Cube Reprojection and Signal Masking Tutorial, version 4¶
Author: Alyssa Bulatek (https://github.com/abulatek)¶
Last updated: May 9, 2022¶
Adapted from a previous tutorial on the same subject by Adam Ginsburg, Eric Koch, Tom Robitaille, and Erik Rosolowsky. This tutorial shows how to take two spectral cubes observed toward the same part of the sky, but at different frequencies, and put them onto the same grid. Then, we use one spectral cube to mask the other, which is helpful for situations where the velocity structure of one cube is more complicated than the other.
Index¶
- Step 0: Some imports and version checks
- Step 1: Download
- Step 2: Open files, collect metadata
- Step 3: Convert to velocity
- Step 4: Spectral Interpolation
- Step 5: Spatial Smoothing and Reprojection
- Step 6: Signal Masking
- Step 7: Moment Maps
- Step 8: Masking Outliers
NB: In this example, we do spectral smoothing and interpolation (step 4) before spatial smoothing and interpolation (step 5), but if you have a varying-resolution cube (with a different beam size for each channel), you have to do spatial smoothing first. This is because smoothing two spectral channels together that have different beamsizes will blend unrelated objects on the sky.
Step 0: Some imports and version checks¶
%pip install -r https://github.com/radio-astro-tools/tutorials/raw/master/requirements.txt
%pip install --pre -U astroquery
Collecting spectral-cube (from -r https://github.com/radio-astro-tools/tutorials/raw/master/requirements.txt (line 14)) Cloning https://github.com/radio-astro-tools/spectral-cube/ to /tmp/pip-install-zqzxh2ui/spectral-cube_95b54c65e8744ff49c181dd3af4e42ec Running command git clone --filter=blob:none --quiet https://github.com/radio-astro-tools/spectral-cube/ /tmp/pip-install-zqzxh2ui/spectral-cube_95b54c65e8744ff49c181dd3af4e42ec
Resolved https://github.com/radio-astro-tools/spectral-cube/ to commit 8ae6705d7e64739cbb08904efc660dbed98874ab
Installing build dependencies ... -
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Getting requirements to build wheel ... -
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Preparing metadata (pyproject.toml) ... -
done Collecting radio-beam (from -r https://github.com/radio-astro-tools/tutorials/raw/master/requirements.txt (line 15)) Cloning https://github.com/radio-astro-tools/radio-beam/ to /tmp/pip-install-zqzxh2ui/radio-beam_c5ff71aefbcb4d409680e51381eb5fb4 Running command git clone --filter=blob:none --quiet https://github.com/radio-astro-tools/radio-beam/ /tmp/pip-install-zqzxh2ui/radio-beam_c5ff71aefbcb4d409680e51381eb5fb4
Resolved https://github.com/radio-astro-tools/radio-beam/ to commit 325cad35689eb71d9959a9e341281713c991fb1d
Installing build dependencies ... -
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Getting requirements to build wheel ... -
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Preparing metadata (pyproject.toml) ... -
done Collecting pvextractor (from -r https://github.com/radio-astro-tools/tutorials/raw/master/requirements.txt (line 16)) Cloning https://github.com/radio-astro-tools/pvextractor/ to /tmp/pip-install-zqzxh2ui/pvextractor_133e866d1092493999a7b5d8c1a9f2fe Running command git clone --filter=blob:none --quiet https://github.com/radio-astro-tools/pvextractor/ /tmp/pip-install-zqzxh2ui/pvextractor_133e866d1092493999a7b5d8c1a9f2fe
Resolved https://github.com/radio-astro-tools/pvextractor/ to commit d5803d85f262981c30c03282fa4b56e748a40331 Running command git submodule update --init --recursive -q
Installing build dependencies ... -
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Getting requirements to build wheel ... done
Preparing metadata (pyproject.toml) ... -
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Note: you may need to restart the kernel to use updated packages.
import matplotlib.pyplot as plt
# # Make plots have dark backgrounds; run this if your notebook is in dark mode
# plt.style.use('dark_background')
# Make sure spectral_cube is up-to-date
from spectral_cube import SpectralCube
import spectral_cube
print(f'spectral_cube version: {spectral_cube.__version__}')
print(f'path to spectral_cube installation: {spectral_cube.__path__}')
# Make sure astropy and reproject are somewhat up-to-date
# Used to require development version of astropy; 4.3.dev1788+ga3263b6 worked for me, but now so does 4.3.1
import astropy
import reproject
print(f'astropy version: {astropy.__version__}')
print(f'reproject version: {reproject.__version__}')
spectral_cube version: 0.6.6.dev10+g8ae6705 path to spectral_cube installation: ['/opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/site-packages/spectral_cube'] astropy version: 6.1.0 reproject version: 0.13.1
Step 1: Download the data¶
The first step is to pick out and download the cubes you will use for the reprojection. Here, we choose to download two cubes from the ALMA Archive, which is a publicly-available database where data taken by ALMA are stored after they exit their proprietary period. If you already have the cubes you will use for this tutorial locally, you can skip this step.
The two cubes we download here both cover the same field at different wavelengths: a water maser in the Galactic Center molecular cloud G0.253+0.016, also known as "The Brick." filename_1
points to a cube containing the H2CS 4(1,3)-3(1,2) line (139.483699 GHz), and filename_2
points to a cube containing the SiO v=5-4 line (217.104984 GHz).
import numpy as np
from astropy.utils.data import download_file
filename_1 = download_file("https://almascience.nrao.edu/dataPortal/member.uid___A001_X1465_X3a33.BrickMaser_sci.spw71.cube.I.manual.image.pbcor.fits",
cache=True)
filename_1
'/home/runner/.astropy/cache/download/url/33718a6f64d8a437fe89bff80278ca22/contents'
filename_2 = download_file("https://almascience.nrao.edu/dataPortal/member.uid___A001_X87d_X141.a_sma1_sci.spw27.cube.I.pbcor.fits",
cache=True)
filename_2
'/home/runner/.astropy/cache/download/url/391fb3a562ec7b94842f7b6e2a55c67e/contents'
Step 2: Load the cubes¶
Now, we will load the cubes into memory using the package spectral_cube
, which we imported in Step 0. As previously mentioned, the two cubes are at different frequencies (139 GHz and 217 GHz). Once we've loaded the cubes in, we'll use the method find_lines
to confirm the presence of the line we expect to find in each of the cubes (H2CS in cube1
, SiO in cube2
).
cube1 = SpectralCube.read(filename_1)
cube1.allow_huge_operations = True
cube1
WARNING: StokesWarning: Cube is a Stokes cube, returning spectral cube for I component [spectral_cube.io.core]
SpectralCube with shape=(75, 250, 250) and unit=Jy / beam: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: FREQ unit_s: Hz range: 139434992275.503 Hz:139503942362.300 Hz
cube2 = SpectralCube.read(filename_2)
cube2.allow_huge_operations = True
cube2
WARNING: StokesWarning: Cube is a Stokes cube, returning spectral cube for I component [spectral_cube.io.core]
SpectralCube with shape=(478, 420, 420) and unit=Jy / beam: n_x: 420 type_x: RA---SIN unit_x: deg range: 266.537002 deg: 266.551600 deg n_y: 420 type_y: DEC--SIN unit_y: deg range: -28.711371 deg: -28.698569 deg n_s: 478 type_s: FREQ unit_s: Hz range: 216957714464.027 Hz:217190639088.700 Hz
# DEBUG
import astroquery
print(astroquery.__version__)
0.4.8.dev9321
cube1.find_lines(chemical_name=' H2CS ')
WARNING: ExperimentalImplementationWarning: The line-finding routine is experimental. Please report bugs on the Issues page: https://github.com/radio-astro-tools/spectral-cube/issues [spectral_cube.spectral_cube]
species_id | name | chemical_name | resolved_QNs | linelist | LovasASTIntensity | lower_state_energy | upper_state_energy | sijmu2 | sij | aij | intintensity | Lovas_NRAO | orderedfreq | lower_state_energy_K | upper_state_energy_K | orderedFreq | measFreq | upperStateDegen | moleculeTag | qnCode | labref_Lovas_NIST | rel_int_HFS_Lovas | unres_quantum_numbers | lineid | transition_in_space | transition_in_G358 | obsref_Lovas_NIST | source_Lovas_NIST | telescope_Lovas_NIST | transitionBandColor | searchErrorMessage | sqlquery | requestnumber |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
int64 | str15 | str16 | str28 | str5 | str5 | float64 | float64 | float64 | float64 | float64 | str7 | int64 | float64 | float64 | float64 | str68 | str69 | str3 | int64 | int64 | str8 | str1 | str28 | int64 | int64 | int64 | str6 | str11 | str13 | str14 | str1 | object | int64 |
210 | H<sub>2</sub>CS | Thioformaldehyde | 4( 1, 3)- 3( 1, 2) | JPL | 0.17 | 16.1465 | 20.79917 | 30.51674 | 11.223 | -4.44732 | -3.2185 | 1 | 139483.41 | 23.23108 | 29.92519 | <span style = 'color: #DC143C'></span> | 139.48341 (0.28), <span style = 'color: #DC143C'>139.48341</span> | 27 | -46003 | 303 | 4 1 3 3 1 2 | 4065687 | 0 | 0 | Lor84a | rho Oph B1 | MMWO 4.9m | datatablegreen | None | 0 | |||
210 | H<sub>2</sub>CS | Thioformaldehyde | 4( 1, 3)- 3( 1, 2) | CDMS | 16.1329 | 20.78557 | 30.59661 | 11.251 | -4.44619 | -3.2182 | 0 | 139483.6816 | 23.21151 | 29.90563 | <span style = 'color: #DC143C'></span> | 139.4836816 (0.05), <span style = 'color: #DC143C'>139.4836816</span> | 27 | -46509 | 303 | 4 1 3 3 1 2 | 4066266 | 0 | 0 | datatablegreen | None | 0 | |||||||
210 | H<sub>2</sub>CS | Thioformaldehyde | 4( 1, 3)- 3( 1, 2) | SLAIM | 0.17 | 16.133 | 20.78568 | 30.59472 | 3.75 | -3.94426 | 0 | 139483.699 | 23.21165 | 29.90578 | 139.483699 (0.017), <span style = 'color: #DC143C'>139.483699</span> | 139.483741 (0.024), <span style = 'color: #DC143C'>139.483741</span> | 8.5 | 0 | 0 | Mar05 | 4( 1, 3)- 3( 1, 2) | 3744487 | 1 | 0 | datatablegreen | None | 0 | ||||||
210 | H<sub>2</sub>CS | Thioformaldehyde | 4(1,3)-3(1,2) | Lovas | 0.17 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0 | 139483.699 | 0.0 | 0.0 | 139.483699 (0.017), <span style = 'color: #DC143C'>139.483699</span> | <span style = 'color: #DC143C'></span> | 0 | 0 | 4(1,3)-3(1,2) | 3734199 | 0 | 0 | Lor84a | rho Oph B1 | MMWO 4.9m | datatablegreen | None | 0 |
cube2.find_lines(chemical_name='SiO')
WARNING: ExperimentalImplementationWarning: The line-finding routine is experimental. Please report bugs on the Issues page: https://github.com/radio-astro-tools/spectral-cube/issues [spectral_cube.spectral_cube]
species_id | name | chemical_name | resolved_QNs | linelist | LovasASTIntensity | lower_state_energy | upper_state_energy | sijmu2 | sij | aij | intintensity | Lovas_NRAO | orderedfreq | lower_state_energy_K | upper_state_energy_K | orderedFreq | measFreq | upperStateDegen | moleculeTag | qnCode | labref_Lovas_NIST | rel_int_HFS_Lovas | unres_quantum_numbers | lineid | transition_in_space | transition_in_G358 | obsref_Lovas_NIST | source_Lovas_NIST | telescope_Lovas_NIST | transitionBandColor | searchErrorMessage | sqlquery | requestnumber |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
int64 | str35 | str29 | str20 | str5 | str4 | float64 | float64 | float64 | float64 | float64 | str7 | int64 | float64 | float64 | float64 | str68 | str68 | str2 | int64 | int64 | str9 | str1 | str24 | int64 | int64 | int64 | str13 | str9 | str13 | str20 | str1 | object | int64 |
21081 | SiO, v = 0-10 | Silicon monoxide (global fit) | J = 5 - 4, v = 0 - 0 | CDMS | 14.4843 | 21.72614 | 47.9909 | 0.0 | -3.28429 | -1.3211 | 1 | 217104.919 | 20.83981 | 31.25927 | <span style = 'color: #DC143C'></span> | 217.104919 (0.002), <span style = 'color: #DC143C'>217.104919</span> | 11 | -44505 | 1202 | 5 0 4 0 | 7980087 | 1 | 0 | Belloche 2013 | Sgr B2(N) | IRAM 30m | datatablelightpurple | None | 0 | ||||
20 | SiO <font color="red">v = 0 </font> | Silicon Monoxide | 5- 4 | CDMS | 1.6 | 14.4843 | 21.72614 | 47.99147 | 5.0 | -3.28429 | -1.3211 | 0 | 217104.98 | 20.83956 | 31.25889 | <span style = 'color: #DC143C'></span> | 217.10498 (0.08), <span style = 'color: #DC143C'>217.10498</span> | 11 | -44505 | 1202 | 5 0 4 0 | 6962437 | 0 | 0 | datatablelightpurple | None | 0 | ||||||
20 | SiO <font color="red">v = 0 </font> | Silicon Monoxide | 5- 4 | CDMS | 1.6 | 14.4843 | 21.72614 | 47.9911 | 5.0 | -3.28429 | -1.3199 | 0 | 217104.98 | 20.83956 | 31.25889 | <span style = 'color: #DC143C'></span> | 217.10498 (0.05), <span style = 'color: #DC143C'>217.10498</span> | 11 | -44505 | 1202 | J=5-4 v=0 | 13358 | 1 | 0 | datatablelightpurple | None | 0 | ||||||
20 | SiO <font color="red">v = 0 </font> | Silicon Monoxide | 5-4 | JPL | 1.6 | 14.4843 | 21.72614 | 48.14651 | 5.0 | -3.28288 | -1.3166 | 1 | 217104.98 | 20.83956 | 31.25889 | <span style = 'color: #DC143C'></span> | 217.10498 (0.08), <span style = 'color: #DC143C'>217.10498</span> | 11 | -44002 | 101 | J=5-4 v=0 | 1167368 | 0 | 0 | Lor84a | OriMC-1 | MMWO 4.9m | datatablelightpurple | None | 0 | |||
20 | SiO <font color="red">v = 0 </font> | Silicon Monoxide | 5-4 | Lovas | 1.6 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0 | 217104.984 | 0.0 | 0.0 | 217.104984 (0.014), <span style = 'color: #DC143C'>217.104984</span> | <span style = 'color: #DC143C'></span> | 0 | 0 | 5-4 v=0 | 3724638 | 0 | 0 | Lor84a | OriMC-1 | MMWO 4.9m | datatablelightpurple | None | 0 | |||||
20 | SiO <font color="red">v = 0 </font> | Silicon Monoxide | 5 - 4 | SLAIM | 14.484 | 21.72584 | 47.6849 | 5.0 | -3.28706 | 0 | 217104.984 | 20.83913 | 31.25846 | 217.104984 (0.014), <span style = 'color: #DC143C'>217.104984</span> | 217.10498 (0.1), <span style = 'color: #DC143C'>217.10498</span> | 11 | 0 | 0 | Man77 | 5 - 4 v=0 | 3932866 | 0 | 0 | datatablelightpurple | None | 0 |
We can see four listings for the H2CS 4(1,3)-3(1,2) line (139.483699 GHz) in cube1
, each from different linelists. We can also see a few different SiO v=5-4 lines (217.104984 GHz), some from the same linelist (CDMS) and some from the other linelists. These printouts are just for verification purposes; we'll use the center frequencies we have written down in Step 1 for each of these transitions instead of the frequencies given in the find_lines
function.
Convert from Jy/beam to kelvin¶
At this point, we'll convert the cubes to be in units of kelvin instead of Jy/beam. This is because we'll be smoothing the cubes spatially in a future step, and the data values will only be preserved if the cubes are in units of kelvin instead of Jy/beam (since the latter units are dependent on the beamsize).
from astropy import units as u
cube1 = cube1.to(u.K)
cube2 = cube2.to(u.K)
WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.to at 0x7ff71b9af370>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils] WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.to at 0x7ff71b9af370>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils]
Step 3: Convert cubes from frequency to velocity¶
Now, we will convert these cubes from frequency units to velocity units using the rest frequencies of the transitions of interest. This is the first step to getting the cubes on a commmon velocity grid: we match up their rest frequencies at 0 km/s. We convert these cubes from frequency to velocity using the method with_spectral_unit
, the documentation for which is available here in the spectral_cube
API.
cube1vel = cube1.with_spectral_unit(u.km/u.s, velocity_convention='radio', rest_value=139.483699*u.GHz)
cube1vel
SpectralCube with shape=(75, 250, 250) and unit=K: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s
cube2vel = cube2.with_spectral_unit(u.km/u.s, velocity_convention='radio', rest_value=217.104984*u.GHz)
cube2vel
SpectralCube with shape=(478, 420, 420) and unit=K: n_x: 420 type_x: RA---SIN unit_x: deg range: 266.537002 deg: 266.551600 deg n_y: 420 type_y: DEC--SIN unit_y: deg range: -28.711371 deg: -28.698569 deg n_s: 478 type_s: VRAD unit_s: km / s range: -118.278 km / s: 203.359 km / s
From the shape of the cube, we can see the H2CS cube is narrower in velocity, so we'll use that as the target spectral reprojection (otherwise, we would have empty velocity channels on either side of the smaller cube that would be getting reprojected). However, the SiO cube is the smaller footprint on the sky.
Do some quicklooks of the peak intensity to see what we expect to see¶
The cubes we are working with are three-dimensional: they have two spatial axes (on the plane of the sky) and one spectral axis. It is hard to visualize them in 3D easily, so we can collapse them into two dimensions in a variety of different ways. Here, we will collapse them along their spectral axis (axis = 0
; spectral_cube
usually makes the spectral axis the first axis as a default). In particular, for any given spatial coordinate in our cube, we will take the value of the brightest pixel along the spectral axis (the peak flux density). This summary image shows us where the bright emission is (spatially) in the cube, and how bright the emission in a certain region is in relation to the brightness of the emission in another region.
peak_fluxdensity1 = cube1.max(axis = 0)
peak_fluxdensity1.quicklook()
WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.max at 0x7ff71b9ac4c0>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils] /opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/site-packages/spectral_cube/spectral_cube.py:439: RuntimeWarning: All-NaN slice encountered out = function(self._get_filled_data(fill=fill,
peak_fluxdensity2 = cube2.max(axis = 0)
peak_fluxdensity2.quicklook()
WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.max at 0x7ff71b9ac4c0>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils]
/opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/site-packages/spectral_cube/spectral_cube.py:439: RuntimeWarning: All-NaN slice encountered out = function(self._get_filled_data(fill=fill,
Step 4: Spectral Interpolation¶
We can safely assume that any two cubes from ALMA are not perfectly matched on a spatial and spectral grid. One cube will have a higher spatial resolution than the other, and one cube will have a finer spectral resolution than the other. We must match these two cubes in two different steps: matching them on a spatial grid (2D), and matching them along their spectral axes (1D). We can treat these as roughly independent steps, and could do either first. However, if your cubes have a varying spatial resolution (different beam size at each spectral channel), you will need to perform spatial smoothing first—you don't want to blend unrelated components of your source by smoothing spectrally before all channels are smoothed to the same spatial resolution. Because each of our cubes have a constant spatial resolution, we will perform spectral smoothing first.
We need to match the spectral resolution of both cubes to each other. We'll start by printing out the spectral resolutions of both cubes.
velocity_res_1 = np.diff(cube1vel.spectral_axis)[0]
velocity_res_2 = np.diff(cube2vel.spectral_axis)[0]
velocity_res_1, velocity_res_2
(<Quantity 2.00262828 km / s>, <Quantity 0.67429189 km / s>)
cube1
, our H2CS cube, has broader channels. We want to smooth cube2
to that channel width before doing the spatial reprojection. To do this, we'll smooth the spectral axis of cube2
using a Gaussian with an appropriate FWHM:
fwhm_gaussian = (velocity_res_1**2 - velocity_res_2**2)**0.5
fwhm_gaussian
Here is the page on smoothing in the spectral_cube
documentation. Importantly, here, we're using a Gaussian to smooth the spectral channels, and then we say that the spectral resolution of the resulting cubes is equal to the FWHM of that Gaussian. However, spectral resolution is defined differently by the ALMA correlator: Adam thinks it's half of the channel width smoothed by a Hanning function and then sampled by the channel width, but the answer lies somewhere in the ALMA documentation. The takeaway: we are not necessarily using the "correct" definition of resolution, but we could figure out what that is, if we wanted to.
from astropy.convolution import Gaussian1DKernel
fwhm_to_sigma = np.sqrt(8*np.log(2))
# We want the kernel in pixel units, so we force to km/s and take the value
spectral_smoothing_kernel = Gaussian1DKernel(stddev = fwhm_gaussian.to(u.km/u.s).value / fwhm_to_sigma)
Then, let's cut down to the same spectral coverage. This is not technically necessary to do, but it is more computationally efficient to smooth over a smaller spectral range. We will do this in a general way, determining the smallest coverage and cutting both cubes based on those values, so no matter which cube has a larger spectral coverage, this code should work.
print(cube1vel.spectral_extrema)
print(cube2vel.spectral_extrema)
lower_limit = np.max([cube1vel.spectral_extrema[0].value, cube2vel.spectral_extrema[0].value])*(cube1vel.spectral_axis.unit)
upper_limit = np.min([cube1vel.spectral_extrema[1].value, cube2vel.spectral_extrema[1].value])*(cube1vel.spectral_axis.unit)
print(lower_limit)
print(upper_limit)
cube1vel_cutout = cube1vel.spectral_slab(lower_limit, upper_limit)
cube2vel_cutout = cube2vel.spectral_slab(lower_limit, upper_limit)
cube1vel_cutout, cube2vel_cutout
[-43.50907945 104.68541315] km / s [-118.27802894 203.35920145] km / s -43.509079452344274 km / s 104.68541315391005 km / s
(SpectralCube with shape=(75, 250, 250) and unit=K: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s, SpectralCube with shape=(221, 420, 420) and unit=K: n_x: 420 type_x: RA---SIN unit_x: deg range: 266.537002 deg: 266.551600 deg n_y: 420 type_y: DEC--SIN unit_y: deg range: -28.711371 deg: -28.698569 deg n_s: 221 type_s: VRAD unit_s: km / s range: -43.432 km / s: 104.913 km / s)
We then smooth with the kernel. Note that this is doing 420x420 = 176400 smoothing operations on a length-221 spectrum: it will take several minutes to run.
cube2vel_smooth = cube2vel_cutout.spectral_smooth(spectral_smoothing_kernel)
WARNING: nan_treatment='interpolate', however, NaN values detected post convolution. A contiguous region of NaN values, larger than the kernel size, are present in the input array. Increase the kernel size to avoid this. [astropy.convolution.convolve]
Now that we've done spectral smoothing, we can resample the spectrum. We include the parameter suppress_smooth_warning = True
in this step, because otherwise we'll see a warning that "the data should be smoothed prior to resampling." Since we have already smoothed the data, this warning can be safely ignored.
cube2vel_spectralresample = cube2vel_smooth.spectral_interpolate(cube1vel_cutout.spectral_axis, suppress_smooth_warning = True)
cube2vel_spectralresample
SpectralCube with shape=(75, 420, 420) and unit=K: n_x: 420 type_x: RA---SIN unit_x: deg range: 266.537002 deg: 266.551600 deg n_y: 420 type_y: DEC--SIN unit_y: deg range: -28.711371 deg: -28.698569 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s
Step 5: Spatial Smoothing and Reprojection¶
Recall that our two cubes started with different beam sizes. Our goal is to match the beam sizes of the two cubes, and then to reproject the cubes onto the same spatial pixel grid. We can start to do this by printing out the beam sizes of the two cubes as they currently stand.
cube1vel_cutout.beam, cube2vel_spectralresample.beam
(Beam: BMAJ=1.29719604986604 arcsec BMIN=1.04247149438736 arcsec BPA=82.95313553702 deg, Beam: BMAJ=0.8935712308515601 arcsec BMIN=0.6649610689789199 arcsec BPA=85.81119797802 deg)
cube1
again has the larger beam, so we'll smooth cube2
to its resolution.
Aside: mixed beams¶
If cube1 and cube2 had different sized beams, but neither was clearly larger, we would have to convolve both to a common beam. In this case, it's redundant and we could have just used cube1's beam, but this is the more general approach:
import radio_beam
common_beam = cube1vel_cutout.beam.commonbeam_with(cube2vel_spectralresample.beam)
common_beam
# # This commented-out approach works in older versions of radio-beam:
# import radio_beam
# common_beam = radio_beam.commonbeam.common_2beams(radio_beam.Beams(beams=[cube1vel_cutout.beam, cube2vel_spectralresample.beam]))
# common_beam
cube2vel_spatialspectralsmooth = cube2vel_spectralresample.convolve_to(common_beam)
cube2vel_spatialspectralsmooth
WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.convolve_to at 0x7ff71b9b40d0>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils]
SpectralCube with shape=(75, 420, 420) and unit=K: n_x: 420 type_x: RA---SIN unit_x: deg range: 266.537002 deg: 266.551600 deg n_y: 420 type_y: DEC--SIN unit_y: deg range: -28.711371 deg: -28.698569 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s
Now, we can reproject cube2
onto the same spatial grid as cube1
.
# these next two lines are a hack to prevent the WCS from trying to convert to frequency
tgt_header = cube1vel_cutout.header
tgt_header['RESTFRQ'] = cube2vel_spatialspectralsmooth.header['RESTFRQ']
cube2vel_reproj = cube2vel_spatialspectralsmooth.reproject(tgt_header)
cube2vel_reproj
WARNING: PossiblySlowWarning: This function (<function BaseSpectralCube.reproject at 0x7ff71b9af520>) requires loading the entire cube into memory and may therefore be slow. [spectral_cube.utils]
SpectralCube with shape=(75, 250, 250) and unit=K: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s
These two cubes are now on an identical grid, and can be directly compared:
cube2vel_reproj, cube1vel_cutout
(SpectralCube with shape=(75, 250, 250) and unit=K: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s, SpectralCube with shape=(75, 250, 250) and unit=K: n_x: 250 type_x: RA---SIN unit_x: deg range: 266.534072 deg: 266.554577 deg n_y: 250 type_y: DEC--SIN unit_y: deg range: -28.713958 deg: -28.695975 deg n_s: 75 type_s: VRAD unit_s: km / s range: -43.509 km / s: 104.685 km / s)
These spectra can now be overplotted as they are in the same unit with the same beam. Using the quicklook maps of the peak intensity from step 2, we know that there is some signal in the central region of our pointing, so we can plot the spectrum for the pixel there. We can approximate the pixel coordinate using the peak intensity map with fewer pixels, since we know both cubes got smoothed to that (coarser) spatial resolution. The center is at about (125, 125).
plt.plot(cube1vel_cutout[:,125,125].spectral_axis, cube1vel_cutout[:,125,125], linestyle = '-', drawstyle = 'steps-mid', label = 'Cube 1')
plt.plot(cube2vel_reproj[:,125,125].spectral_axis, cube2vel_reproj[:,125,125], linestyle = '-', drawstyle = 'steps-mid', label = 'Cube 2')
plt.xlabel(cube1vel_cutout.spectral_axis.unit)
plt.ylabel(cube1vel_cutout.unit)
plt.legend()
plt.show()
WARNING: SliceWarning: Slice (slice(None, None, None), None) cannot be used on this 1-dimensional array's WCS. If this is intentional, you should use this <class 'spectral_cube.lower_dimensional_structures.OneDSpectrum'>'s ``array`` or ``quantity`` attribute. [spectral_cube.lower_dimensional_structures] WARNING: SliceWarning: Slice (slice(None, None, None), None) cannot be used on this 1-dimensional array's WCS. If this is intentional, you should use this <class 'spectral_cube.lower_dimensional_structures.OneDSpectrum'>'s ``array`` or ``quantity`` attribute. [spectral_cube.lower_dimensional_structures]
Do a quick continuum subtraction¶
If we looked at the spectra we calculated in the previous step and saw that the baseline level of the spectra were not at zero, we would want to perform continuum subtraction to subtract off that non-zero baseline (since we're only working with the spectral lines for our purposes). Though our above spectra look like they are likely already continuum-subtracted, we can perform the following steps anyway. First, we'll calculate the median values of each of the cubes along the spectral axes. This gives us the approximate continuum value in each pixel. The median is more resistant to the presence of the spectral lines than the mean would be.
# Calculate the median value along the spectral axis
med1 = cube1vel_cutout.median(axis=0)
med1.quicklook()
/opt/hostedtoolcache/Python/3.10.14/x64/lib/python3.10/site-packages/spectral_cube/spectral_cube.py:439: RuntimeWarning: All-NaN slice encountered out = function(self._get_filled_data(fill=fill,