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A Cm-Wavelength Search for Pre-Biotic Molecules
in Starburst Galaxies: A Case Study for Zw 049.057
Minchin R., Catinella B., Ghosh, T., Lebron M., Lerner M.S., Momjian E.
O’Neil, K., Salter, C.J. (NAIC , NRAO, UPR and MPIfA)
Interstellar organic species mostly form on the surface of dust grains and are then released into the gas phase by
heating events. Organic species known as pre-biotic molecules may combine to form amino acids; one example is
the combination of methanimine (CH2NH) and HCN on the path to glycine. Both CH2NH and HCN can be
detected at cm-wavelengths, as we have demonstrated in Arp 220 (Salter et al 2008, AJ, 136, 389). Following this
discovery, we have initiated observations of 40 Arp 220-like galaxies with Arecibo and the GBT. C-band
observations with a frequency range of 4.4 - 5.2 GHz cover both the CH2NH line and the HCN v2=1, J=4
transition. Observations and analysis are ongoing for both telescopes. In the data reduced so far, both CH2NH
and HCN have been found in a number of sources, as have other molecules such as H2CO and excited OH, and
hydrogen recombination lines. In this poster we present a case study for Zw 049.057.
The Sample
-cm Excited OH Absorption Lines
Formaldehyde (H2CO)
Our sample of 40 Arp-220-like galaxies
were selected from the lists of;
• OH Megamaser galaxies as compiled
by Chen et al. (2007)
• Starbursts with detected H2CO masers
or absorption (Araya et al. 2004)
4660-MHz
satellite line
• ULIRGS with direct evidence of
nuclear starbursts (Armus et al.1990)
• FIR-luminous merging galaxies
(Condon et al. 1991)
Spectra were recorded from 8 × 100MHz C-band band-passes that included
lines either seen in Arp 220, or felt to be
good candidates for detection. The
objective was to obtain ~1 hr of ONsource integration on each target. The
spectra here have ~30 km/s resolution.
a
4750-MHz
main line
The H2CO line in Zw 049.057 was previously
detected by Araya et al. (2004) and Baan et al.
(1993). However, our line shape appears
somewhat different to that of Araya et al. The
possibility of variability needs investigation.
Methanimine (CH2NH)
Zw 049.057
Zw 049.057 (IRAS 1507+0724; z =
0.012999) appears as a highly inclined
disk with a smooth light distribution at
large radii, but a bright arm of star
formation ~1“ south of the 2.2-μm peak.
4765-MHz
satellite line
Hydrogen Cyanide (HCN; v2=1)
The λ6-cm lines of the excited-OH
Transitions 2П½,,J=1/2,F=0-1,1-1 and
1-0 have intensity ratios close to 1:2:1,
the LTE values. The OH lines at 4750
& 4765 MHz were heavily affected by
RFI, and only 20 min of good ON-source
data was usable.
ZW 049.057 is classified as both an HII
and a Luminous IR Galaxy (LIRG), and
is also a strong OH Megamaser emitter.
The galaxy was one of the sources
included in the Arecibo part of the
sample, and 1.25 hr of ON-source data
were acquired for the object.
A Molecular Comparison with Arp 220
• All molecular lines seen in our bands in Arp 220 are also detected in Zw 049.057.
• Excited-OH absorptions in both galaxies display LTE, with the Zw 049.057 line integrals ~12%
the stronger. The HCN v2=1 absorption in Zw 049.057 is a factor of ~2.5 times the weaker.
• In each of the two galaxies, the H2CO and CH2NH emission lines have similar relative
strengths, with the absolute integrated luminosities of these lines being ~6 × stronger in Arp 220
than in Zw 049.057.