvestigate CII at Early Times (ALPINE). Using exponential profile fits, we measure the effective radius of the [C ii] line (r e, [CII] ) for individual galaxies and compare them with the rest-frame ultra-violet (UV) continuum (r e,UV ) from Hubble Space Telescope images. The effective radius r e, [CII] almost always exceeds r e,UV by factors of ∼ 2-3 and the ratio of r e,[CII] /r e,UV increases as a function of M star . We do not find strong evidence for a displacement over 1-kpc scale among the [C ii] line, the rest-frame UV, and FIR continuum. We identify 30% of isolated ALPINE sources as having an extended [C ii] component over 10-kpc scales detected at 4.1σ-10.9σ beyond the size of rest-frame UV and far-infrared (FIR) continuum. One object has tentative rotating features up to ∼ 10 kpc, where the 3D model fit shows the rotating [C ii]-gas disk spread over 4 times larger than the rest-frame UV-emitting region. Galaxies with the extended [C ii] line structure have high star-formation rate (SFR), stellar mass (M star ), low Lyα equivalent-width, and more blue-shifted (red-shifted) rest-frame UV metal absorption (Lyα line), as compared to galaxies without such extended [C ii] structures. Although we cannot rule out the possibility that a selection bias towards luminous objects may be responsible for such trends, the extended [C ii] line structure appears even around several objects whose SFR and M star are as small as those without the extended structure. Deeper observations are essential to test whether the extended [C ii] line structures are ubiquitous to high-z star-forming galaxies.

A kiloparsec view of a typical star-forming galaxy when the Universe was ~1 Gyr old. Part I. Outflow, halo, and interstellar medium properties

“…The ratio R eff,[CII] /R eff,UV we measure in HZ4 is lower than the median value found in the ALPINE sample of R eff,[CII] /R eff,UV = 2.3, but comparable to the ratio of ∼ 1.3 measured in the two ALPINE galaxies with stellar masses slightly lower than HZ4 (Fujimoto et al 2020). The good correspondence between the [C ii], dust, and rest-frame UV continuum emission in the central R ∼ 3 kpc disk suggests that the main source of heating is the photoelectric effect on dust grains.…”

“…The fact that ejected gas remains bound to the galaxy is key for the formation and maintenance of the circumgalactic medium (CGM). Evidence for this comes directly from the existence of extended [C ii] halos around z ∼ 5 galaxies (e.g., Fujimoto et al 2019Fujimoto et al , 2020, which we also detect in HZ4.…”

“…Spatial offsets of this magnitude between Hα or UV and dust emission have been observed in z ∼ 2−3 sub- millimeter galaxies (SMGs; e.g., Hodge et al 2016;Chen et al 2017), and main-sequence galaxies at z 4 (e.g., Maiolino et al 2015;Carniani et al 2017;Ginolfi et al 2020a). Fujimoto et al (2020), for example, find an average offset between the [C ii] and UV emission of 23 ALPINE galaxies of ∼ 0.15 ′′ . The physical origin of these offsets could be related to dust obscuration or the ionization state of the gas (e.g., Katz et al 2017), or stellar feedback destroying molecular clouds around star forming regions (Vallini et al 2015).…”

“…One of the most exciting new results of ALMA observations of main-sequence galaxies at z ∼ 4 − 5 is the detection of an extended [C ii] emission component or halo. These [C ii] halos extend significantly beyond the stellar disk and are potentially part of the circumgalactic medium (e.g., Fujimoto et al 2019Fujimoto et al , 2020Ginolfi et al 2020b).…”

“…traced by the rest-frame UV/optical emission, and can be considered part of the CGM (Fujimoto et al 2019(Fujimoto et al , 2020Ginolfi et al 2020b).…”