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

An Investigation of the Molecular-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 RodríguezDr. Laurent Loinard (UNAM - México)Dr. Tommy Wiklind (STScI - USA)

IntroductionIntroduction

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.

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 :

• Ionizing continuum radiation (UV)• Balmer lines • near-infrared, mid-infrared• Dust emission (far-infrared)• Molecular emission (CO)• Radio continuum

Examples :

• Ionizing continuum radiation (UV)• Balmer lines • near-infrared, mid-infrared• Dust emission (far-infrared)• Molecular emission (CO)• Radio continuum


Several diagnostics are correlatedSeveral diagnostics are correlated

FIR-RC correlation RC-CO correlationFIR-CO correlationCondon1992 Paladino et al. (2006)Tutui et al. (2002)


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).

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 GasMolecular Gas

The molecular gas is mainly traced by The molecular gas is mainly traced by 1212CO emissionCO emission

Since it is an optically thick line, the line does no Since it is an optically thick line, the line does no give any information about density give any information about density

Absorption lines can be used as tracer of cold Absorption lines can be used as tracer of cold molecular gas since the excitation characteristics molecular gas since the excitation characteristics looks differentlooks different

But the absence of radio continuum sources limits the But the absence of radio continuum sources limits the use of absorption linesuse of absorption lines

The 6-cm HThe 6-cm H22CO line is an absorption line against the CO line is an absorption line against the Cosmic Microwave Background, seems to offers an Cosmic Microwave Background, seems to offers an alternative … alternative …

The molecular gas is mainly traced by The molecular gas is mainly traced by 1212CO emissionCO emission

Since it is an optically thick line, the line does no Since it is an optically thick line, the line does no give any information about density give any information about density

Absorption lines can be used as tracer of cold Absorption lines can be used as tracer of cold molecular gas since the excitation characteristics molecular gas since the excitation characteristics looks differentlooks different

But the absence of radio continuum sources limits the But the absence of radio continuum sources limits the use of absorption linesuse of absorption lines

The 6-cm HThe 6-cm H22CO line is an absorption line against the CO line is an absorption line against the Cosmic Microwave Background, seems to offers an Cosmic Microwave Background, seems to offers an alternative … alternative …

Formaldehyde (H2CO)Formaldehyde (H2CO)

Several transitions 2-mm, 2-cm & Several transitions 2-mm, 2-cm & 6-cm 6-cm

At 6-cm (4829.660 Mhz) :At 6-cm (4829.660 Mhz) :

Was discovered in 1969 Was discovered in 1969 (Palmer et al. 1969)(Palmer et al. 1969)

Low excitation energy ( ~ 1.7 K)Low excitation energy ( ~ 1.7 K)

It is an absorption line against It is an absorption line against the Cosmic Microwave Background the Cosmic Microwave Background (CMB)(CMB)

Several transitions 2-mm, 2-cm & Several transitions 2-mm, 2-cm & 6-cm 6-cm

At 6-cm (4829.660 Mhz) :At 6-cm (4829.660 Mhz) :

Was discovered in 1969 Was discovered in 1969 (Palmer et al. 1969)(Palmer et al. 1969)

Low excitation energy ( ~ 1.7 K)Low excitation energy ( ~ 1.7 K)

It is an absorption line against It is an absorption line against the Cosmic Microwave Background the Cosmic Microwave Background (CMB)(CMB)

Energy-level diagram (Townes & Cheung 1969)

Formaldehyde (H2CO)Formaldehyde (H2CO)

Townes & Cheung 1969 used a classical calculation for collisional Townes & Cheung 1969 used a classical calculation for collisional excitationexcitation

The collisional pumping mechanism is more effective at high The collisional pumping mechanism is more effective at high collision rates (Evans et al. 1975), however they showed that collision rates (Evans et al. 1975), however they showed that the mechanism would still be effective at low temperaturesthe mechanism would still be effective at low temperatures

More precise calculations in Garrison et al. (1975) suggest a More precise calculations in Garrison et al. (1975) suggest a smaller effect at very low kinetic temperaturessmaller effect at very low kinetic temperatures

This leaves open the possibility that high-density, cold This leaves open the possibility that high-density, cold molecular gas may be detected using Hmolecular gas may be detected using H22COCO

Townes & Cheung 1969 used a classical calculation for collisional Townes & Cheung 1969 used a classical calculation for collisional excitationexcitation

The collisional pumping mechanism is more effective at high The collisional pumping mechanism is more effective at high collision rates (Evans et al. 1975), however they showed that collision rates (Evans et al. 1975), however they showed that the mechanism would still be effective at low temperaturesthe mechanism would still be effective at low temperatures

More precise calculations in Garrison et al. (1975) suggest a More precise calculations in Garrison et al. (1975) suggest a smaller effect at very low kinetic temperaturessmaller effect at very low kinetic temperatures

This leaves open the possibility that high-density, cold This leaves open the possibility that high-density, cold molecular gas may be detected using Hmolecular gas may be detected using H22COCO

Structure of the H2CO molecule (Townes & Cheung 1969)

Introduction to the target sourcesIntroduction 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


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 sourcesIntroduction to the target sources

L1204/S140 Region :

Photodisociated region caused by a very close nearby B0V starClose star forming region

L1204/S140 Region :

Photodisociated region caused by a very close nearby B0V starClose star forming region

ObservationsObservations

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’.

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’.

ResultsResults


143 positions

H2CO absorption at 10 %

No H2CO emission


143 positions

H2CO absorption at 10 %

No H2CO emission

ResultsResults

l = 182o, b = 0ol = 182o, b = 0o

H2CO 12COH2CO 12CO

63 positions

ResultsResults

l = 190o, b = 0ol = 190o, b = 0o

H2CO 12COH2CO 12CO

101 positions

ResultsResults

Proving the nature of the H2CO absorption toward the Anticenter Proving the nature of the H2CO absorption toward the Anticenter

Grey scale :

H2CO absorption

Contours : 21-cm radio

continuum

Grey scale :

H2CO absorption

Contours : 21-cm radio

continuum

First panel :

-12 km/sec < v < -5 km/sec

Second panel :


Third panel :


L1204/S140 Region

Photodissociated Region

Results 1)

2)

3)

ResultsResults

L1204/S140 Region :

l = 107o b = 5.3o

72 positions

H2CO peak is at 10’ offset of 12 CO peak

L1204/S140 Region :

l = 107o b = 5.3o

72 positions

H2CO peak is at 10’ offset of 12 CO peak

H2CO 12COH2CO 12CO

Relation of the H2CO CMB absorption to CO(1-0) emission


Galactic Anticenter :Galactic Anticenter :

Points : both tracer

Open circles : CO

Intensity ratio

I(H

2CO

) K

km

/se

c

I(12CO) K km/sec



L1204/S140 Region :L1204/S140 Region :

Points : both tracer

Open circles : CO

Intensity ratio

I(H

2CO

) K

km

/se

c

I(12CO) K km/sec

ConclusionsConclusions

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 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

PublicationsPublications

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)

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 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 …

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 planWork plan

• 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

• 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

100

408 MHz

1420 MHz

10’

Work planWork plan

• Clean the bright point sources

• Get the index spectral map for every region

• Obtain the pure non-thermal images combining the two wave lengths

• Compare them quantitatively with the far-infrared data from IRAS

• Compare our local correlation FIR-RC with the global correlation

• Clean the bright point sources

• Get the index spectral map for every region

• Obtain the pure non-thermal images combining the two wave lengths

• Compare them quantitatively with the far-infrared data from IRAS

• Compare our local correlation FIR-RC with the global correlation

GoalsGoals

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.

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.

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

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