Starformation on Galactic Scales: NormalGalaxies

Gordon J. Stacey

Cornell University

Broad Definition of Normal Galaxies - Need starformation todetect in the far-IR - include M83, NGC 6946, M51, and NGC 891

Focus on infrared observations with an emphasis on far-infraredspectroscopy

n Airborne Observations: Paving the way

n ISO: A new perspective

n SOFIA: Resolving (some) of the issues

First Far-IR Line from Normal Galaxies:The [CII] 158 µm Line from Milky Way

Stacey et al. 1985

• [CII] Line Predicted to Dominate Cooling of AtomicClouds (Dalgarno and McCray 1972)

• Early Lear Jet Observations showed that the linewas very bright and extended over Galactic GMCs(Russell et al. 1979)

• Cut Across the Galactic Plane With Lear Jet

• Very Good Correlation with CO Emission

• Most (70%) [CII] Line Emission Arises From thePhotodissociated Surfaces of Molecular Clouds

• Insufficient HI Column Density -- not atomicclouds

• Insufficient free-free emission measure -- notextended low density (ELD) aka WIM HIIRegions

• L[CII] ~ 6 ×× 107 L¤¤, or 0.3 % Lfar-IR

• M[CII] ~ 109 M¤¤, or ~ 10% MISM

First KAO Observations:[CII] Emission from Galaxies

[CII] Emission from ~ 6 Starburst Nuclei (Crawfordet al. 1985), from 20 galaxies that included NormalGalaxies & some mapping (Stacey et al. 1991)

• [CII] line is a dominant cooling line: ~ 0.1 to 1%of the far-infrared luminosity

• Most [CII] does not arise from atomic clouds orHII regions: there is insufficient HI columndensity, and the EM is too small.

• [CII] line produced in PDRs that may compriseup to ~ 40% of the ISM

• Much of the 21 cm line emission may arise fromthese PDRs --- atomic gas may be the product, notprecursor, of star formation

Stacey et al 1991

The [CII]/CO (1 → 0) Line Intensity Ratio

The [CII] and CO Line Intensity are Correlated

• The line ratio, R ~ 4400 for starburst nuclei andGalactic Starformation Regions

⇒⇒ Elevated Tex for (1 →→ 0) line affecting the COline intensity to H2 mass conversion factor, X

• Ratio, R ~ 1200 for non-starburst galaxies andGalactic GMCs

Forms a link between ISM in normal galaxies andGalactic GMCs, and, lends some confidence in X

for normal galaxies

[CII]/CO line ratio may be a good indicator ofstarformation activity

Examples or Exceptions:

LMC: R ~ 40,000 Understood in terms ofmetalicity effects (more below...)

NGC 6240: R ~ 1600 Not a starburster? --- but seeISO results...

Stacey et al 1991

Photodissociation Regions:Theoretical Context

Models address the brightness of the[CII] Line, and its observed correlationwith the far-infrared continuum and theCO (1 →→ 0) line (Teilens & Hollenbach1985, Wolfire, Hollenbach, & Tielens1989)

• N(C+) limited by extinction of ionizingphotons to AV ~ few

• Heating in PDRs by photoelectriceffect -- up to ~ 1% efficient

• Gas cooling by [CII] and [OI] (63 µµm)lines

• Far-IR continuum arises in PDRs

• H2 Self-shielding brings the H/H2transition forward

• Co lies in a relatively thin transitionregion between C+ and CO

Hollenbach and Tielens, 1998

Ratios (YCO ≡≡ ICO/Ifar-IR, Y[CII] ≡≡ I[CII]/Ifar-IR,)eliminate beam filling factors

• Typically, χχUV ⇔⇔ Go ~ 100 to few 1000, andn(H2) ~ few 103 for galaxies

• The [CII]/CO ratio is constant for starburstnuclei and Galactic starformation regions:explained as a saturation effect:

• for n(H2) > 3 ×× 103, and Go > 3 ×× 103,ICOand I[CII] approach a constant

• [CII]: Tgas > Tlevel, n > ncrit, N(C+) ~constant because of dust attenuation

• CO: Optically thick ∴∴ sensitive toTgas, but the line arises from deeper,cooler regions of the cloud that arerelatively insensitive to Go.

• Comparing Go to Ifar-IR yields the beamfilling factor

Photodissociation Regions: Physical Parameters

Assuming [CII], CO and the far-IR continuum arise in PDRs, one can use their ratiosto derive the physical conditions of the emitting clouds (Wolfire et al. 1989,Sternberg and Dalgarno, 1989, Stacey et al 1991):

Stacey et al 1991

[CII] Imaging of GalaxiesComplete images of “normal” spiralgalaxies with FIFI (NGC 6946, M83, M51,NGC 891)

• [CII] ubiquitous and bright ~ 1% Lfar-IR

• [CII] Enhanced in Spiral Arms:Emission likely from dense PDRs

• Likewise, Nuclear emissiondominated by dense PDRs

• But… diffuse component betweenthe spiral arms. Likely associated with:

• Atomic clouds (n(H) ~ 30 to 100cm-3, Tgas ~ 100 K) or,

• Low-density (ne ~ few cm-3) HIIregions

Possibly in near equal proportions

If dominated by atomic cloud emission,then we have measured the coolingrate of atomic clouds!

Madden et al. 1993

HI [CII] far-IR CO

KAO work included [CII] maps of theLMC (Poglitsch et al 1995, Israel et al.1996) and IC 10 (Madden et al. 1997).These galaxies have metalicities ~ 1/6to 1/4 solar. These studies find:

• [CII] line is typically very bright. Formany regions, comparisions with CO(1→→ 0), and the far-IR continuum indicateemission from standard PDRs.

• However, in several regions, e.g. the30 Doradus region of the LMC, the[CII]/CO(1 →→ 0) ratio is anomalouslyhigh -- up to 60,000!

• The excess [CII] is not coming fromatomic or ionized gas - not enough.

• The relatively bright [CII] emission isbest understood as a metalicity effect

Poglitsch et al. 1993

[CII] Mapping of Dwarf Irregulars

30 Doradus Region: [CII] Emission(black) overlayed on CO (1 →→ 0)

Metalicity Effects on Molecular Clouds

• N(C+) not affected by z, since N(C+) governed bydust extinction.

• But, the linear penetration of carbon ionizing (andCO photodissociating) photons is much larger(assuming the dust to gas ratio scales with z).

•Therefore, the CO emitting core of the low metalicitycloud is relatively small

•However, the size of the molecular cloud itself isessentially unchanged since the H2 molecule is self-shielding.

⇒⇒ one can have CO free molecular clouds -- greatlyaffecting the CO luminosity to molecular massconversion factor (cf. Cohen et al. 1988, andJohansson et al. 1990).

Solar z cloud

Low z cloud

CO Core

H2 Cloud

CO Core

PDR

PDR

KAO Multi-line Observations: Starburst Galaxies

Due to sensitivity issues, there very very fewobservations of normal galaxies in lines other than[CII]

However, extensive studies of starburst systemssuch as M82 (Lord et al. 1996), and NGC 253, andNGC 3256 (Carral et al. 1994) in multiple lines wereundertaken:

e.g [CII], [OI] (63 & 145 µµm), [OIII] (52 and 88 µµm),[NII] (122 and 205 µµm), [NIII] (57 µµm), [SiII] (35 µµm),and [SIII] (18 and 33 µµm).

This ensemble of lines can more fully characterizethe physical conditions of the ISM and the localstellar population:

• abundances (e.g. [NIII]/[OIII] line ratio)

• densities (e.g. [NII], [OIII], and [SIII] pairs)

• gas pressure (e.g [OI] pair)

• Stellar UV field strength and hardness (e.g.[NII]/[NIII] and [SIII]/[OIII] pairs)

Rubin, 1989

Results, M82Neutral Lines: [CII], [OI], [SiII]

Lord et al. 1996, Lugten et al. 1986

• Twin emission lobes, with diameters ~ 125 pc

• Highly fragmented ISM: Large number (~ 3 ×× 105)of cloudlets

• T ~ 230 K, n ~ 104 cm-3 ⇒⇒ P ~ 2-3 ×× 106 cm-3K

• rclump ~ 0.4 - 1.0 pc ⇒⇒ Mclump ~ 600 M¤¤

• MPDR >10% Mmolecular

• Go ~ 103

• Held in pressure equilibrium by HII regions

• φφVolume~ 0.1, φφArea~ 7 to 20

• Fragmentation, photoionization, andphotodissociation will soon destroy the natalenvironment, ending the starburst

• [SiII] Arises from HII regions, with solar Siabundances, half in grains…

Lord et al. 1996

Results, M82Ionized Gas Lines: [OIII], [SIII], [NII], [NIII],

[SiII]

Houck et al. 1982, Lugten et al. 1986, Duffy et al.1987, Colbert et al. 1999

• Ionized gas within neutral emissionlobes

• Effective Temperature of ionizing stars ~35,000 K

Ratio of [NII]/[NIII]

Ratio of [OIII]/[SIII] lines

• ne ~ 200 cm-3: much higher than theMilky Way

• MHII ~ 4 ×× 107 M¤¤

• O++/N++ ~ 3.8 -- higher than inner Galaxy(~ 0.9-1.4, Dinnerstein et al. 1984)

• Lower N abundance?

• Ionization effect-- more N++ forlower Teff starsCarral et al. 1994

The Milky Way GalaxyCOBE Launched in 1988

FIRAS Experiment (Wright et al. 1991):

• Michelson Interferometer: 1 to 100 cm-1 at ∆ν∆ν ~ 0.7 cm-1

• Beam size ~ 7 degrees

• Confirmed estimates of [CII] and Far-IR Luminosities

(Stacey et al. 1985, Gispert et al. 1982)

L[CII] ~ 5.2 ×× 107 L¤¤,, and Lfar-IR ~ 1.8 ×× 1010 L¤¤,

or 0.3 % Lfar-IR

• Surprisingly:

• [NII] 122 µµm: L[NII] ~ 8 ×× 106 L¤¤, or 0.045 % Lfar-IR

• [NII] 205 µµm: L[NII] ~ 5 ×× 106 L¤¤, or 0.028 % Lfar-IR

• Ionization Potential of N ~ 14.5 eV ⇒⇒ Ionized Gas

• Line ratio ⇒⇒ Low density (~ 10 to 35 cm-3) HII Regions

• [CII]/[NII] ratios indicate ~ 50% [CII] ELD HII Regions!

Wright et al. 1991

Petuchowski & Bennet 1993

The Milky Way Galaxy: Balloon Experiments

Balloon-borne Infrared CarbonExplorer (BICE)

• Survey in [CII] line:

• 350° < l < 25°

• |b| < 3°

• Beam: 15’

• Ubiquitous, but tightlycorrelated with far-IRcontinuum

• Diffuse component in [CII]

• I[CII]/Ifar-IR ~ 0.6%, for diffuseregions, but

• I[CII]/Ifar-IR ~ 0.2%, forstarformation regions

• I[CII]/Ifar-IR very low forGalactic Center Region

Nakagawa et al. 1998

Results from the Infrared SpaceObservatory (ISO)

ISO Made significant Ccontributions to our understandingof the normal galaxies with each instrument

ISOPHOT

ISOCAM

SWS

LWS

Here I will focus on LWS Spectroscopy

• 60 cm cryogenic space telescope

• (strangely) Constant beam at ~ 70” for LWS)

• Improved (5 to 10 times) sensitivity over KAO

• Access to entire spectrum from 43.0 to 196.9 µµm

• Spectral Resolution ~ 200 for the LWS grating

• First results published in A&A 315 (1996)

ISO LWS Observations of Normal Galaxies: NGC 5713

ISO Key Project, the Interstellar Medium in Normal Galaxies (Helou et al. 1996).

NGC 5713 is an isolated Sbc spiral galaxy at 24 Mpc, multiple spiral arms

Lfar-IR ~ 2 ×× 1010 L¤¤, confined to 30” region

• LWS L02 spectra of 6 lines: L[CII]/Lfar-IR ~ 0.7% L[OI]/Lfar-IR ~ 0.3%

L[OIII]/Lfar-IR ~ 0.2%

• Weak [NII] ⇒⇒ very little [CII] from ELD HII regions

Also true for Virgo cluster spirals (Smith & Madden, 1997)

• Strong [OI] very little [CII] from atomic clouds

• PDR Origin for [OI] & [CII]lines, n ~ 1.6 ×× 104, Go ~ 630

• Lower limit on [OIII] 88/[OIII]52 µµm ratio ⇒⇒ ne< 1000 cm-3

• Lower limit on [NIII] 57/[NII]122 µµm ratio ⇒⇒ Teff > 33,500 to35,000 K

Lord et al. 1996

Deficit of [CII] Emission in Normal Spiral Galaxies

Malhotra et al. 1997

Observed [CII] from 30 normal starforming galaxies

Take R = L[CII]/Lfar-IR: cooling of gas to that of dust, ie. photoelectic heating efficiency

• Most (2/3) of the sample fall in a narrow range R ~ (2-7) ×× 10-3, but..

• R varies by a factor of 40: Remaining 1/3 show decreasing R with increasingF(60)/F(100), and with increasing starformation activity, as measured by Lfar-IR/LB

• Three far-IR bright galaxies undetected at R < 0.5 to 2 ×× 10-4

• Trend due to decreased photoelectric heating efficiency with increasing Go due tobuild up of dust grain charge --- Lowest ratios ⇒⇒ Go/n > 10 cm3

Low R could also be due to:

• Self absorption in [CII]

• Strong far-IR fromregions weak in [CII], e.g.dense HII regions, AGNtype UV fields, or soft UVradiation fields

• Analogue with weak [CII]from ULIGS (Luhman et al.1998)

ISO Mapping of Fine-Structure Lines in Normal Galaxies

KAO [CII] Map (Madden et al. 1993)

55” beam

ISO [CII] Map (Contursi et al. 1999)

80” beam

LWS Fine-structure Line Mapping of M83

KAO: 55” Beam (Geis ISO: 80”Beam (Stacey et al. 1999)

L[CII] ~ 8.8 ×× 107 L¤¤ ⇔⇔ 0.3% Lfar-IR

As part of the LWS Core Program, the ISM in Normal Spiral Galaxies (Smith et al.)M83 was mapped (7 ×× 8 grid, 45” spacing) in the:

[CII] 158 µµm, [OI] 63 & 145 µµm, [OIII] 88 µµm, and [NII] 122 µµm lines

[CII] 158 µµm Line

Overall Properties, Spiral Arms

[CII]: 158 µµm [NII]: 122 µµm[OI]: 63 µµm

• Total line flux ~ 2.1 × 108 L¤, or 0.7% Lfar-IR

• [CII]/[NII] line ratio ⇒ ~ 37% (nucleus), ~30% (spiral arms), and 27% (inter-arm regions) of the [CII] arises from ELD HII regions• [CII]/[NII]/[OI] line ratios the same across spiral arms -- mix in the ISM(PDRs/ELD/atomic clouds) constant across arms and inter-arm regions• Large [OI]/[CII] ratio ⇒ Very little [CII] from “atomic clouds”. The bulk of the[CII] emission arises from PDRs• I[OI] 63/I[OI] 146 ~ 20, I[OI] 63/I[CII] ~ 0.7 ⇒⇒ Go surprisingly high (~ 3000), nH ~ 3000 cm-3

• Spiral arms/inter-arm contrast highest for [OIII] 88 µµm line ⇒⇒ earliest type starsreside in the spiral arms ⇒⇒ star formation occurs in the spiral arms.

• At bar/spiral arm interfaces, [OI], [CII], & [OIII] strongly enhanced ⇒⇒ greatlyenhanced starformation activity

[OI] 146 µµm and [OIII] lines as strong as they are at the nucleus!!

• I[OI] 63/I[OI] 146 is very small ( ~ 8 ±± 1.5) at the SW bar-spiral arm intersection⇒⇒ Go ~ 105, and nH ~ 104 cm-3 ⇔⇔ Orion interface region 0.2 pc from ΘΘ1C

• The SW bar region strong in Hαα and CO as well (e.g. Kenney & Lord, 1991)

Orbit crowding likely triggers a massive burst of starformation

[CII]: 158 µµm [OIII]: 88 µµm[OI]: 63 µµm

Bar-Spiral Arm Intersections

[OI] 63 µµm [OIII] 88 µµm [NII] 122 µµm

[OI] 145 µµm

[CII] 158 µµm

M83 Nucleus

.

M83 Nucleus

• I[OIII] 52/I[OIII] 88 ~ 1.05 ⇒⇒ ne ~250 cm-3 - ⇔⇔M82, >> the Galaxy (ne~3 cm-3).

• I[NIII]/I[NII] ~ 0.9 - << M82 (~ 2.1, Colbert etal. 1998) ⇒⇒ MS headed by later type stars(Teff ~ 35,000 K, O9, Rubin 1985) in M83than for M82 (Teff ~ 37,000, O8) ⇒⇒ olderpopulation of stars.

• I[NIII]/I[OIII] 52 µµm ~ 0.67 ⇒⇒ N/O ~ 2.4 solar(Rubin, 1985) ~ abundance ratios in theinner Galaxy (Lester et al. 1987). EnhancedN/O ⇒⇒ more processing of the ISM.

• I[OI] 63/I[OI] 146 ~ 17, I[CII]/I[OI] 63 ~ 0.9 ⇒⇒ PDRsin the nucleus are:

• dense, nH ~ 1000 cm-3

• exposed to high Go, ~ 1000

•Total line flux (dominated by the [CII] and [OI] 63 µµm lines) is ~ 0.5% Lfar-IR .

Far-IR Spectra of “Normal”Galaxies

M83 Nucleus

M51 Nucleus

[OI] 63 µµm [OIII] 88 µµm [NII] 122 µµm

[OI] 145 µµm

[CII] 158 µµm.

.

.NGC 891 Nucleus

OH 119 µµm

Far-IR Spectra of Starburst orULIG Galaxies

Fischer et al. 1999

UIR Band Emission in Normal GalaxiesISOPHOT-S Observations of NGC 891

• NGG 891: edge on Sb spiral at ~ 9 Mpc

• UIR bands at 6.2, 7.7, 8.6, and 11.3 µµm arebright: ~ 9% of total LfaR-IR

• Traceable over the entire galactic disk

• Likely arises from PDRs, can obtain very highspatial resolution images of PDRs

ISOPHOT-S Spectra of 43 Normal Galaxies

UIR emission is ubiquitous, but depend weakly ongalaxy parameters such as the far-IR color ⇒⇒emitting particles are not in thermal equilibrium

Redshift indicator for galaxies at z ~ 3 with SIRTF

Mattila, Lehtinen, & Lemke 1999

(Helou et al. 1999)

Pure Rotational H2 Line Emission from Normal Galaxies

• Pure rotational H2 line emissionobserved from the inner regions of NGC6946

• Tgas ~ 170 K, Mgas ~ 5 ×× 106 M¤¤

• Warm component ~ 5 to 10% of the totalmolecular mass

• Confined to a 135 pc diameter region thatis heated by a modest nuclear starburst

Valentijn et al. 1996

Fine-Structure Lines: [OI] 63 and 145 µµm, [CII],[OIII] 51.8 and 88 µµm, [NII] 122 and 205 µµm, [NIII]

FIFI-LS

• [CII] Beam 14”

• [OI] Beam 6”

• Scale Height of Neutral ISM --- Energetics

• SNR ~ 100 in 15 minutes

• Map in [CII] in 2 to 3 hours

• Map in [NII] in 20 hours

[CII] Beam

[OI] Beam

FIFI-LSFOV

SOFIA Imaging of Spiral Galaxies:NGC 891

SOFIA Imaging of Face-on Spiral Galaxies: M83

KAO Map in [CII] 55” Beam (Geis

6” Resolution CO (1-0) MapOverlayed on false-color HI (Rand

Lord, & Higdon 1999

SOFIA Will Easily Resolve [OI] and [CII] Line Emission from Spiral Arms⇔⇔ Starformation in Spiral Galaxies

SAFIRE Field of View:Would require ~ 10pointings, each 1 hour

Imaging of M83 inSubmillimeter Lines

Mid-J rotational lines of COtrace warm, dense ISM inPDRs near starformationregions--ratio with 1-0 traces nand Go (Kaufman et al. 1999)

[CI] lines trace mass and gastemperature in PDR (or withinmolecular clouds?)

Broad lines best detected bydirect detection multiplexedspectrometer, e.g. SAFIRE

Bradford et al. 2000

The End? -- No, the Beginning...