HI Regions

Karina Caputi

Interstellar Medium 2015-2016 Q3Rijksuniversiteit Groningen

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The ISM around a Young Star

Picture credit: http://www.astro.cornell.edu

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Ionisation in HI Regions

mainly metals are ionisedC is the fourth most abundant element of the ISM

Neutral fraction of C atoms:

x(C0) =!rr(C+)ne

!rr(C+)ne + P (C0)

...but X rays and cosmic rays can ionise HI

Ionised fraction of H atoms:

x(H+) =[(! + " + xM )2 + 4!]1/2 ! (! + " + xM )

2, ! =

PCR(1 + #s)

$rrnH, " " $gr

$rr

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Heating of HI Regions

dominated by e- produced by photoelectric effect on dust grains

Heating Rate:

⇣CR(1 + �s)nH [1� x(H+)] = ↵rr(H+)n2H [x(H+) + x(M+)]x(H+) + ↵gr(H+)n2

Hx(H+) , (4)

where ↵rr and ↵gr are the rate coe�cients for radiative recombination, and grain-assisted re-combination, respectively, of H in this case.

Note: M are all species with ionisation poetential < 13.6 eV, i.e. the most-abundant metals.

This is a quadratic equation for x(H+) with solution:

x(H+) =[(� + � + xM )2 + 4�]1/2 � (� + � + xM )

2, (5)

where

� ⌘ ⇣CR(1 + �s)↵rrnH

(6)

� ⌘ ↵gr

↵rr(7)

The coe�cient ↵gr depends on the charge state of the grains, which in turn depends on theelectron density ne.

1.4 Warm HI regions: ionisation of hydrogen

In WNM, T ⇡ 5000 K. Most H ionisation is due to cosmic rays, as it is in CNM. The percentageof ionised H is very small here too: only ⇠ 2% of H is ionised in warm HI regions.

2 HI clouds: heating and cooling

2.1 Heating of HI gas

As we saw in a previous lecture, the photoelectric e↵ect on dust grains dominates theheating of di↵use HI in the Milky Way. The typical work function for graphite is 4.50 eV, sophotons with this energy or more can photoionise large carbonaceous grains.

The heating rate due to dust is:

�pe

nH⇡ 1.4⇥ 10�26 erg

s

n(8� 13.6 eV)3⇥ 10�3cm�3

� h�absi10�21cm2

hY i0.1

(hEpei � hEci)1 eV

, (8)

where n(8� 13.6 eV) is the number density of photons with energies within 8� 13.6 eV,h�absi is the total dust photoabsorption cross section per H nucleon, averaged over the 8-13.6 eV

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Cooling of HI Regions

mainly through [CII] emission at 158 µm and [OI] emission at 63 µm

Picture credit: KINGFISH Cambridge website (R. Kennicutt et al.)

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Contribution of different lines to cooling rate of HI

Picture credit: B. Draine’s book (fig. 30.1 - upper panel)

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Two phases for HI regions

Picture credit: B. Draine’s book (fig. 30.2b)

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Tspin determination with background sources

Picture taken from M. Hogerheijde‘s lecture

7.3 H I emission-absorption studiesStudy extended cloud in front of extragalactic radio source

Observe two positions (on-source and off-source = blank)

Assume that cloud is uniform ⇒ properties of H I are the same in source and blank positions

Radiative transport | 21cm line | HI studies | Two-phase ISM | Optical absorption line widths | & observations

H I emission-absorption studies (cont’d)Measure on-line and off-line at each position

Toff (blank) = Tbg

Ton(blank) = Tbg e!! + TS (1 ! e

!! )

Ton(source) = Tsrc e!! + TS (1 ! e

!! )

Toff (source) = Tsrc

Ton(source) ! Ton(blank)

Toff (source) ! Toff (blank)=

(Tsrc ! Tbg) e!!

Tsrc ! Tbg= e

!!

Ton(source) ! Toff (blank)

1 ! e!!+ Toff (blank) = TS ! Tbg + Tbg = TS

Radiative transport | 21cm line | HI studies | Two-phase ISM | Optical absorption line widths | & observations

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18Thursday, October 21, 2010

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Observations of HI Regions

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21cm Radiation

directly measure HI column density

Kalberla et al. (2005)

Galactic HI

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HI Absorption in NGC7469

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HI in a Cosmological Context

Picture credit: J. Liske (ESO)

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HI Distribution in the Milky Way

Nakanishi & Sofue (2003)

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Absorption Lines in the Optical Regime

fringing, which can become quite severe in some cases beyondk ! 8500 8. As we explain later, we only perform line meas-urements on individual spectra at k < 8500 8, and all themeasurements between k " 8000 and 8500 8 have beenmanually checked on an individual basis.

Stacked spectra can give a good qualitative idea of the meanoptical spectra of 24 !m galaxies. Figures 4, 5, and 6 show therest-frame average stacked spectra of our 24 !m galaxies with"L24 !m" <1010 L#, 10

10 L# < "L24 !m" <1011 L#, and "L24 !m" >1011 L#, respectively, in different redshift bins. We constructedthe stacked spectra on sets of 38Y89 galaxies, depending on theredshift and IR luminosity bin. The zCOSMOS BLAGNs havebeen excluded for the stacking. The wavelength resolution in allthe stacked spectra is 28 (rest frame).We only show the spectra upto rest-frame wavelengths corresponding to observed k P 85008;i.e., we excluded the regions most affected by fringing. However,as we show in x 7, even the longest wavelength regions showreasonably good average spectra when sufficient numbers ofsources are stacked. This is due to the fact that the fringing patternadds incoherently for different sources, so no systematic noiseis propagated into the average stacked spectra.

We obtained the average spectrum in each bin by renormalizingthe individual rest-frame spectrum of each source to the averagevalue of a featureless region of the continuum (which was chosendepending on the redshift bin). In this way, all the individualspectra in a given redshift and IR luminosity bin are put on a samescale before stacking.We smoothed out regions lying on top of themain atmospheric absorption lines, except when source emissionlines were present. At each rest-frame wavelength, we excludedthe 5% smallest and largest values before computing the average.This sigma-clipping procedure helps to clean the stacks for pos-sible remaining spurious lines in the individual spectra.

These average spectra of 24 !m galaxies in different redshiftand IR luminosity bins show the following:

1. As expected, all emission lines characterizing star-forminggalaxies are present. We note that these emission lines are aproperty of the average spectra of IR galaxies, but we do notnecessarily observe all these lines in every spectrum on an indi-vidual basis (because, e.g., they are much extincted by reddeningin some cases). On the average spectra, we also see some ab-sorption lines characteristic of old stellar populations, as Na Dor Ca ii H and K. This means that different generations of starsare present in many IR galaxies.

2. The average line ratios vary as a function of IR luminosityand redshift. We further analyze this point in x 7.

3. High-order Balmer absorption lines are clearly present in24!mgalaxies. These lines are produced by short-living (P1Gyr)A-type stars. This indicates that 24 !m galaxies not only areinstantaneously forming stars but have been also forming starsfor some time during the last gigayear. We explore this issue inmore detail in x 9.

Although the stacked spectra give a good qualitative idea ofthe average spectral properties of 24 !m galaxies of different IRluminosities and redshifts, they do not contain information aboutthe variety of strengths of the spectral features among 24 !m gal-axies in a same redshift and luminosity bin.

We measured the fluxes and equivalent widths (EWs) of emis-sion lines in our zCOSMOS spectra by direct integration on therest-frame spectra.We do notmeasure emission lines lying beyondobserver-frame k " 85008, to avoid being severely affected byfringing. For lines at observer-frame 8000 8 < k < 8500 8 ,we manually checked the measurements on an individual basis

Fig. 4.—Composite average zCOSMOS spectra of 24 !m galaxies with"L24 !m

" < 1010 L# in different redshift bins. From top to bottom, the numbers ofstacked galaxies are 74, 64, and 38, respectively.

OPTICAL SPECTRA OF 24 !m GALAXIES IN COSMOS. I. 943No. 2, 2008

Fig. 6.—Composite average zCOSMOS spectra of 24 !m galaxies with"L24 !m" > 1011 L! in different redshift bins. From top to bottom, the numbersof stacked galaxies are 42, 86, and 89, respectively.

Fig. 5.—Composite average zCOSMOS spectra of 24 !m galaxies with1010 L! < "L24 !m" < 1011 L! in different redshift bins. From top to bottom, thenumbers of stacked galaxies are 70, 57, and 42, respectively.

Caputi et al. (2008)

Average optical spectra of mid-IR-selected galaxies

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Absorption Lines in Rest UV Spectra

Steidel et al. (2010)

Average optical spectra of UV-selected galaxies at z~2.3The Circum-Galactic Medium at z = 2−3 9

FIG. 5.— Composite rest-frame far-UV spectra for two independent samples of z ! 2.3 galaxies. The top panel is an average of the 89 spectra in the H!sample, with R = 800, after normalizing each to the same relative intensity in the range 1300–1500 Å. The bottom panel is a composite of 102 galaxy spectraobtained with higher spectral resolution (R = 1330), shifted into the rest frame using equations 2 and 4, and scaled as the first sample before averaging.

composites presented above: maximum blue-shifted veloci-ties of |vmax ! 800 km s−1 , roughly independent of ionizationlevel.

4.3. Trends with Baryonic MassReturning to the trends in the centroid velocities of the IS

lines noted in the previous section, it is instructive to exam-ine the mean line profiles of composite spectra selected byimplied baryonic massMbar, the parameter most significantlylinked to the observed kinematics of the interstellar absorp-tion features. Figure 10 shows the comparison between thehalves of the H! sample that are above and below an inferredMbar = 3.7" 1010 M!, the sample median. There are cleardifferences in the Ly! emission line strength, which is simi-lar to that seen for sub-samples of different metallicity as inErb et al. (2006a). The peak of the Ly! emission line profileis shifted by ! +200 km s−1 (from ! +400 to ! +600 km s−1 )for the higher mass sub-sample relative to that of the lower-mass sub-sample.14 The profiles of the low-ionization inter-stellar lines (CII "1334 is in the cleanest spectral region and

14 No significant correlation was found between Mbar and !vLy! in §3,but many of the galaxies in the higher mass sub-sample had Ly! emissionthat was too weak to measure, resulting in a very small sample.

illustrates it best) may be indicating the root cause of the kine-matic trends discussed in §3: the higher mass sample exhibitsstronger IS absorption at or near v# 0, while the profiles arenearly identical in their behavior near v = −|vmax|. This “ex-cess” low velocity material in the higher-mass sub-sample – ashift of ! 200 km s−1 in the red wing of the IS line profiles–systematically shifts the centroid of the IS velocity distribu-tion by ! +100 km s−1 relative to the lower mass sub-sample,while the blue wing of the profile exhibits no clear trend withMbar. The excess low-velocity material in the higher masssub-sample is not obvious in the C IV absorption profile, forwhich the 2 profiles appear to be nearly identical for v >

# 0(note that C IV was not used to measure !vIS for any of thegalaxies in the sample, because of the dependence of the rest-wavelength for the blend on the relative strength of the linesof the doublet).Fig 11 shows the residual apparent optical depth!# (v) for

3 relatively isolated low-ionization transitions for the high–and low– Mbar sub-samples. By this we mean the additionaloptical depth as a function of velocity that when added to theline profiles of the low-Mbar sub-sample would produce lineprofiles identical to those of the high-Mbar sub-sample, i.e.,

Ihm(v) = Ilm(v)e−!! (v) (5)

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Emission of Dust within HI Regions

Maps taken from http://mwmw.gsfc.nasa.gov

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