Metal content of the circumgalactic medium around star-forming galaxies at z ∼ 2.6 as revealed by the VIMOS Ultra-Deep Survey

¹ Insituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, 2340000 Valparaíso, Chile
e-mail: hugo.mendez@postgrado.uv.cl
² Dipartimento di Fisica e Astronomia Galileo Galilei, Università degli Studi di Padova, Vicolo del L’Osservatorio 3, 35122 Padova, Italy
³ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán, 1305 La Serena, Chile
⁵ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
⁶ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁸ INAF-IASF, Via Bassini 15, 20133 Milano, Italy
⁹ Department of Astronomy, University of Massachusetts Amherst, 710 N. Pleasant St, Amherst, MA 01003, USA
¹⁰ Departamento de Ciencias Fisicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹³ Gemini Observatory, NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA
¹⁴ Department of Physics and Astronomy, University of California, Davis, One Shields Ave., Davis, CA 95616, USA
¹⁵ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile
¹⁶ European Southern Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago, Chile

Received: 30 October 2021
Accepted: 10 June 2022

Abstract

Context. The circumgalactic medium (CGM) is the location where the interplay between large-scale outflows and accretion onto galaxies occurs. Metals in different ionization states flowing between the circumgalactic and intergalactic mediums are affected by large galactic outflows and low-ionization state inflowing gas. Observational studies on their spatial distribution and their relation with galaxy properties may provide important constraints on models of galaxy formation and evolution.

Aims. The main goal of this paper is to provide new insights into the spatial distribution of the circumgalactic of star-forming galaxies at 1.5 < z < 4.5 (⟨z⟩∼2.6) in the peak epoch of cosmic star formation activity in the Universe. We also look for possible correlations between the strength of the low- and high-ionization absorption features (LIS and HIS) and stellar mass, star formation rate, effective radius, and azimuthal angle ϕ that defines the location of the absorbing gas relative to the galaxy disc plane.

Methods. The CGM has been primarily detected via the absorption features that it produces on the continuum spectrum of bright background sources. We selected a sample of 238 close pairs from the VIMOS Ultra Deep Survey to examine the spatial distribution of the gas located around star-forming galaxies and generate composite spectra by co-adding spectra of background galaxies that provide different sight-lines across the CGM of star-forming galaxies.

Results. We detect LIS (C II and Si II) and HIS (Si IV, C IV) up to separations ⟨b⟩ = 172 kpc and 146 kpc. Beyond this separation, we do not detect any significant signal of CGM absorption in the background composite spectra. Our Lyα, LIS, and HIS rest-frame equivalent width (W₀) radial profiles are at the upper envelope of the W₀ measurements at lower redshifts, suggesting a potential redshift evolution for the CGM gas content producing these absorptions. We find a correlation between C II and C IV with star formation rate and stellar mass, as well as trends with galaxy size estimated by the effective radius and azimuthal angle. Galaxies with high star formation rate (log[SFR/(M_⊙ yr⁻¹)] > 1.5) and stellar mass (log[M_⋆/M_⊙] > 10.2) show stronger C IV absorptions compared with those low SFR (log[SFR/(M_⊙ yr⁻¹)] < 0.9) and low stellar mass (log[M_⋆/M_⊙] < 9.26). The latter population instead shows stronger C II absorption than their more massive or more star-forming counterparts. We compute the C II/C IVW₀ line ratio that confirms the C II and C IV correlations with impact parameter, stellar mass, and star formation rate. We do not find any correlation with ϕ in agreement with other high-redshift studies and in contradiction to what is observed at low redshift where large-scale outflows along the minor axis forming bipolar outflows are detected.

Conclusions. We find that the stronger C IV line absorptions in the outer regions of these star-forming galaxies could be explained by stronger outflows in galaxies with higher star formation rates and stellar masses that are capable of projecting the ionized gas up to large distances and/or by stronger UV ionizing radiation in these galaxies that is able to ionize the gas even at large distances. On the other hand, low-mass galaxies show stronger C II absorptions, suggesting larger reservoirs of cold gas that could be explained by a softer radiation field unable to ionize high-ionization state lines or by the galactic fountain scenario where metal-rich gas ejected from previous star formation episodes falls back to the galaxy. These large reservoirs of cold neutral gas around low-mass galaxies could be funnelled into the galaxies and eventually provide the necessary fuel to sustain star formation activity.