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

Working Group:

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

A Broad Look at Nearby Galaxies

Nearby Galaxies at Millimeter Wavelengths

Mapping Nearby Galaxies With the IRAM 30m

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?

Cosmology

Nearby galaxies give you:

spatial resolution sensitivity, wavelength coverage input for simulations baseline for comparison

… compared to high z / simulation.

Galactic Star Formation

Nearby galaxies give you:

wider range of environments zoomed out view / statistics no distance ambiguity easier to isolate a particular environment

… compared to the Milky Way.

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GALEX view of M81 HST view of M51

HST view of M64

SINGS IRAC

NGC 3627 NGC 3351 NGC 7793 NGC 2976

swiped from A.P.o.D. & NASA heritage websites

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HST view of M51

Spitzer view of the SMCK. Gordon et al.

VLA (HI) view of NGC 2403F. Walter

30m Map of M51K. Schuster et al.

30m Map of M63

Cartoon Anatomy of A Galaxy

Dark Matter HaloHot Ionized Halo Gas

Stellar Disk

Stellar Bulge

Warm Ionized Gas

Young stars

DustMolecular Gas

Atomic Gas

How To Study the Cartoon AnatomyDark Matter Halo:kinematics Hot Ionized Halo Gas:

X-Rays, Absorption

Stellar Disk and Bulge:Optical, NIR

Warm Ionized Gas:line emission, radio/mm-cont

Young stars:UV, optical cont

Dust: IR emission, opt/UV absorption

Molecular Gas:mm lines,(especially CO),UV absorption,dust

Atomic Gas:21cm line,UV and radio absorption

Galaxy Components Observable with the 30mDark 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 andmillimeter dust continuum

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 neutralcold neutral

neutralAtomic (HI)

0.5 cm-3 50 cm-3

104 K102 K

21cm lineIR cooling lines

molecularmolecular(H2)

100+ cm-3 20 K

Molecular lines(CO, HCN, HCO+, CS, etc.)Dust, H2 rotational lines

Cartoon Breakdown of the ISM

* 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.

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 neutralcold neutral

neutralAtomic (HI)

0.5 cm-3 50 cm-3

104 K102 K

21cm lineIR cooling lines

molecularmolecular(H2)

100+ cm-3 20 K

Molecular lines(CO, HCN, HCO+, CS, etc.)Dust, H2 rotational lines

Cartoon Breakdown of the ISM

* 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 GalaxyUV Optical near-IR mid-IR far-IR sub-mm

0.1 1 10 100 1000Wavelength (m)

Flux (log

F n)

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

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.

Dale+ 2007

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.Remember: mm & radio don’t carry appreciable energy!

Turning the SED into Physical Information

UV

Continuum:Young Stars

UV Absorption:HI, H2, metals

Optical

Continuum and Absorption:Stellar Mass, Age, Metallicity

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

Continuum:Hot/small dust

Band Features:PAH modes

Far-IR

Continuum:Dust

Lines:Atomic ISM Cooling

Millimeter

Continuum:Dust, Ionized Gas

Lines:Molecular Mass,Dens., Temp.

Radio

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 MatterStellar Luminosity (left: B band, right: K band)

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.

Meyer+ 2008 following Tully & Fisher 1977

Scaling Relations: Starlight and Dark Matter

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.

Velocity Dispersion

Stellar Luminosity

Face On

Edge On

Jorgensen+ 1996After Faber & Jackson 1976

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.

Scaling Relations: Size and Luminosity/Mass

The size of a galaxy (here stellar half-light) is a clear function of its mass.

Galaxy Size (Optical)

Scatter About Relation

Stellar 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

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).

Optical/UV Color

Blue

Red

Blue

Red

Scaling Relations: Mass and Star FormationStar Formation per Stellar Mass

Stellar Mass

Salim+ 2007

Star Formers Non-star Formers

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.

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 Star Formation

Scaling Relations: Mass and Metallicity

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Tremonti+ 2004Lee, Bell, and Somerville 2008-2009

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.

Metal Abundance (Gas Phase)

Stellar Mass

Metal Abundance (Stellar)

Stellar Mass

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: Mass and Metallicity

Scaling Relations: Gas and Star FormationKennicutt 1998

Stars Formed per Area per Time

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 FormationWong & Blitz 2002 Kennicutt+ 2007Bigiel+ 2008

Inside galaxy disks star formation correlates with CO (H2) more clearly than HI

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Blue: HIBlack & Green: CO

Stars Formed per Area per Time

HI per Area HI per Area

CO per Area CO per Area

Scaling Relations: Gas and Star Formation

Gao & Solomon 2004

Emission from dense gas (HCN) shows a linear correlation with star formation

Even where the correlation between CO and star formation is non-linear

Wu et al. 2005

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Infrared Luminosity ~ Star Formation Rate

Emission From High Density Molecular Gas

Galaxies

Milky Way Cores

Infrared Luminosity ~ Star Formation Rate

Emission From High Density Molecular Gas

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.

Scaling Relations: Gas and Star Formation

Scaling Relations: Radio and FIR Emission

Condon 1992 Yun+ 2001

(Non-thermal) Radio continuum luminosity correlates very tightly with IR luminosity

1.4 GHz (mostly nonthermal) Continuum Luminosity

Infrared Luminosity (IRAS Satellite)

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?”

Scaling Relations: Stellar Mass and Gas

Low mass galaxies have more HI relative to stellar mass than high mass galaxies

HI Mass (21cm) per Stellar Luminosty (B-band)

Hubble Type

S0 S0a Sa Sab Sb Sbc Sc Scd Sd Sm Im

Low MassHigh Mass

Roberts & Haynes 1994

Scaling Relations: Stellar Mass and Gas

The ratio of H2 to stellar mass does not vary strongly in relatively massive galaxies

CO per Stellar Luminosty (B-band)

Stellar Luminosity [Magnitudes]

Scaling Relations: Stellar Mass and Gas

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

CO per Stellar Luminosty (B-band)

Stellar Luminosity [Magnitudes]Star Formation per Area per Time

CO per Unit Area

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