Transcript Investigation of the FIR-Radio correlation at small scales in the Galaxy

An
Investigation
of
the
Molecular-FIR-Radio correlation
at small scales in the Galaxy
Mónica Ivette Rodríguez
Dr. Laurent Loinard (UNAM - México)
Dr. Tommy Wiklind (STScI - USA)
Introduction
The main goal of studying spiral galaxies is to understand how stars form and
how the star formation is related to dynamical and physical conditions in the
interstellar medium through several different diagnostics.
Examples :
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Ionizing continuum radiation (UV)
Balmer lines
near-infrared, mid-infrared
Dust emission (far-infrared)
Molecular emission (CO)
Radio continuum
Introduction
Several diagnostics are correlated
FIR-RC correlation
Condon1992
FIR-CO correlation
Tutui et al. (2002)
RC-CO correlation
Paladino et al. (2006)
Introduction
These correlations hold when viewing galaxies on global scales, however the emission
mechanisms and the processes driving the emission are different.
The physical bases for understanding the molecular-FIR-RC correlation is not well
understood, and several effects can modify the basic correlation such as density waves,
etc.
In my PhD. Project I will study these correlations, most notably, the far-infrared and radio
continuum correlation on scales corresponding to the size of small molecular clouds.
I will also study the possibility that the correlation between CO and far-infrared
luminosities is caused by strong selection effects: molecular CO emission is only
detected in warm regions. An alternative is searching for very cold molecular gas using
the anomalous 6-cm line of formaldehyde (H2CO).
Molecular Gas
The molecular gas is mainly traced by
12CO
emission
Since it is an optically thick line, the line does no give any information
about density
Absorption lines can be used as tracer of cold molecular gas since the
excitation characteristics looks different
But the absence of radio continuum sources limits the use of absorption
lines
The 6-cm H2CO line is an absorption line against the Cosmic Microwave
Background, seems to offers an alternative …
Formaldehyde (H2CO)
Several transitions 2-mm, 2-cm & 6-cm
At 6-cm (4829.660 Mhz) :
Was discovered in 1969
(Palmer et al. 1969)
Low excitation energy ( ~ 1.7 K)
Energy-level diagram (Townes & Cheung 1969)
It is an absorption line against the Cosmic
Microwave Background (CMB)
Formaldehyde (H2CO)
Townes & Cheung 1969 used a classical calculation for collisional excitation
The collisional pumping mechanism is more effective at high collision rates (Evans
et al. 1975), however they showed that the mechanism would still be effective
at low temperatures
More precise calculations in Garrison et al. (1975) suggest a smaller effect at very
low kinetic temperatures
This leaves open the possibility that high-density, cold molecular gas may be
detected using H2CO
Structure of the H2CO molecule
(Townes & Cheung 1969)
Introduction to the target sources
Galactic Anticenter :
The Galactic non-thermal background is faint in this direction
The velocity gradient is small in this direction, enhancing the probability of
detection
Introduction to the target sources
L1204/S140 Region :
Photodisociated region caused by a very close nearby B0V star
Close star forming region
Observations
The observations were obtained during three sessions (January 2004, September - October
2004, May 2005) with the 25.6m telescope of the Onsala Space Observatory (OSO)
At 6 cm, the angular resolution of the 25 m is 10’.
Results
Galactic Anticenter :
143 positions
H2CO absorption
at 10 %
No H2CO emission
Results
l = 182o, b = 0o
63 positions
H2CO
12CO
Results
l = 190o, b = 0o
101 positions
H2CO
12CO
Results
Proving the nature of the H2CO absorption toward the Anticenter
Grey scale :
H2CO absorption
Contours :
21-cm radio
continuum
Results
1)
L1204/S140 Region
Photodissociated Region
2)
First panel :
-12 km/sec < v < -5 km/sec
Second panel :
3)
-10 km/sec < v < -5 km/sec
Third panel :
-12 km/sec < v < -10 km/sec
Results
L1204/S140 Region :
l = 107o b = 5.3o
72 positions
H2CO peak is at 10’ offset
of 12 CO peak
H2CO
12CO
Relation of the H2CO CMB
absorption to CO(1-0) emission
Intensity ratio
Galactic Anticenter :
Open circles : CO
I(H2CO) K km/sec
Points : both tracer
I(12CO) K km/sec
Relation of the H2CO CMB
absorption to CO(1-0) emission
Intensity ratio
L1204/S140 Region :
Points : both tracer
I(H2CO) K km/sec
Open circles : CO
I(12CO) K km/sec
Conclusions
The excitation characteristics of both lines are similar
H2CO and 12CO lines trace warm, dense molecular gas
The H2CO absorption line is not a viable tracer of cold
molecular gas
The question that clouds of cold and dense molecular gas may exist
remains open
Publications
The results of the H2CO observations toward the Galactic Anticenter were
presented in the article :
“Anomalous H2CO Absorption Toward the Galactic Anticenter :
A Blind Search for Dense Molecular Gas “
(Rodriguez el al., 2006 Astro-ph/0607616)
(submitted and accepted to ApJ)
The results of the H2CO observations toward the Galactic Cloud L1204 will
presented in the article :
“Anomalous H2CO Absorption in the L1204/S140 Region and a Comparison with
CO(1-0) emission”
(to be submitted to ApJ)
Future Work
The work plan for the up coming year, will be focused in the behavior of far-infrared and
continuum correlation, on scales corresponding to the size of the small molecular
cloud.
Following the calorimeter theory (Voelk 1989) such correlation is not expected at local
scales
Hoernes, Berhuijsen & Xu (1998) showed that it still holds at scales of about 1 kpc in
M31
Murphy et al. 2005 combined new Spitzer data with archival radio observations of M51
conclude that this correlation still holds at 750 pc
Then the scale of infrared-radio remains unknown …
Such correlation is indeed needed to explain the overall radio-infrared correspondence
We proposed to study it at much smaller scales …
Milky Way
Example
Comparison between the IRAS 100 m image and the
408 MHz radio image of the Galactic region around
(135,+2)
Work plan
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Get 1.4 GHz and 408 MHz and IRAS images of the Dominion Radio Astrophysical
Observatory (DRAO) (Taylor et al. 2003)
Identify several prototypical Galactic regions, we proposed 20-25 examples
Infrared
Radio continuum
60 
100 
408 MHz
1420 MHz
60 
408 MHz
10’
100 
1420 MHz
60 
408 MHz
100 
1420 MHz
10’
Work plan
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Clean the bright point sources
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Get the index spectral map for every region
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Obtain the pure non-thermal images combining the two wave lengths
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Compare them quantitatively with the far-infrared data from IRAS
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Compare our local correlation FIR-RC with the global correlation
Goals
Hoernes, Berhuijsen & Xu (1998) proposed that if the FIR-RC still holds on
small scales there should be a strong coupling between the interstellar gas
located in the clouds traced by IRAS and the magnetic field
If we confirm the existence of a tight radio/infrared correlation at parsec scales,
we shall attempt to explain it using similar models, and be able to put
stronger constraints on the theoretical models
If the results are in agreement with our expectations, we will consider extending
our studies over the entire Galactic disk.