dust and molecules in early galaxies: prediction and strategy for observations

Dust and Molecules in Early Galaxies: Dust and Molecules in Early Galaxies: Prediction and Strategy for ObservationsPrediction and Strategy for Observations

Tsutomu T. TAKEUCHILaboratoire d’Astrophysique de Marseille, FRANCE

Part I General Introduction

Part II Dust Emission from Forming Galaxies

Part III IR Absorption Line Measurement of H2 in Early Galaxies

Summary

Contents

Part I General Introduction

Heavy element production

Star formation

Dust formation

UV FIRDust

Short wavelength photons are scattered and absorbed by dust grains and re-emitted as FIR radiation.

Dust and Molecules in Early Galaxy Evolution Dust and Molecules in Early Galaxy Evolution

1. Absorption and Re-Emission of Radiation by Dust

Dust works as a catalyst for the formation of molecular hydrogen (H2).

Molecular formation is closely related to the star formation activity. Especially in an early phase of galaxy evolution, H2 molecules are very important coolant of gas to contract to form stars.

Dust controls the early stage of star formation history in galaxies! (Hirashita et al. 2002; Hirashita & Ferrara 2002).

Without dust, star formation does not proceed effectively.

2. Dust as a Catalyst of H2 Formation

Motivation

For understanding the physics of galaxy formation and early evolution, and testing various models, it is crucial to measure their physical quantities related to the metal enrichment, dust production, and molecular gas amount.

1. Young systems with active dust production

We focus on the two systems:

2. Dense systems with little metal/dust

Observation of the continuum radiation from dust.

Measurement of H2 through IR absorption lines.

Part II Dust Emission from Forming

Galaxies

Dust Emission Model of Forming Galaxies Dust Emission Model of Forming Galaxies

1. Model for dust production and radiation

1.1 Dust supply in young galaxies

Dust formation in low-mass evolved stars (RGBs, AGBs, and SNe Ia) is negligible in a galaxy we consider here, because of the short timescale (age < 108 yr). Dust destruction can also be negligible by the same reason.

We can safely assume that only SNe II contribute to the dust supply in young, forming galaxies.

We solve the star formation, evolution of the strength of UV radiation field, metal enrichment, and dust production in a self-consistent manner.

1.2 Basic framework of our dust emission model

1. Dust supply: Nozawa et al. (2003)

2. Star formation history: one-zone closed-box model with the Salpeter IMF (0.1 < M < 100 Msun).

3. Supernova rate: calculated from star formation rate

Galaxy formation and evolution

Physics of dust grains

1. Optical properties of grains: Mie theory

2. Specific heat of grains: multidimensional Debye model

3. Radiative processes, especially stochastic heating of small grains are properly considered and included

A little more about Nozawa et al. (2003)

Nozawa et al. (2003) proposed a theoretical model of dust formation by SNe II whose progenitors are initially metal-free.

Two extreme scenarios are considered for the internal structure of the helium core of the SN progenitor.

Unmixed case: the original onion-like structure of the elements is preserved.

Mixed case: all the elements are uniformly mixed in the He core.

We show the results for both cases in the following.

Based on the SFR and dust size spectrum, the total SED is constructed by a superposition of the radiation from each grain species.

Construction of the SED

Spherical SF region with radius rSF surrounded by dust

Radiation field strength is calculated from LOB and rSF

LOB evolves according to the SF history.

Considering the self-absorption by dust, the final SED is obtained as

2. Results

Infrared SED (rSF=30pc, SFR=1.0Msunyr-1)

Infrared SED (rSF=100pc, SFR=1.0Msunyr-1)

The extinction curve of forming galaxiesAge t = 107yr.

We will use these extinction curves also in Part III.

Blue compact dwarf (BCD)

Distance : 54 Mpc

Very metal-poor : Z ~ 1/41 Zsun

Very active star formation:

SFR = 1.7 Msun yr-1 (Hunt et al. 2001)

Very young stellar population: < 5 Myr (Vanzi et al. 2000)

Very hot dust: Tdust > 80 K

Very strong extinction: AV = 12-30 mag

3. Observational Implications3.1 A Local ‘Young Galaxy’ SBS 0335-052

Comparison of the models with the observed SED

The observed dust SED is roughly reproduced by the model of unmixed case.

If gas collapses on the free-fall timescale with a SF efficiency SF (we assume SF=0.1), SFR of a galaxy is basically evaluated as follows (Hirashita & Hunt 2004):

Typical physical parameters for high-z small galaxies

3.2 Quest for Forming Dusty Galaxies

If we consider a small clump with gas mass of 108 Msun and adopt rSF = 30pc and 100pc, we have a typical SFR of 10 Msunyr-1. We use these values for the estimation of dust emission from a genuine young galaxy.

Expected flux for a forming subgalactic clump at high-z

Herschel confusion limits by Lagache et al. (2003).

ALMA detection limits (64 antennas, 8 hours).

Natural huge telescope: gravitational lensing

If we consider a strong gravitational lensing by a cluster at zlens=0.1–0.2 with dynamical mass of 5×1014 Msun, it becomes feasible to detect such galaxies (magnification factor ~ 30–40).

We can expect 1–5 events for each cluster at these redshifts.

Expected flux for a forming subgalactic clump at high-z II

Part IIIIR Absorption Line Measurement

of H2 in the Early Galaxies

H2 molecules: the predominant constituent of dense gas.

IR Absorption Line Measurement of HIR Absorption Line Measurement of H22 in the in the

Early GalaxiesEarly Galaxies1. Basic Idea

Local Universe

Molecules containing heavy elements (e.g., CO, etc) are good tracers of the amount of H2.

High-z systems in their first star formation

They are very metal-poor, and we need a special technique for measuring the amount H2 directly..

Petitjean et al. (2000) and subsequent studies showed a direct measurement of H2 in UV absorption lines. Their target is damped Lyman- absorbers (DLAs).

Transition probability A of ionizing/dissociation lines is so large that they are useful for detecting thin layers and small amounts of the molecular gas, but not useful for detecting dense gas clouds, as those of our interest.

Then, H2 has well-known vibrational and rotational transitions in the IR. Their transition probabilities are very small because the H2 is a diatomic molecule of two identical nuclei, and has no allowed dipole transitions.

The vib-rotaional and rotational line emission of H2 are useful for analyzing dense (n > 10 cm-3) and hot (T > 300 K) gas.

Unfortunately, direct measurement of the H2 emission lines is very difficult for distant galaxies (Ciardi & Ferrara 2001).

If, however, there is a strong IR continuum source behind or in the molecular gas cloud, absorption measurements of these transition lines can be possible (Shibai, Takeuchi, Rengarajan, & Hirashita 2001, PASJ, 53, 589)!

Such observation will be feasible by the advent of the proposed space missions for large IR telescope, like SPICA, etc.

SPICA: One of the Observational Possibilities

SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is the next-generation IR mission, which is to be launched by the Japanese HIIA rocket into the L2 point.

This mission is optimized for M- and FIR astronomy with a large (3.5 m), cooled (4.5 K) telescope. The target year of launch is 2010.

http://www.ir.isas.jaxa.jp/SPICA/index.html

2. Calculation

Assume a uniform cool gas cloud with kTex << hthen the optical thickness of the line absorption is expressed as

where u and l: upper and lower states, gu and gl: degeneracy of each state, Aul: Einstein’s coefficient, Nl: column density of the molecules in the lower state, and V: line width in units of velocity.

Assumption: almost all the molecules occupy the lowest energy state.

Absorption line flux in the extinction free case is

where is a line width in units of frequency, S: continuum flux density of the IR source behind the cloud. Subscript 0 means extinction-free.

We consider V=100 kms-1 for the line, and S=10mJy as a baseline model. If the line optical thickness is smaller than 0.01, it is very difficult to detect. We therefore assume line,0

=0.01.

The dust extinction is introduced as

where A/AV: extinction curve, and AV/NH: normalization. We use the Galactic extinction (Mathis 1990) as a baseline model. We also see the effect of different extinction curves. The extinction scales with metallicity Z.Using this, absorption line flux with extinction is obtained as

Summary of the parameters for this calculation

The detection limits is for SPICA (Ueno et al. 2000).

(Shibai et al. 2001, PASJ, 53, 589)

Considered hydrogen lines in the IR

3. Results

3.1 Absorption lines vs. dust extinction (metallicity)

Z = 1 Zsun Z = 0.01 Zsun

If the metallicity is one solar, we cannot detect these lines because of strong extinction. But if Z = 0.01 Zsun, they can be detected by SPICA.

Solid line is the result for the Galactic extinction, while the dotted line is the mixed-case extinction curve, and dashed line is for the unmixed-case one. The result is sensitive to the extinction curve when the column density of the gas is high.

3.2 Effect of extinction curve

3.3 Possible background source

We consider QSOs, especially lensed ones. If we put some known QSOs at z=5, they have flux densities around 10mJy.

Considering a 60K blackbody, 17, 28, and 112 m lines will be suitable for this observation, but 2 m line is hard to detect because of the weak continuum.

17, 28m: IR 112m: submm

SPICAALMA

4. What can we learn?

Consider a protogalactic cloud of M ~ 1011 Msun. Since the radius R is a few kpc in this case, we have its column density

where f : gas mass fraction of molecular clouds. This fraction can be very high (~ 1) when NH is high enough (Hirashita & Ferrara 2005).

Its evolution occurs in a free-fall timescale, much shorter than the cosmic evolution timescale, e.g., Hubble time.

Observed properties are specific to the redshift at which the cloud absorption is measured.

We obtain z and V (velocity dispersion) of primordial gas clouds from this observation. These quantities tell us their dynamical evolution through the structure formation theory.

Collapse of a massive cloud (M ~1011Msun) at z <5:

Basically observed in the IR, SPICA will be useful

Population III objects (M ~ 106-9Msun) at z > 5:

Observed in the submm, ALMA will be required

Summary

1. Summary of dust emission model

1. We constructed a model for the SED of forming galaxies based on a new theory of dust production by SN II.

2. The model (unmixed case) roughly reproduced the observed SED of a local low-metallicity dwarf SBS0335-052, which has a peculiar strong and MIR-bright dust emission.

3. We also calculated the SED of a very high-z forming small galaxy. Although it may be intrinsically too faint to be detected even by ALMA, gravitational lensing can make it possible.

2. Summary of IR Absorption Measurement of H2

1. We proposed a method to measure the amount of H2 in primordial low-metallicity cloud in absorption in an IR spectra of QSOs.

2. If the metallicity of the cloud is low (Z ~ 0.01 Zsun), dust extinction is expected to be so weak that 17 and 28m lines are detectable by SPICA for objects at z < 5. Small very high-z population III objects will be detected by ALMA.

3. By this method, we can trace back the dynamical evolution of early collapsing objects at very high redshifts.

Dust grain species produced by SN II

Grain size spectrum of dust produced by SN II

(Nozawa et al. 2003, ApJ, 598, 785)

Chemical evolution (a little more)

Closed-box model is assumed.

where SFH is assumed to be constant, and we adopted Salpeter IMF

Time evolution of the mass of ISM

Remnant mass (fitting formula)

Important transitions of H2 molecules

Equivalent width of some IR lines

The second line follows by the optically thin condition.

Herschel

SPICA

SPICA sensitivity

H2 17m line for various Z