astro-legacy-archive/wmap-ilc-9yr

WMAP Nine-Year ILC Map

This dataset contains the WMAP nine-year Internal Linear Combination (ILC) map, a full-sky estimate of cosmic microwave background (CMB) temperature anisotropy with one row for each HEALPix pixel. The WMAP Science Team formed the map from a weighted linear combination of the mission's five smoothed frequency-band maps. The weights preserve the CMB signal while minimising Galactic foreground emission.

The combination was computed separately in 12 sky regions: 11 regions in the Galactic plane and region 0 covering the outer Galactic plane and high Galactic latitudes. The region boundaries were smoothed with a 1.5-degree kernel, and the released map includes a Monte Carlo bias correction.

Intended uses

The ILC map provides a compact full-sky view of the CMB anisotropy for visualisation, large-angular-scale studies, and machine-learning work with spherical data. The WMAP team describes it as a reliable estimate of the CMB signal on angular scales greater than ~10 degrees, where instrument noise is negligible. Quantitative analysis on smaller scales should instead use the WMAP frequency-band maps and likelihoods because the small-scale structure introduced by the bias correction remains uncertain.

Mission	WMAP
Product	Nine-year Internal Linear Combination CMB anisotropy map
Release	DR5 (map version PASS 5)
Observation period	2001-2010 (nine years)
ILC map file creation date	2012-05-25
LAMBDA product delivery date (both files)	2012-11-11
Pixelisation	HEALPix, NSIDE 512 (3,145,728 pixels), NESTED ordering
Coordinate frame	Galactic
Effective angular resolution	1 degree
Units	mK thermodynamic

How to use

python -m venv .venv && .venv/bin/pip install datasets huggingface_hub pyarrow

from datasets import load_dataset
dataset = load_dataset("astro-legacy-archive/wmap-ilc-9yr", "wmap_ilc_9yr_v5", split="train")
row = dataset.select([0]).select_columns(["TEMPERATURE"])[0]
print(row["TEMPERATURE"])

0.05137988179922104

from huggingface_hub import hf_hub_download
import json
import pyarrow.parquet as pq
path = hf_hub_download(repo_id="astro-legacy-archive/wmap-ilc-9yr", repo_type="dataset", filename="wmap-ilc-9yr.parquet")
print(json.loads(pq.ParquetFile(path).schema_arrow.metadata[b"healpix_map"])["healpix"]["ordering"])

NESTED

The data example selects one column from one row of the main-map config. Use the wmap_ilc_rgn_defn_9yr_v5 config for the companion region definition. The healpix_map document embedded in each Parquet schema self-describes its geometry, field units, coverage, provenance, and source headers; consumers can resolve coordinates and interpret coverage from those fields without materialising either complete config.

Dataset structure

The table contains 3,145,728 rows at NSIDE 512, one for each pixel in the full-sky map. The wmap_ilc_9yr_v5 config is about 13.3 MB.

column	type	unit	description
TEMPERATURE	float32	mK, thermodynamic	CMB temperature anisotropy
N_OBS	float32	counts	Uniformly 1.0; placeholder inherited from the source format

Row i is HEALPix pixel i in NESTED ordering. For example, row 0 is {"TEMPERATURE": 0.0514, "N_OBS": 1.0}.

This dataset contains no nulls. It covers the full sky. The measured temperature ranges from -0.452 mK to 0.442 mK, with a median of 0.0032 mK. The N_OBS column is uniformly 1.0 in this product and carries no information; it is present in the source and uniformly 1.0.

The source TUNIT mK, thermodynamic denotes thermodynamic millikelvin.

The Parquet file has three row groups. Pixel number is the source row position.

Pixel coordinates are Galactic, as stated by SKYCOORD = 'Galactic' in the preserved primary header.

Region definition

The wmap_ilc_rgn_defn_9yr_v5 config contains the companion map that defines the 12 ILC construction regions and is about 0.14 MB. N_OBS is the per-pixel region assignment and takes exactly the integer values 0-11; region 0 covers the outer Galactic plane and high latitudes. TEMPERATURE is a bitmask of the regions whose weight computation used that pixel; 0, which covers most of the sky, means none. Measured over the file, 85.1% of pixels (2,677,571 of 3,145,728) have value 0, while 468,157 pixels have any bit set. Nonzero values are powers of two and their sums. The source FITS header states: "The TEMPERATURE field is a bitmask defining the set of pixels used to compute the weights for each region." It also states: "N_obs defines the ILC regions 0-11 to which the weights are applied."

Region 0, the outer Galactic plane and high latitudes, holds 2,798,684 pixels. Regions 1 through 11 respectively hold 128,768, 10,712, 16,525, 51,270, 24,522, 48,831, 16,927, 8,598, 9,357, 11,259, and 20,275 pixels.

Both TEMPERATURE and N_OBS are stored as float32 despite holding integer values. Both inherit TUNIT counts from the sky-map table format, but that unit is not meaningful for a bitmask or region index. The file's own header contains the per-region band-weight table. Its primary-header HISTORY states: "This is actually the DR2 file with header updates; the same algorithm was used for the DR5 version of the ILC."

• Source file: wmap_ilc_rgn_defn_9yr_v5.fits
• Source SHA-256: 620db1c4e5b1972b3174a18cdfd49882bc6134e68a3635370ef01d813db089ff (25,176,960 bytes).
• File creation date: 2012-11-07. The LAMBDA product delivery date is 2012-11-11 for both this file and the ILC map.

Provenance

• Product page: WMAP nine-year ILC products
• Source file: wmap_ilc_9yr_v5.fits
• Source SHA-256: 4501090f824f9140e916339f9e8e490333f6ffe4257a6ee6a02526c8640099a9 (25,174,080 bytes; delivery 2012-11-11).
• Projected image versions are available from the product page but are not included here.

The WMAP release at LAMBDA remains the authoritative source. The file carries a versioned metadata document (schema astro-legacy-archive/healpix-map 2.0.0, Parquet key healpix_map) holding the source HEALPix and coordinate fields, the source pixel rule, per-column source TTYPE/TFORM/TUNIT, table-level HDU/EXTNAME provenance, the verbatim primary and extension headers, source URL + SHA-256 + retrieval date, and the regeneration command. The Parquet field metadata carries per-column descriptions and units, visible via PyArrow schema inspection; datasets.load_dataset strips field metadata, so Dataset.features shows types only.

Each selected bintable reconstructs in source column and row order with identical TFORM, shape, dtype, and value bits. FITS container serialisation is outside that promise; the original headers are preserved verbatim in the embedded metadata.

Considerations for use

• The effective angular resolution is 1 degree because the ILC is constructed from smoothed frequency-band maps.
• The map is a reliable estimate of the CMB signal on angular scales greater than ~10 degrees, with negligible instrument noise, according to the WMAP team.
• On smaller angular scales, the structure associated with the Monte Carlo bias correction remains uncertain. Use the frequency-band maps and likelihoods for quantitative small-scale analysis.
• The ILC weights were selected in 12 sky regions to preserve the CMB while minimising Galactic foregrounds.
• The N_OBS values are uniformly 1.0 and do not provide an observation count or spatial coverage measure for this product.

Licensing and credit

HEASARC states verbatim:

HEASARC materials are all available freely for your use.

This is a free-use statement rather than a named formal licence.

WMAP is a NASA mission, and this map is a product of the WMAP Science Team hosted by LAMBDA. No explicit licence text has been published. LAMBDA requests the following acknowledgment:

We acknowledge the use of the Legacy Archive for Microwave Background Data Analysis (LAMBDA), part of the High Energy Astrophysics Science Archive Center (HEASARC). HEASARC/LAMBDA is a service of the Astrophysics Science Division at the NASA Goddard Space Flight Center.

Citations

Please cite the WMAP nine-year final maps and results paper:

@article{Bennett2013,
  author  = {Bennett, C. L. and Larson, D. and Weiland, J. L. and
             Jarosik, N. and Hinshaw, G. and Odegard, N. and
             Smith, K. M. and Hill, R. S. and Gold, B. and
             Halpern, M. and Komatsu, E. and Nolta, M. R. and
             Page, L. and Spergel, D. N. and Wollack, E. and
             Dunkley, J. and Kogut, A. and Limon, M. and
             Meyer, S. S. and Tucker, G. S. and Wright, E. L.},
  title   = {Nine-year Wilkinson Microwave Anisotropy Probe (WMAP)
             Observations: Final Maps and Results},
  journal = {The Astrophysical Journal Supplement Series},
  volume  = {208},
  pages   = {20},
  year    = {2013},
  doi     = {10.1088/0067-0049/208/2/20},
  eprint  = {1212.5225}
}

Errata and requests

To report an erratum, propose a correction, or ask a question about this dataset, open a Discussion or Pull Request in its Community tab; posting requires a free Hugging Face account. Dataset requests and archive-wide questions may be posted in the README space's Community tab or the Community tab of any dataset. Confirmed corrections are made in the source conversion tools and published by rebuilding the dataset.