[Paper Review] Spectroscopy of Ultra--Steep Spectrum Radio Sources: A Sample of z>2 Radio Galaxies

H. J. A. Röttgering, R. van Ojik+4 more|arXiv (Cornell University)|Aug 12, 1996

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study presents spectroscopic observations of 64 ultra-steep spectrum (USS) radio galaxies, identifying 29 at z > 2, with three exceeding z = 3. The authors find a strong correlation between steeper radio spectra and higher redshifts, and confirm that Lyα emission dominates with rest-frame equivalent widths up to 1000 Å, indicating photoionization by a central source. Velocity shifts between Lyα and higher-ionization lines (up to 1000 km s⁻¹) are likely due to associated H I absorption, though kinematic or obscuration effects may also contribute.

ABSTRACT

We present spectroscopic observations for 64 radio galaxies having ultra steep radio spectra. Twenty-nine objects have redshifts $z>2$, the largest redshifts being almost 4. Our ultra steep spectrum (USS) criterion ($\alpha < -1$) has proven to be the most efficient way of finding distant radio galaxies. We find that even among the USS sources, there is a strong statistical correlation between the spectral index and redshift. The most distant radio galaxies within the USS sample have the steepest radio spectra. In our sample there are 3 radio galaxies at $z>3$ compared with 26 at $2 < z < 3$. However, the present data do not allow us to decide whether there is a decrease in co-moving source density at the highest redshifts. We have analyzed the spectra of the 30 objects with the highest redshifts ($z>1.9$). For these high redshift radio galaxies, Ly$\alpha$ is almost always the dominant emission line, with a rest frame equivalent width ranging from $\sim100$ \AA\ to more than 1000 \AA. The equivalent widths of the most important emission lines (Ly$\alpha$, C\,{\small IV}, He\,{\small II}, C\,{\small III}]) are found to correlate strongly with each other. The large rest frame equivalent widths and the correlation between the equivalent widths of the emission lines, confirm that photoionization by a central continuum source is most likely the dominant ionization mechanism. There are significant velocity differences between the various emission lines of our high redshift radio galaxies; in particular the Ly$\alpha$ line is shifted with respect to the higher ionization lines. Velocity shifts range from 100 to almost 1000 km s$^{-1}$ in some cases. Simulations show that the effects of associated H\,{\small I} absorption on the Ly$\alpha$ emission line may be

Motivation & Objective

• To identify and characterize high-redshift radio galaxies using ultra-steep spectrum (USS) selection.
• To determine the efficiency of the USS criterion (α < −1) in finding z > 2 radio galaxies.
• To analyze emission line properties, particularly Lyα, in high-redshift radio galaxies (HZRGs).
• To investigate the origin of velocity shifts between Lyα and higher-ionization emission lines.
• To assess the co-moving source density of USS radio galaxies at z > 3 and test for evolution.

Proposed method

• Selection of 64 radio sources using the ultra-steep spectrum criterion (α < −1) from the Leiden intermediate-flux compendium and 4C sample.
• High-resolution VLA imaging (1.5′′) to obtain accurate positions for optical identification.
• R-band CCD imaging with 2-m telescopes to confirm optical counterparts.
• Low-resolution spectroscopy using 4m-class telescopes to measure redshifts and emission line properties.
• Spectral analysis to determine redshifts, rest-frame equivalent widths, and velocity shifts between emission lines.
• Simulations of H I absorption effects on Lyα profiles to assess their role in observed velocity shifts.

Experimental results

Research questions

• RQ1 Is the ultra-steep spectrum criterion (α < −1) an efficient method for identifying high-redshift radio galaxies?
• RQ2 What is the correlation between radio spectral index and redshift in USS sources?
• RQ3 What causes the observed velocity shifts between Lyα and higher-ionization emission lines (e.g., C IV, He II) in high-redshift radio galaxies?
• RQ4 Is photoionization by a central continuum source the dominant ionization mechanism in these systems?
• RQ5 Is there evidence for a decrease in co-moving source density at z > 3?

Key findings

• The USS criterion (α < −1) is highly efficient for identifying high-redshift radio galaxies, with 29 of 64 sources at z > 2.
• A strong statistical correlation exists between steeper radio spectra and higher redshifts, with the most distant sources (z ≈ 3.8) having the steepest spectra.
• Lyα is the dominant emission line in high-redshift radio galaxies (z > 1.9), with rest-frame equivalent widths ranging from ∼100 Å to over 1000 Å.
• The equivalent widths of Lyα, C IV, He II, and C III] are strongly correlated, supporting photoionization by a central continuum source as the dominant ionization mechanism.
• Significant velocity shifts (100–1000 km s⁻¹) are observed, primarily with Lyα shifted relative to higher-ionization lines, consistent with the effects of associated H I absorption systems.
• Other mechanisms, such as outflows or obscuration, may contribute in at least some sources, though H I absorption is the most likely dominant cause.