[Paper Review] Molecules as tracers of galaxy evolution: an EMIR survey. I. Presentation of the data and first results

F. Costagliola, S. Aalto+11 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Jan 11, 2011

Astrophysics and Star Formation Studies43 references59 citations

TL;DR

This study uses EMIR broadband observations at 3 mm to survey molecular gas in 23 galaxies, identifying chemical tracers of galaxy evolution. Key findings include low HNC/HCN ratios inconsistent with PDR/XDR models, bright HC3N in HCO+-faint sources suggesting embedded star formation over AGN activity, and correlations between line ratios and dust temperature or IR luminosity.

ABSTRACT

We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected transitions are the J=1-0 lines of CO, 13CO, HCN, HNC, HCO+, CN, and C2H. We also detect HC3N J=10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC3N detections are in objects with HCO+/HCN<1 and warm IRAS colours. Galaxies with the highest HC3N/HCN ratios have warm IRAS colours (60/100 μm>0.8). The brightest HC3N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state.We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. Bright HC3N emission in HCO+-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of embedded starformation, instead of AGN activity.

Motivation & Objective

• Investigate molecular gas properties in 23 galaxies to trace galaxy evolution using molecular line emission.
• Test chemical signatures of starbursts and AGN activity through line intensity ratios.
• Assess the reliability of HCN/HCO+ and HNC/HCN ratios as tracers of X-ray-dominated regions (XDRs) and PDRs.
• Explore the connection between molecular emission, dust temperature, and IR luminosity in luminous infrared galaxies (LIRGs).
• Identify molecular species such as HC3N and CN as potential indicators of physical conditions in galactic nuclei.

Proposed method

• Conducted 3 mm continuum and line observations using the EMIR broadband receiver on the IRAM 30 m telescope.
• Measured line intensities of key transitions: CO(1-0), 13CO(1-0), HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0), C2H(1-0), and HC3N(10-9).
• Constructed line intensity ratio diagrams (e.g., HNC/HCN, HCO+/HCN) to compare with PDR and XDR chemical models.
• Applied principal component analysis to identify dominant trends in molecular emission across the sample.
• Correlated molecular line ratios with dust temperature and IRAS colors to assess evolutionary stage.
• Used spatially unresolved single-dish data to infer global gas conditions, acknowledging limitations from beam-averaged emission.

Experimental results

Research questions

• RQ1 Can HCN/HCO+ and HNC/HCN line ratios reliably distinguish between XDR-dominated AGN and PDR-dominated starburst environments?
• RQ2 What is the significance of bright HC3N emission in galaxies with low HCO+ emission, and does it indicate embedded star formation?
• RQ3 How do line intensity ratios like CO/13CO and CN/C18O correlate with dust temperature and IR luminosity?
• RQ4 Why are HNC/HCN ratios consistently below 0.5 in the sample, and can existing chemical models explain this?
• RQ5 To what extent are observed trends driven by optical depth, excitation, or true abundance variations in molecular gas?

Key findings

• HC3N J=10–9 was detected in five galaxies: IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068, with the strongest emission in IC 860.
• Galaxies with the highest HC3N/HCN ratios have warm IRAS colors (60/100 μm > 0.8), and all HC3N detections occur in sources with HCO+/HCN < 1.
• HNC/HCN line ratios are consistently below 0.5 across the sample, a result not explained by current PDR or XDR chemical models.
• HCO+ and HNC intensities are anti-correlated: low HCO+/HCN ratios coincide with high HNC/HCN ratios, suggesting different chemistry or excitation conditions.
• The CO/13CO intensity ratio is positively correlated with dust temperature and typically ~20, exceeding the Galactic value.
• CN and C18O emission are positively correlated, possibly due to shared abundance trends or enrichment from starburst activity.