Next Generation Very Large Array Memo No. 103 Spectroscopy of High Redshift Galaxies with the ngVLA C.L. Carilli

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Next Generation Very Large Array Memo No. 103

Spectroscopy of High Redshift Galaxies with the ngVLA

C.L. Carilli

National Radio Astronomy Observatory, Socorro, NM, USA

Marcel Neeleman

National Radio Astronomy Observatory, Charlottesville, VA, USA

Abstract

We present simulations of the capabilities of the ngVLA to image

at ∼0.75 kpc resolution (0.085”), molecular line emission from star

forming disk galaxies at high redshift. The results are compared to the

current capabilities of ALMA. ALMA can detect the integrated emis-

sion, and determine the velocity gradient and size across the brighter

emission regions of the galaxy. The ngVLA is a factor ∼6 more sen-

sitive at the adopted spatial and velocity resolution. This sensitivity

is needed to recover the detailed column density distribution, velocity

ﬁeld, and velocity dispersion at full resolution. The ngVLA will enable

detailed analysis of spectral line proﬁles at 0.75 kpc resolution, even

in relatively faint regions. The ngVLA will trace the rotation curves

to large radii, and recover sub-structure in the disks, such as clumps,

spiral arms, bars, and rings. Detection of these features is crucial in

order to assess how cold gas precipitates the formation of stars at high

redshift.

1 Introduction

A number of memos have investigated the capabilities of the ngVLA to

image the molecular and other redshifted millimeter line emission from high

redshift galaxies (ngVLA memos 8, 13, 15, 501). In this work, we explore

1https://ngvla.nrao.edu/page/memos

arXiv:2210.13584v1 [astro-ph.GA] 24 Oct 2022

this important science case (KSG 32) in more detail, incorporating a realistic

model based on the spatial and dynamical properties of a nearby galaxy, and

employing the latest ngVLA conﬁguration. We compare the results to the

capabilities of the current ALMA.

The principle application will be in the imaging of CO line emission

from intermediate to high redshift galaxies at 0.085” ∼0.75kpc (Carilli

and Walter 2013). However, the results are generally applicable to any line

emission (or absorption), from high redshift galaxies of similar line strength

and spatial distribution. Possibilities include: (i) [CI] 492 GHz at z∼4 to

5 (Valentino et al. 2018), (ii) H2O absorption against the CMB at z∼4 to

7 (Riechers et al. 2022), and (iii) extreme redshift (z∼20) ﬁne structure

line emission (Carilli et al. 2017).

2 Model

As an input model, we adopt the HI 21cm emission cube from the THINGS

survey of the galaxy NGC 2366 (Walter et al. 2008). The galaxy is classiﬁed

as a barred irregular dwarf galaxy. This galaxy shows a rotating HI disk that

is modestly disturbed, with a possible outer warp (van Eymeren et al. 2009).

The integrated line proﬁle is ﬂat-top, not a clear double horn. Such a proﬁle

is more characteristic of those seen for CO emission from high redshift disk

galaxies, for which classic double horn line proﬁles are uncommon (Gonzales-

Lopez et al. 2019). The THINGS survey has a resolution of a few hundred

parsecs, and provides reasonably high HI 21cm signal-to-noise spectra across

the disk, on scales out to twice the radius of the main optical galaxy in the

case of NGC 2366.

The importance of the input model is simply to provide a dynamical

template that could be representative of a real galaxy at high redshift. The

strength, size, and full velocity width then have to be adjusted to match

those expected for a disk galaxies at high redshift. We adopt parameters

that are representative of the massive main sequence star forming disk galax-

ies seen in ASPECS – the deepest blind, large cosmic volume search for

molecular gas to date (Walter et al. 2016):

•Size: full extent = 0.8” (∼7 kpc)

•Line FWHM = 400 km/s

•Line peak = 1.2 mJy

2https://ngvla.nrao.edu/page/scibook

•Integrated ﬂux = 414 mJy km/s (derived from Gaussian ﬁt)

•Central frequency = 86 GHz (eg. CO 2-1 at z= 1.7, or 3-2 at z= 3.0)

As an example, if this were CO 2-1 emission at z= 1.7, the molecular

gas mass would be 1.6×1011 Mfor Milky Way excitation and gas mass-to-

line luminosity conversion, or 1.9×1010 Mfor starburst galaxy conditions

(Carilli & Walter 2013).

The line strength, source size, and line width are also comparable to

what was seen in the recent discovery of H2O absorption against the CMB

at z= 6.3 by Riechers et al. (2022). Hence, the results of these simulated

observations can be used as a guide to the capabilities of the ngVLA to

provide spatially resolved observations of this new cosmic phenomenon – a

phenomenon that can be used to determine the evolution of the temperature

of the CMB to very high redshift. Multiple lines of sight through a given

source may prove crucial in mitigating systematic uncertainties due to source

line and continuum spatial structure.

On a more speculative note, the 86 GHz model corresponds to [CII] 158

µm line at z= 21, with a line luminosity of 2.6×109L. Such a [CII]

luminosity is at the high end of the distribution of [CII] luminosities for

galaxies discovered in the ALPINE and REBELS surveys at z∼4 to 7 (Yan

et al. 2020; Bouwens et al. 2022). Whether such galaxies exist at z= 21

remains unknown.

The input image cube is blanked at 3σin each channel to remove noise.

The frequency structure of the cube is then set to a channel resolution of

2.5 MHz = 8.7 km s−1, with a total of 66 channels.

3 Simulations and Imaging

SIMOBSERVE in CASA was used for the simulations. We simulate a four

hour synthesis with 20 second records, and scale the noise for a total inte-

gration time of 20 hours (ie. 5 observations).

We employ the ngVLA Rev D conﬁguration, including the Core and Spi-

ral components (ngVLA memo 92). For ALMA, we use the c9 conﬁguration

(longest baseline of 13.6 km). This ALMA conﬁguration results in a natural

weighted synthesized beam FWHM of 0.091” ×0.077”.

TCLEAN was used to image and deconvolve the spectral cubes. A chan-

nel width of 12.5 MHz (44 km s−1) was used for the image cubes (width =

5 relative to the input measurement set). The cubes were cleaned to a

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NextGenerationVeryLargeArrayMemoNo.103SpectroscopyofHighRedshiftGalaxieswiththengVLAC.L.CarilliNationalRadioAstronomyObservatory,Socorro,NM,USAMarcelNeelemanNationalRadioAstronomyObservatory,Charlottesville,VA,USAAbstractWepresentsimulationsofthecapabilitiesofthengVLAtoimageat0:75kpcresolution(0:08...

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Next Generation Very Large Array Memo No. 103 Spectroscopy of High Redshift Galaxies with the ngVLA C.L. Carilli

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