nearby galaxies (mostly) at mm and ir wavelengths adam leroy (mpia heidelberg) christof buchbender...

Nearby Galaxies(mostly) at mm and IR wavelengths

Adam Leroy (MPIA Heidelberg)Christof Buchbender (IRAM Granada)

Topics

A Broad Look at Nearby Galaxies

Observing Nearby Galaxies at Millimeter Wavelengths

Mapping nearby galaxies with the IRAM 30m

Working Group:

Mapping the bulk distribution of molecular gas in a bright nearby spiral galaxy.

Goal: An overview of what you can observe in nearby galaxies at millimeter wavelengths. Discuss typical intensities from several perspectives.

Specific Topics:

o What you see studying galaxies at mm wavelengths.

o Intensities at extragalactic distances: clouds, galaxies, chunks of galaxies.

o Millimeter continuum: origin and use (very briefly).

Apologies in advance (but not really): I’m going to be very “CO-centric” (though not “CO-exclusive”). The reason, which I hope you’ll appreciate is that CO - by far the brightest mm line - is already very sensitivity limited at extragalactic distances.

Observing Nearby Galaxies at mm s

What There Isn’t To See In The Milky Way: H2

CO

Dust

H2

UV absorption studies: great! But requires background source, probe pencil beam.Rotational line emission (IRS), sensitive only to gas down to T~100K (most H2 much colder).

Next most common molecule after H2.Standard tracer of H2 in high-mass galaxies.At low metallicities C and O less abundant and dust shielding weak. CO suppressed?Comes with velocity information - dynamics offer another way to trace mass.

Probes total gas column modulo dust-to-gas ratio.Absorption tough because of lack of clear background screen.Emission limited by finite resolution of IR telescopes.

rays Cosmic ray hits nucleus, produces ray… modeled to yield total nuclei column density.Major resolution and sensitivity challenge beyond Milky Way, modeling complex…

H2 molecule lacks a dipole moment and most H2 is too cold to excite the lowest rotational transitions. This makes tracing the star-forming part of the interstellar medium a challenge.

What There Is To See In The Milky Way: CO

Short answer for mm line work: Giant Molecular Clouds Dame+ 01

Galactic Ring Survey Jackson+ 06

Orion Molecular CloudWilson+ 05

(Giant) Molecular Clouds

Key properties of molecular clouds*:

o Distinct (in real and P-P-V space) dense clouds containing a significant mass of H2.

o Clumpy (multiscale structure / cloud-clump-core) and turbulent.

o High mass (giant) clouds gravitationally bound, low mass clouds confined by external pressure.

o Sites of all present-day star formation.

o Power law mass spectrum, dN/dM M-1.5 N (M > M) M-0.5

(top-heavy, but maybe variable across the Local Group)

o Obey a set of basic scaling relations (“Larson’s Laws”)

* Reasonably established, but there are open questions on each point here.

Review: Blitz ‘93

What There Is To See In Nearby Galaxies

M33 (Engargiola+ 03, BIMA)

Color: Atomic gas

Green dots: Bright CO

LMC (Fukui+ 99, NANTEN)

Color: Stars and H

Contours: CO

What There Is To See In Nearby Galaxies

M31 (Nieten+ 06; IRAM 30m)

Left: CO from Andromeda

Giant Molecular Clouds in Galaxies

o Typical sizes: few 10s of parsecs

o Line widths: few km/s

o Surface density (brightness): ~100 Msun pc-2 (10-20 K km s-1)

o Mass: ~105 to few times 106 Msun

o To first order, cloud in other galaxies look like Milky Way GMCs:

Radius [parsecs]

Lin

e W

idth

[k

m s

-1]

CO Luminosity

Vir

ial M

ass 2

R [

Msu

n]

Bolatto+ ‘08

Starbursts and Associations of GMCs

Resolution and sensitivity matched to a single GMC are largely limited to the Local Group (especially for a single dish telescope).

Shen & Lo ‘95 (M82, left), Roslowsky & Blitz (M64, right); Wilson+ (Antennae, bottom)

Starbursts show GMC-like properties (or more extreme) over scales of few hundred pc to kpc. No longer analogs to collections of MW GMCs.

Aalto+ ‘99: collections of GMCs in M51

On various scales, most mapping of other galaxies relies on getting several clouds per beam.

Well Beyond CO

R. Genzel (1991)

The low rotational transitions of 12CO are the best available simple tracer of distribution of H2 mass. More sophisticated mm spectroscopy can reveal physical conditions (density, temperature) and refine estimates of H2.

Well Beyond CO

Martin+ ‘06: IRAM 30m

NGC 253 (nearby starburst)

111 lines, 25 species

When things are bright and dense enough, there’s lots of information there! (see S. G-B)

But CO is Still Your Benchmark / Starting Point

Sutton+ ‘85: 1mm line survey of Orion A

Pointing towards the bright part of a nearby GMC:

12CO J=2-1 : Tpeak ~ 110 K

Next brightest line:

13CO J=2-1 : Tpeak ~ 35 K

Other lines down by larger factors.

…

The same thing is true in galaxies:

o 12CO J=2-1 (230 GHz) and J=1-0 (115 GHz) are the brightest lines.

o Typically this is followed by the same transitions in 13CO (lower ; intensity down by a factor of ~6 in the Milky Way).

o Then HCN, HCO+, CN, HNC, CS, C18O, etc.

o Intensities for these species seldom much higher than 1/10 CO.

Example: HCN in Nearby Galaxies

log 10

HC

N t

o 12

CO

Lin

e R

atio

Courtesy A. Usero (in prep.)log10 12CO Intensity [K km s-1]

The numbers on the y-axis are low!

Line ratio in a normal galaxy disk ~ 0.03.

Observations towards ~ kpc2 regions in the disks of nearby, massive, star-forming galaxies.

What’s Been Done (very roughly)

Millimeter line work on nearby galaxies:

o CO pointings or sampling towards ~500 - 1000 galaxies.

o Complete/resolved CO maps of ~100 nearby galaxies.

o HCN/HCO+ pointings towards ~100 nearby galaxy centers.

o HCN/HCO+ maps of a handful of nearby galaxies.

o Multitransition studies of a handful of nearby galaxy centers.

o CO maps able to identify individual GMCs in a handful of galaxies.

Looking at Other Galaxies: Numbers

For the next few slides we’ll look at other the intensity of other galaxies as observed with the 30m. We’ll do this in three ways:

o In terms of individual molecular clouds …

o In terms of integrated molecular mass (luminosity) …

o In terms of surface density …

We’ll focus on the 12CO J=1-0 transition. As we’ve just discussed, this (and the corresponding J=2-1 transition) is the brightest available transition by a factor of several.

Looking at Other Galaxies: Units

UnitWhat Does It Measure?

Notes

K

brightness temperature (a way to

phrase specific intensity)

• be careful when switching to and from Jy beam-1 (factor of 2)!

K km/s integrated intensity

• brightness temp. integrated over velocity.• with XCO yields a surface density.• usually quoted as an average over a telescope beam.

K km/s arcsec2 flux

• brightness temp. integrated over velocity and angular area.• independent of telescope beam.

K km/s pc2 luminosity

• brightness temp. integrated over velocity and physical area• with XCO yields an integrated molecular mass.

Looking at Other Galaxies: Conversion Factors

XCO : assume a linear conversion between CO and H2 to hold on large scales. This “X-factor” is the conversion.

Milky Way value ( rays, dust emission, virial mass):

XCO = 1.5 - 3 1020 cm-2 (K km s-1)-1

But be careful! XCO is an approximation. It does not hold perfectly within Galactic clouds and different values are used in starburst and dwarf galaxies.

Using XCO and the units just described, we can then convert:

o Integrated Intensity [K km/s] to Surface Density [Msun pc-2]

o Luminosity [K km/s pc2] into Molecular Mass [Msun]€

ΣH2 Msun pc-2[ ] = 3.2 I co K km s-1

[ ] Xco

2×1020 cm -2 (K km s-1)-1

⎛

⎝ ⎜

⎞

⎠ ⎟

€

MH2 Msun[ ] = 3.2 Lco K km s-1 pc2[ ]

Xco2×1020 cm-2 (K km s-1)-1

⎛

⎝ ⎜

⎞

⎠ ⎟

N.B., this is only H; apply another factor of ~ 1.36 to include He.

Looking at Other Galaxies: GMC Perspective

How bright are giant molecular clouds in other galaxies?

What is the line-average intensity (in K) you expect pointing the 30m at a GMC in a nearby galaxy?

1. Luminosity of a GMC K km s-1 pc2

2. Line width of a GMC km s-1

3. Size of a cloud (or the beam) pc2

Line-average intensity K


Line width (FWHM) ~ 3 - 20 km s-1

v varies systematically with mass.

For this exercise we’ll take

vFWHM ~ 10 km s-1

for all cases.

(N.B., plot at left is using RMS line width)

Luminosity of Molecular Clouds

Highest mass Milky Way GMCs ~ 106 K km s-1 pc2

High Mass SF Region (like Orion) ~ 105 K km s-1 pc2

Low Mass SF Region (Like Taurus) ~ 104 K km s-1 pc2

Low mass, pressure-confined cloud ~ 103 K km s-1 pc2

Radius [parsecs]

Lin

e W

idth

[k

m s

-1]


Physical area being integrated over:

o GMC area (via mapping) if bigger than telescope beam.o Telescope beam if bigger than cloud size.

Angular area of 30m beam ~ 520 / 115 GHz2 arcsec2

In parsecs2 vs. distance:

Local Group

Nearest Cluster

ULIRGS

Other Groups

Typical cloud size ~ 30 pc

Area ~ 103 pc2

30m beam relevant scale (i.e. clouds unresolved) beyond Local Group.

Different case for interferometers.

€

Abeam pc2[ ] ≈1.2×104

115 GHz

ν

⎛

⎝ ⎜

⎞

⎠ ⎟2

d

1 Mpc

⎛

⎝ ⎜

⎞

⎠ ⎟

2

Area of a typical GMC


Take line width ~ 10 km s-1 for all cases.

Take cloud luminosity.

Take 30m physical beam size (function of distance).

1 hour ON source with EMIR(single pixel 30m receiver) at 115 GHz, 10 km s-1 channel:

~ 3 mK

Recall radiometer formula:

t-0.5

Ref. + overheads add x 2-4

€

ICO K[ ] ≈ 0.8 Lco

105 K km s-1 pc2

⎛

⎝ ⎜

⎞

⎠ ⎟

d

1 Mpc

⎛

⎝ ⎜

⎞

⎠ ⎟

−2Δv

10 km s-1

⎛

⎝ ⎜

⎞

⎠ ⎟−1

~3 in 1h ON

Avera

ge I

nte

nsit

y A

cro

ss L

ine [

K]


Some things to take away:

o Individual CO-emitting structures are faint at extragalactic distances.

o Individual clouds only resolved inside the Local Group by the 30m.

o Individual low mass clouds only detectable inside Local Group.

o Individual high mass clouds detectable with effort in nearest other groups.

o Observing distant galaxies requires averaging many clouds inside a beam (that’s okay, beam has large spatial area too).

We’ve used 12CO as our example, scale to get other lines.

Looking at Other Galaxies: Galaxy Perspective

How bright are whole galaxies in CO?

Spatial size of the 30m beam gets very big at extragalactic distances.

It is easy to find yourself with most of a galaxy inside a beam.

In this case, the relevant thing is the CO luminosity of the galaxy, LCO.

How to make a good guess at LCO?

o We’ve already seen that many galaxy properties are strongly covariant with one another. This include CO luminosity.

o Both stellar luminosity and infrared luminosity (or some other tracer of star formation rate) are good places to start.


CO

per

Ste

llar

Lu

min

osty

(B

-ban

d)

Stellar Luminosity [Magnitudes]

Dots: relatively massive star-forming galaxies.

For relatively massive (M* > 1010 Msun) star-forming (blue) galaxies, CO per unit starlight is relatively fixed.

There are important exceptions to this, especially o low metallicity dwarfs o gas-poor ellipticals o CO-bright mergers… but it remains a good starting point.


For actively star-forming galaxies, CO is clearly related to star formation (above traced by IR, but this works for other tracers, too).

Details vary a bit:o In normal disks, a fixed CO-to-IR ratio is probably pretty much OK.o Very vigorous star-formers have less CO per IR.o Quiescent and low-metal galaxies may also have less CO per IR.

Gao & Solomon ‘04

Sta

r Form

ati

on

Rate

[fr

om

IR

]

H2 Mass from CO


A simple scaling from the Milky Way is often a good place to start. From Allen’s:

Property Milky Way Value

H2 Mass

(CO Luminosity)

2-6 109 Msun

(0.5 - 1.4 109 K km s-1 pc2)

Star Formation Rate ~ 3 Msun yr-1

Stellar Mass(B band Luminosity)

4-8 1010 Msun

(2.3 1010 Lsun)

Then (very) roughly:

o H2-per-SFR ~ 1-2 Gyr (nearby spirals also yield ~ 2 Gyr)o H2-per-Stellar Mass ~ 0.03 - 0.1


Let’s run the same exercise we did with the clouds: LCO <ICO>

o take H2-per-Stellar Mass ~ 0.05o take XCO = 2 1020 cm-2 (K km s-1)-1

o assume the galaxy fits inside the 30m beam.o assume FWHM = 200 km s-1*

* A (much) better way is to use TF and inclination to estimate line width.

Avera

ge I

nte

nsit

y A

cro

ss L

ine [

K]

Distance [Mpc]

1 hour ON source with EMIR(single pixel 30m receiver) at 115 GHz, 200 km s-1 channel:

~ 0.7 mK

Recall radiometer formula:

t-0.5

Ref. + overheads add x 2-4

~3 in 1h ON



o When all CO emission from a galaxy is in the beam, the 30m can detect galaxies out to a large distance, though the time investment is still not trivial.

o We have ignored metallicity effects, enhancement due to intereactions. In reality, low mass galaxies are hard to see at all and mergers can be very bright.

o Unlike GMCs, it isn’t always straightforward to get all of the galaxy inside the beam. We’ll talk a bit about the intermediate case next…

We’ve used 12CO as our example, scale to get other lines.

Looking at Other Galaxies: Surface Density

Often, especially when mapping, you are in the intermediate regime:

Many GMCs per beam, but not a sizable piece of the galaxy.

Relevant quantity here is H2 surface density or CO surface brightness.

We’ve talk about how we might estimate the CO luminosity of a galaxy, but how is this luminosity distributed?

HERA (30m) maps of some normal spiral galaxies at ~10 Mpc. The spatial resolution is a kpc and the mass in each resolution element is more than 106 Msun, implying collections of GMCs.


A typical CO scale length in a massive star forming galaxy (like the Milky Way)

Size of a typical Milky Way Giant Molecular Cloud

We’re in this regime from just beyond the Local Group (can see clouds in SMC, LMC, M33, M31) to several 10s of Mpc (past which even big galaxies are essentially point sources).


In star-forming galaxies, CO surface brightness varies strongly with radius.

Right:

o Profiles of integrated intensity (black points) vs. galactocentric radius for 7 galaxies mapped by HERA on the 30m.

o Lines show exponential fits.

o Higher (gray) profile shows peak surface brightness in the ring.


Azimuthally averaged variations in CO emission tend to track those in stellar surface density or star formation.

Typical CO scale length ~ 0.2 r25.

CO Scale Length [kpc]

Scale

Len

gth

At

Oth

er

[kp

c]

Radius

CO

In

ten

sit

y (

poin

ts)

an

d S

tellar

Pro

file

(lin

e)

+ a

rbit

rary

off

set

Regan+ 01


Kiloparsec-scale averages of CO emission tend to track IR emission or other tracers of recent star formation. For a local guess of the CO surface brightness this is probably a reasonable place to start (but beware metallicity issues).

HERA 30m (color, Bigiel+ ‘08) + Literature (gray, Young+ ‘95, Elfhag+ ‘96, Murgia+ ‘02, Leroy+ ‘05)

CO Surface Brightness

Recen

t S

tar

Form

ati

on

(IR

or

RC

)



o Azimuthally averaged, CO emission looks pretty similar to stars (there are important differences, but this is a good place to start): an exponential decline with a scale length ~0.2 to 0.25 times the optical radius.

o The exponential decline is a mix of filling factor (e.g., arms vs empty space) and decline in the peak integrated intensity (arms get fainter).

o The local star formation rate or IR surface brightness are reasonable ways to guess at the surface brightness of CO on fairly large (~ kpc) scales.

We’ve used 12CO as our example, scale to get other lines or use the well-established HCN-IR (or an analogous HCO+-IR) relations.

Millimeter Continuum

We’ve focused on lines, what about the continuum at 1-3mm?(though many 30m observing modes deliberately filter out continuum via references).

o 3mm is a low point in the SED of a (non-”monster”) galaxy. Emission from longer than ~ 1mm makes up < 10-4 of the bolometric luminosity.

o Dust emission dominates at shorter : R-J tail + -dependent emissivity

o Synchrotron dominates at longer : declines with Fn ~ -0.8

o Thermal free free “fills in the gap”: emission from ionized gas, p-e collisions

Condon ‘92

M82

(Difference is nu*Fnu on left vs. Fnu on right)

Dust Continuum

Dust emission dominates at shorter : R-J tail + -dependent emissivity

The Sombrero galaxy at optical (right/bottom) and millimeter wavelengths (top left: LABOCA 870 mm, top right: MAMBO2/30m 1.2mm). Millimeter continuum traces dust, getting notably weaker with wavelength (RJ tail + emissivity). Vlahakis+ 2008.

€

Fν = 2ν 2kT

c2

⎛

⎝ ⎜

⎞

⎠ ⎟1− e−τ( ) with τ ∝ λ-β

o usually (very) optically thin o = 1 - 2o know/estimate T and (the mass absorption coefficient relating to mass) can get dust mass.o more points: sophisticated fits to models or constrain , , T simultaneously

Dust Continuum

Dust emission utility:

o dust and gas well mixed: dust is an optically thin, relatively robust tracer of the ISM

o dust SED fueled by young star formation (mostly), estimate recent SF (better near 60 or 100 m)

o disentangling dust temperature and mass allows estimate of heating radiation field

o dust to gas ratio relevant to many aspects of star formation and interpreting observations

The Sombrero galaxy at optical (right/bottom) and millimeter wavelengths (top left: LABOCA 870 mm, top right: MAMBO2/30m 1.2mm). Millimeter continuum traces dust, getting notably weaker with wavelength (RJ tail + emissivity). Vlahakis+ 2008.

Free Free Continuum

Thermal free free emission (electrons accelerated by protons):

€

≈ 3.3 ×10-7 104 K

Te

⎛

⎝ ⎜

⎞

⎠ ⎟

1.35

1 GHz

ν

⎛

⎝ ⎜

⎞

⎠ ⎟2.1

nenp∫ dl

TB = Te 1− e−τ( )

(N.B. these equations already assume RJ approximation and h < kTe; okay for mm … but there is a cutoff in the spectrum once h ~ kTe).

Temperature of electrons in the emitting (HII) region Observed frequency

Emission measure: integrated density squared along the line of sight ( # of recombinations).Observed brightness temperature.

See Tielens ‘05

Millimeter Continuum

Physical information from thermal free free emission:

o Estimate the rate of ionizing photons hitting the region producing the emission.

o Emission measure number of recombinations, so…

€

Nuvs-1

⎛

⎝ ⎜

⎞

⎠ ⎟≥ 6.3×1052

Te104 K

⎛

⎝ ⎜

⎞

⎠ ⎟-0.45

ν

1 GHz

⎛

⎝ ⎜

⎞

⎠ ⎟0.1

Lff

1020 W Hz -1

⎛

⎝ ⎜

⎞

⎠ ⎟

(Note the unit change on Lff!)

See Condon ‘92

… why is this interesting? Ionizing photons are produced only by young stars, making this an effective, easily interpreted tracer of recent star formation (in practice this is slightly easier at longer wavelength than in the millimeter, e.g. K band).

Wrap Up

1. Millimeter spectroscopy of other galaxies:

• Best available tool to study distribution of H2. • CO strongest line by several factors (ncrit, abundance).

• Weaker lines give physical conditions (or fraction of dense, excited gas).

2. Sensitivity calculations / feasibility estimates:

• through the lens of GMCs.

• through the lens of H2 surface densities.

• for whole galaxies.

Optically thinner, high excitation, high ncrit lines: down by factor 5-40+ …

3. Continuum:

• mixture of free-free and dust. Weak, often filtered out by design. Carries info about ionizing radiation, dust temperature and dust mass (AW).

nearby galaxies (mostly) at mm and ir wavelengths adam leroy (mpia heidelberg) christof buchbender...

Documents

o mass

o intensities

o distinct

o sites

galaxies o typical sizes

highmass galaxies

o high mass giant clouds

chunks of galaxies