---
OA_place: publisher
OA_type: diamond
_id: '19930'
abstract:
- lang: eng
  text: We present an analysis of the UV continuum slope, β, using a sample of 726
    galaxies with z > 4, selected from a mixture of JWST ERS, GTO, and GO observational
    programs. We considered only spectroscopic data obtained with the low-resolution
    (R ∼ 30 − 300) PRISM/CLEAR NIRSpec configuration. Studying the correlation between
    β and MUV, we find an overall decreasing trend, described by β = ( − 0.055 ± 0.017)MUV + ( − 2.98 ± 0.34).
    This is consistent with previous studies, where brighter galaxies show redder
    β values. However, when analyzing the trend in separate redshift bins, we find
    that at high redshift the relation becomes much flatter and is consistent with
    a flat slope within 1σ. Furthermore, we find that β tends to decrease with redshift,
    following β = ( − 0.075 ± 0.010)z + ( − 1.496 ± 0.056). This is consistent with
    most recent results showing a steepening of the spectra at higher z. We selected
    a sample of galaxies with extremely blue slopes (i.e., β < −2.6). Such slopes
    are steeper than predicted by stellar evolution models – even for dust-free, young,
    metal-poor populations – when the contribution of nebular emission is included.
    We selected 44 extremely blue galaxies (XBGs) and investigated the possible physical
    origin of their steep slopes by comparing them to a subsample of redder galaxies
    (matched in Δz = ±0.5 and ΔMUV = ±0.2). We find that XBGs have younger stellar
    populations, stronger ionization fields, lower dust attenuation, and lower but
    not pristine metallicity (∼10% Z⊙) compared to red galaxies. However, these properties
    alone cannot explain the extreme β values. Using indirect inference of Lyman continuum
    escape with the most recent models, we estimated the escape fraction fesc > 10%
    in at least 25% of the XBGs, whereas all the red sources exhibit much lower fesc
    values. A reduced nebular continuum contribution – resulting from either a high
    escape fraction or a bursty star formation history – is likely the origin of the
    extremely blue slopes.
acknowledgement: We acknowledges support from the INAF Large Grant for Extragalactic
  Surveys with JWST and from the PRIN 2022 MUR project 2022CB3PJ3 – First Light And
  Galaxy aSsembly (FLAGS) funded by the European Union – Next Generation EU. PS acknowledges
  INAF Mini Grant 2022 “The evolution of passive galaxies through cosmic time”. Part
  of the research activities described in this paper were carried out with the contribution
  of the Next Generation EU funds within the National Recovery and Resilience Plan
  (PNRR), Mission 4 – Education and Research, Component 2 – From Research to Business
  (M4C2), Investment Line 3.1 – Strengthening and creation of Research Infrastructures,
  Project IR0000034 – “STILES – Strengthening the Italian Leadership in ELT and SKA”.
  RA acknowledges support of Grant project PID2023-147386NB-I00 funded by MICIU/AEI/10.13039/501100011033
  and by ERDF/EU, and the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/50110001103.
article_number: A234
article_processing_charge: No
article_type: original
arxiv: 1
author:
- first_name: D.
  full_name: Dottorini, D.
  last_name: Dottorini
- first_name: A.
  full_name: Calabrò, A.
  last_name: Calabrò
- first_name: L.
  full_name: Pentericci, L.
  last_name: Pentericci
- first_name: Sara
  full_name: Mascia, Sara
  id: edaf889c-c7cd-11ef-ab1b-bb28c431bd29
  last_name: Mascia
- first_name: M.
  full_name: Llerena, M.
  last_name: Llerena
- first_name: L.
  full_name: Napolitano, L.
  last_name: Napolitano
- first_name: P.
  full_name: Santini, P.
  last_name: Santini
- first_name: G.
  full_name: Roberts-Borsani, G.
  last_name: Roberts-Borsani
- first_name: M.
  full_name: Castellano, M.
  last_name: Castellano
- first_name: R.
  full_name: Amorin, R.
  last_name: Amorin
- first_name: M.
  full_name: Dickinson, M.
  last_name: Dickinson
- first_name: A.
  full_name: Fontana, A.
  last_name: Fontana
- first_name: N.
  full_name: Hathi, N.
  last_name: Hathi
- first_name: M.
  full_name: Hirschmann, M.
  last_name: Hirschmann
- first_name: A. M.
  full_name: Koekemoer, A. M.
  last_name: Koekemoer
- first_name: R. A.
  full_name: Lucas, R. A.
  last_name: Lucas
- first_name: E.
  full_name: Merlin, E.
  last_name: Merlin
- first_name: A.
  full_name: Morales, A.
  last_name: Morales
- first_name: F.
  full_name: Pacucci, F.
  last_name: Pacucci
- first_name: S.
  full_name: Wilkins, S.
  last_name: Wilkins
- first_name: P.
  full_name: Arrabal Haro, P.
  last_name: Arrabal Haro
- first_name: M.
  full_name: Bagley, M.
  last_name: Bagley
- first_name: S. L.
  full_name: Finkelstein, S. L.
  last_name: Finkelstein
- first_name: J.
  full_name: Kartaltepe, J.
  last_name: Kartaltepe
- first_name: C.
  full_name: Papovich, C.
  last_name: Papovich
- first_name: N.
  full_name: Pirzkal, N.
  last_name: Pirzkal
citation:
  ama: 'Dottorini D, Calabrò A, Pentericci L, et al. Evolution of the UV slope of
    galaxies at cosmic morning (z &#62; 4): The properties of extremely blue galaxies.
    <i>Astronomy &#38; Astrophysics</i>. 2025;698. doi:<a href="https://doi.org/10.1051/0004-6361/202453267">10.1051/0004-6361/202453267</a>'
  apa: 'Dottorini, D., Calabrò, A., Pentericci, L., Mascia, S., Llerena, M., Napolitano,
    L., … Pirzkal, N. (2025). Evolution of the UV slope of galaxies at cosmic morning
    (z &#62; 4): The properties of extremely blue galaxies. <i>Astronomy &#38; Astrophysics</i>.
    EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202453267">https://doi.org/10.1051/0004-6361/202453267</a>'
  chicago: 'Dottorini, D., A. Calabrò, L. Pentericci, Sara Mascia, M. Llerena, L.
    Napolitano, P. Santini, et al. “Evolution of the UV Slope of Galaxies at Cosmic
    Morning (z &#62; 4): The Properties of Extremely Blue Galaxies.” <i>Astronomy
    &#38; Astrophysics</i>. EDP Sciences, 2025. <a href="https://doi.org/10.1051/0004-6361/202453267">https://doi.org/10.1051/0004-6361/202453267</a>.'
  ieee: 'D. Dottorini <i>et al.</i>, “Evolution of the UV slope of galaxies at cosmic
    morning (z &#62; 4): The properties of extremely blue galaxies,” <i>Astronomy
    &#38; Astrophysics</i>, vol. 698. EDP Sciences, 2025.'
  ista: 'Dottorini D, Calabrò A, Pentericci L, Mascia S, Llerena M, Napolitano L,
    Santini P, Roberts-Borsani G, Castellano M, Amorin R, Dickinson M, Fontana A,
    Hathi N, Hirschmann M, Koekemoer AM, Lucas RA, Merlin E, Morales A, Pacucci F,
    Wilkins S, Arrabal Haro P, Bagley M, Finkelstein SL, Kartaltepe J, Papovich C,
    Pirzkal N. 2025. Evolution of the UV slope of galaxies at cosmic morning (z &#62;
    4): The properties of extremely blue galaxies. Astronomy &#38; Astrophysics. 698,
    A234.'
  mla: 'Dottorini, D., et al. “Evolution of the UV Slope of Galaxies at Cosmic Morning
    (z &#62; 4): The Properties of Extremely Blue Galaxies.” <i>Astronomy &#38; Astrophysics</i>,
    vol. 698, A234, EDP Sciences, 2025, doi:<a href="https://doi.org/10.1051/0004-6361/202453267">10.1051/0004-6361/202453267</a>.'
  short: D. Dottorini, A. Calabrò, L. Pentericci, S. Mascia, M. Llerena, L. Napolitano,
    P. Santini, G. Roberts-Borsani, M. Castellano, R. Amorin, M. Dickinson, A. Fontana,
    N. Hathi, M. Hirschmann, A.M. Koekemoer, R.A. Lucas, E. Merlin, A. Morales, F.
    Pacucci, S. Wilkins, P. Arrabal Haro, M. Bagley, S.L. Finkelstein, J. Kartaltepe,
    C. Papovich, N. Pirzkal, Astronomy &#38; Astrophysics 698 (2025).
date_created: 2025-06-29T22:01:15Z
date_published: 2025-06-01T00:00:00Z
date_updated: 2026-02-16T12:11:39Z
day: '01'
ddc:
- '520'
department:
- _id: JoMa
doi: 10.1051/0004-6361/202453267
external_id:
  arxiv:
  - '2412.01623'
  isi:
  - '001510826300019'
file:
- access_level: open_access
  checksum: 100f897d468de9d0113277c870035b62
  content_type: application/pdf
  creator: dernst
  date_created: 2025-06-30T08:22:08Z
  date_updated: 2025-06-30T08:22:08Z
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file_date_updated: 2025-06-30T08:22:08Z
has_accepted_license: '1'
intvolume: '       698'
isi: 1
language:
- iso: eng
month: '06'
oa: 1
oa_version: Published Version
publication: Astronomy & Astrophysics
publication_identifier:
  eissn:
  - 1432-0746
  issn:
  - 0004-6361
publication_status: published
publisher: EDP Sciences
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'Evolution of the UV slope of galaxies at cosmic morning (z > 4): The properties
  of extremely blue galaxies'
tmp:
  image: /images/cc_by.png
  legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode
  name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)
  short: CC BY (4.0)
type: journal_article
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
volume: 698
year: '2025'
...