Nearby Galaxies
(mostly) at mm and IR wavelengths
Adam Leroy (MPIA Heidelberg)
Christof Buchbender (IRAM Granada)
Liberally plundering .ppt by:
Eric Bell, Hans-Walter Rix, James Graham
Topics
A Broad Look at Nearby Galaxies
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.
A Broad Look at Nearby Galaxies
Goal: Briefly survey the components of galaxies, how these are observed, how they
relate to one another, and how mm and IR observations fit into the picture.
Gloss over: nuclei (S.G-B.), detailed phase balance (C.K.), B. Fields (C.T.)
Specific Topics:
o Why study nearby galaxies?
o Key components of a galaxy and its ISM? Which are observable from the 30m?
o What does the zoomed out SED of a galaxy look like? Where do IR and mm fit in?
o Scaling relations in nearby galaxies and their relation to IR and mm work.
Apologies in advance (but not really): I’ve tried to stay IR and mm focused, but I may drift a bit
into other wavelengths in the interests of a more complete cartoon.
Why Study Nearby Galaxies?
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Galactic Star Formation
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Cosmology
Nearby galaxies give you:
Nearby galaxies give you:
 wider range of environments
 zoomed out view / statistics
 no distance ambiguity
 easier to isolate a particular environment
 spatial resolution
 sensitivity, wavelength coverage
 input for simulations
 baseline for comparison
… compared to the Milky Way.
… compared to high z / simulation.
NGC 3627
NGC 3351
NGC 7793
NGC 2976
SINGS IRAC
HST view of M64
GALEX view of M81
HST view of M51
swiped from A.P.o.D. & NASA heritage websites
VLA (HI) view of NGC 2403
F. Walter
Spitzer view of the SMC
K. Gordon et al.
30m Map of M63
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30m Map of M51
K. Schuster et al.
HST view of M51
Cartoon Anatomy of A Galaxy
Dark Matter Halo
Hot Ionized Halo Gas
Warm Ionized Gas
Atomic Gas
Young stars
Stellar Bulge
Dust
Stellar Disk
Molecular Gas
How To Study the Cartoon Anatomy
Dark Matter Halo:
kinematics
Hot Ionized Halo Gas:
X-Rays, Absorption
Warm Ionized Gas:
line emission,
radio/mm-cont
Young stars:
UV, optical cont
Dust:
IR emission,
opt/UV absorption
Stellar Disk and Bulge:
Optical, NIR
Molecular Gas:
mm lines,
(especially CO),
UV absorption,
dust
Atomic Gas:
21cm line,
UV and radio
absorption
Galaxy Components Observable with the 30m
Dark Matter Halo:
using kinematics traced by line emission
Warm Ionized Gas:
via mm free-free continuum
Dust:
via the millimeter continuum
Molecular Gas:
using millimeter lines and
millimeter dust continuum
Cartoon Breakdown of the ISM
Phase
H State
Density
Temp.
Emission Diagnostics*
hot
ionized
10-2 cm-3
106 K
X-ray
warm (H II)
Ionized (HII)
1 cm-3
104 K
optical emission lines
warm neutral
cold neutral
neutral
Atomic (HI)
0.5 cm-3
50 cm-3
104 K
102 K
21cm line
IR cooling lines
molecular
molecular
(H2)
20 K
Molecular lines
(CO, HCN, HCO+, CS, etc.)
Dust, H2 rotational lines
100+ cm-3
* In addition to these diagnostics, absorption against background sources from the UV to the
radio is an incredibly powerful diagnostic of physical conditions in the ISM.
Cartoon Breakdown of the ISM
Phase
H State
Density
Temp.
Emission Diagnostics*
hot
ionized
10-2 cm-3
106 K
X-ray
warm (H II)
Ionized (HII)
1 cm-3
104 K
optical emission lines,
mm continuum
warm neutral
cold neutral
neutral
Atomic (HI)
0.5 cm-3
50 cm-3
104 K
102 K
21cm line
IR cooling lines
molecular
molecular
(H2)
20 K
Molecular lines
(CO, HCN, HCO+, CS, etc.)
Dust, H2 rotational lines
100+ cm-3
* In addition to these diagnostics, absorption against background sources from the UV to the
radio is an incredibly powerful diagnostic of physical conditions in the ISM.
Spectral Energy Distribution of A Galaxy
Right: SED of a massive, metal-rich
star-forming galaxy (like ours):
o energy ~ half optical (stellar black
body), half IR (dust black body) …
o shape ~ mix of black bodies
(broad), and narrow features (lines) …
o other shape at very long 
(synchrotron, thin free free) …
o mm and radio emission is a footnote
(useful as a tracer of conditions).
swiped from E. Bell
Spectral Energy Distribution of A Galaxy
UV Optical near-IR mid-IR
far-IR
sub-mm
NGC 6822: A star-forming, low metal, low
dust, low mass Local Group dwarf:
- high UV relative to IR
- high UV relative to optical / near-IR
- low IR relative to optical / near-IR
Flux (log  Fn)
Dominated by young star-light.
NGC 7331: A star-forming, metal-rich, low
dusty, spiral galaxy (like the Milky Way):
- low UV relative to IR
- low UV relative to optical / near-IR
- high IR relative to optical / near-IR
Dominated by reprocessed young star-light.
NGC 4594: A very early-type spiral (almost
elliptical).
- comparable UV and IR
- low UV relative to optical / near-IR
- low IR relative to optical / near-IR
Dominated by old starlight.
0.1
Dale+ 2007
1
10
Wavelength (m)
100
1000
Remember: mm & radio don’t
carry appreciable energy!
Turning the SED into Physical Information
UV
Optical
Continuum:
Young Stars
Continuum and Absorption:
Stellar Mass, Age, Metallicity
UV Absorption:
HI, H2, metals
Emission Lines:
Warm Ionized Medium
Near-IR
Continuum:
Stellar Mass
Absorption:
Dust mass (hard)
Lines:
As optical
Turning the SED into Physical Information
Mid-IR
Far-IR
Millimeter
Radio
Continuum:
Hot/small dust
Continuum:
Dust
Continuum:
Dust, Ionized Gas
Band Features:
PAH modes
Lines:
Atomic ISM Cooling
Lines:
Molecular Mass,
Dens., Temp.
Continuum:
SN Remnants,
B Field
Lines:
HI Column
Same galaxy, axes, longer wavelength range (from NED)
Scaling Relations
1. “How galaxies are” …
… these are basic observational facts about galaxies.
2. Drive science …
… how do scaling relations evolve with z?
… what physics cause them?
3. Affect observations; e.g., …
… low metals means less dust, less CO
… high mass means high CO/HI, red means little SF
Scaling Relations: Starlight and Dark Matter
Stellar Luminosity (left: B band, right: K band)
Meyer+ 2008 following Tully & Fisher 1977
Maximum Rotation Velocity (from HI profile)
The stellar luminosity of a spiral galaxy is tightly correlated circular velocity:
Circular velocity driven by the mass of the dark matter halo hosting the
galaxy. So halo mass and galaxy mass are intimately related. Considering all
baryons (not just stars) needed to make it work for low-mass galaxies.
Scaling Relations: Starlight and Dark Matter
Jorgensen+ 1996
Velocity Dispersion
After Faber & Jackson 1976
Face On
Edge On
Stellar Luminosity
Ellipticals also show basic relations between star light and dark matter:
“Fundamental plane” or Faber Jackson relation. Best-fit relation for ellipticals
has three (rather than two) parameters.
Scaling Relations: Starlight and Dark Matter
Relation to Millimeter and IR astronomy:
o Millimeter lines can be used to trace galaxy kinematics
(and thus the dark matter distribution).
o If you know the mass / optical magnitude of a galaxy,
you can guess its line width with reasonable accuracy.*
o If you know the line width of a galaxy from line
observations, you can estimate its distance or at least
check for consistency.*
o Ellipticals, more ambiguous…
* With the caveat that CO is more compact than HI and may not trace
the whole potential.
Scatter About Relation
Galaxy Size (Optical)
Scaling Relations: Size and Luminosity/Mass
Stellar Mass
The size of a galaxy (here stellar half-light) is a clear function of its mass.
Scaling Relations: Size and Luminosity/Mass
Relation to Millimeter and IR astronomy:
o The size of the molecular gas disk is fairly tightly
coupled to the size of the stellar disk. So this is is also
(roughly) a way to guess the distribution of H2.
o Exceptions: LIRG/ULIRGs and ellipticals tend to have
central molecular disks with scales of hundreds of
parsecs, not matched to stellar disk.
o Why? Related to ability to build a stellar disk.
Scaling Relations: The Galaxy CMD
Red
Blue
Blue
Optical/UV Color
Red
Salim+ 2007 (following lots of SDSS stuff, e.g., Kauffmann, Blanton, Hogg)
The galaxy population is strongly bimodal:
Most galaxies are either blue star formers or “red and dead” (with a less
populated “green valley” in between).
Scaling Relations: Mass and Star Formation
Salim+ 2007
Non-star Formers
Star Formation per Stellar Mass
Star Formers
Stellar Mass
Star formation is largely a function of the stellar mass of a galaxy:
Low-mass galaxies show more star formation per unit mass. More massive
are bimodal, a mixture of red non-star formers and star formers.
Scaling Relations: Mass and Star Formation
Relation to Millimeter and IR astronomy:
o The millimeter continuum (free free) and infrared (dust)
continuum both allow us to measure the amount of
recently formed stars without worrying about dust.
Infrared is absolutely key to many current star formation
tracers.
o Millimeter lines are the most straightforward way to
trace the star-forming ISM. Although it isn’t in this plot
directly, tracing the distribution and evolution of gas in
galaxies is key to understanding why galaxy populations
have this basic behavior.
Scaling Relations: Mass and Metallicity
Lee, Bell, and Somerville 2008-2009
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Stellar Mass
Metal Abundance (Stellar)
Metal Abundance (Gas Phase)
Tremonti+ 2004
Stellar Mass
Low mass galaxies have less heavy elements relative to their mass:
There is a strong relationship between stellar mass and heavy element
abundance (gas phase & stellar) spanning many orders of magnitude.
Scaling Relations: Mass and Metallicity
Relation to Millimeter and IR astronomy:
o The infrared continuum is a key tracer of the distribution of
dust and the dust-to-gas ratio is intimately related to heavy
element enrichment (e.g., you need dust to see IR!).
o Along similar lines, millimeter line tracers of the ISM are key
to robustly measure the dust-to-gas ratio in large systems.
o CO (and other molecules) are known to be suppressed
relative to other galaxy components at low metallicity. A
robust guess as to the metallicity is helpful to plan
observations.
o In reverse: the effect of metallicity on the ISM and star
formation is of considerable interest. This relation allows one
to readily guess metallicity from mass.
Scaling Relations: Gas and Star Formation
Stars Formed per Area per Time
Kennicutt 1998
Gas (HI + H2) per Area
More gas means more star formation for actively star-forming galaxies:
Averages over galaxy disks yield a tight correlation between star formation
rate and gas content.
Scaling Relations: Gas and Star Formation
Stars Formed per Area per Time
Wong & Blitz 2002
Bigiel+ 2008
Kennicutt+ 2007
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CO per Area
CO per Area
Blue: HI
Black & Green: CO
HI per Area
HI per Area
Inside galaxy disks star formation correlates with CO (H2) more clearly than HI
Gao & Solomon 2004
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Emission From High Density Molecular Gas
Infrared Luminosity ~ Star Formation Rate
Infrared Luminosity ~ Star Formation Rate
Scaling Relations: Gas and Star Formation
Wu et al. 2005
Galaxies
Milky Way Cores
Emission From High Density Molecular Gas
Emission from dense gas (HCN) shows a linear correlation with star formation
Even where the correlation between CO and star formation is non-linear
Scaling Relations: Gas and Star Formation
Relation to Millimeter and IR astronomy:
o Both axes… IR is key to trace recent star formation
(and mm can help).
o Millimeter lines almost the exclusive tracer of molecular
gas distribution.
o Combinations of lines (ideally up to the sub-mm) can
give physical conditions (density, temp.) in the H2.
o It’s almost impossible to study the relationship between
gas and star formation without integrally involving the IR
and millimeter lines.
1.4 GHz (mostly nonthermal) Continuum Luminosity
Scaling Relations: Radio and FIR Emission
Condon 1992
Yun+ 2001
Infrared Luminosity (IRAS Satellite)
(Non-thermal) Radio continuum luminosity correlates very tightly with IR luminosity
Scaling Relations: Radio and FIR Emission
Relation to Millimeter and IR astronomy:
o The IR part of the “radio-IR” correlation.
o Cartoon of star formation, supernova, cosmic rays,
synchrotron implies a connection to star forming gas (but
beware “conspiracies”)… mm lines may help address
“why?”
HI Mass (21cm) per Stellar Luminosty (B-band)
Scaling Relations: Stellar Mass and Gas
Roberts & Haynes 1994
S0
S0a
Sa
Sab
Sb
Sbc
Sc
Scd
Sd
Sm
Im
Hubble Type
High Mass
Low Mass
Low mass galaxies have more HI relative to stellar mass than high mass galaxies
CO per Stellar Luminosty (B-band)
Scaling Relations: Stellar Mass and Gas
Stellar Luminosity [Magnitudes]
The ratio of H2 to stellar mass does not vary strongly in relatively massive galaxies
CO per Stellar Luminosty (B-band)
Star Formation per Area per Time
Scaling Relations: Stellar Mass and Gas
Stellar Luminosity [Magnitudes]
CO per Unit Area
The ratio of CO to stellar mass or star formation does vary strongly at low metallicity
Red circles: low mass, low metallicity galaxies
But is this because you have less CO or less H2?
Mizuno+ 01; Wilke+ 03; Young+ 95; Kennicutt 98; Elfhag+ 96; Gondhalekar+ 98; Boker+ 01; Murgia+ 02; Taylor+ 98; Leroy+ 05
Scaling Relations: Stellar Mass and Gas
Relation to Millimeter and IR astronomy:
o Obviously (again) millimeter lines are key tracers of
molecular mass.
o At the same time a warning that millimeter lines are not
perfect tracers of H2.
o Small galaxies have less CO/HI and more HI/stars, why?
Wrap Up
1. Why are nearby galaxies interesting?
Environment, Statistics, Perspective, (plus very pretty!)
2. What are the major constituents of galaxies?
Young/Old Stars, Gas (HII, HI, H2), Dark Matter
3. What does the zoomed-out SED of a galaxy look like?
Quiescent/Star-Forming, Embedded/Unobscured
4. How do you pull physical information about #2 from #3?
Radio/mm Lines & Continuum, Dust Emission, Starlight
5. What are some of the basic galaxy scaling relations?
Tully-Fisher, Mass-SFR, Mass-Metallicity, Gas-SFR, Mass-Gas