Euclid Quick Data Release (Q1). Spectroscopic unveiling of highly ionised lines at z = 2.48-3.88

TL;DR

The paper introduces the first systematic identification and characterization of high-ionisation [NeV]$\\lambda$3426 emitters at $z$ between $2.5$ and $3.8$ using Euclid Q1 spectroscopy, exploiting purely spectral diagnostics to surpass traditional selection limitations. By requiring simultaneous detections of [NeV]$\\lambda$3426 and [OII]$\\lambda$3727 via direct integration and Gaussian fitting, the authors assemble a robust sample of $65$ NeV candidates and construct a matched control sample for comparison. Stacked spectroscopy confirms a strong [NeV] signal in the NeV targets and a non-detection in the control, while host galaxies tend to be more massive and show a modest difference in the 4000 Å break, suggesting differences in star-formation histories. The work also finds limited cross-identification with X-ray and radio data, indicating these high-ionisation sources may represent a population not readily captured by shallow multi-wavelength surveys, and sets the stage for follow-up in upcoming Euclid data releases.

Abstract

This study explores a rare population of sources in a currently uncharted region of spectroscopic redshift space in the Euclid Quick Data Release (Q1), and is intended potentially to support upcoming spectroscopic studies. Our goal is to identify and investigate a population of sources characterised by highly ionised emission lines in their spectra, which are indicative of active galactic nucleus activity, extreme shock phenomena, or Wolf--Rayet stars. A comprehensive visual inspection of spectra is conducted to ensure the reliability of the sample, focusing on the simultaneous detection of both NeV and OII emission-line measurements, a condition that restricts the Euclid spectroscopic redshift range to z=2.48--3.88. To characterise this population, we analysed the morpho-spectrophotometric properties of their host galaxies. This allowed for a direct comparison with control sources that exhibit similar OII properties and spectroscopic redshifts, but not NeV lines. We identify sources solely based on spectroscopic criteria in the redshift range beyond the Halpha regime. Encompassing 65 potential NeV candidates, the resulting sample delivers the first systematic probe of these NeV candidate emitters at high redshift. We found a good agreement, within 1$σ$, between the spectral measurements calculated using both direct integration and Gaussian fitting methodologies. The NeV candidates exhibit colours similar to bright QSOs, with only a few in the tail of very red quasars. We observed a higher stellar mass content, a lower continuum around the 4000A break, and a similar Sérsic index distribution compared to the control sample. This unique sample paves the way for a wide range of scientific investigations, which will be pursued in the forthcoming data releases.

Figures (5)

• Figure 1: Comparison of the fluxes measured via the Gaussian fit (GF, $x$-axis) and the direct integration (DI, $y$-axis) for the [Nev]$\lambda$3426 and [Oii]$\lambda$3727 emission lines used in this analysis (squares and circles, respectively). QSO- and galaxy-classified [Nev]$\lambda$3426 emitters are plotted with red and blue symbols, respectively. The dashed line represents the $x=y$ relation. The bands represent the $\pm$1 $\sigma$ distributions of each category.
• Figure 2: Distributions of the median stellar mass and the Sérsic index for the objects in the target sample (red) and control sample (grey). The dashed lines indicate the median value, while the vertical dotted lines and shaded bands represent the interquartile ranges(IQRs) of the distributions. The error bars represent the maximum dispersion in the parameters estimated via the bootstrap resampling and the IQR. The hatched bars represent the sources excluded from the analysis.
• Figure 3: Top: $(\IE - \YE)$ versus $(\JE - \HE)$ diagram showing the [Nev]$\lambda$3426 galaxy-classified targets and [Nev]$\lambda$3426 QSO-classified target sources (in blue and red circles) plotted with the spectroscopically identified quasars in F25 (green). We overplot the region proposed in Q1-SP027 that identifies the QSO candidates (black-solid line), with the stellar locus found below this boundary at approximately $-$$< 0.1$ represented by purple contours in their Fig. 6. The grey points are the compact sources in Q1-SP027. Bottom: $(\YE - \HE)$ versus $(\JE - \HE)$ diagram with the region proposed in Q1-SP023 that identifies the red QSO (black-dashed line). Symbols are as in the upper panel.
• Figure 4: Distribution of the [Nev]$\lambda$3426- ($x$-axis) and [Oii]$\lambda$3727-flux ($y$-axis) for the candidate emitters as a function of the 4000 Å break (D$_{\rm n}$4000, in colours when measured, in grey otherwise). Symbols are as in Fig. \ref{['fig:gfdi']}.
• Figure 5: Rest-frame smoothed spectrum (3-pixel window) of [Ne v] candidate emitters, shown in black, compared with that of the high-quality spectroscopic control sample of 50 [Oii]$\lambda$3727 candidates in blue. The grey shaded bands delineate the wavelength intervals of the continuum used to calculate the 4000 Å break index.