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Sci TR-31

Updates: 14.05.2012 (Aybüke)

3.1 Galaxy Formation and Evolution

There are still many open questions in the astrophysical history of the Universe, in the context of galaxy formation and evolution and the era of re-ionisation:

Galaxy Formation and Evolution

  • What was the nature of the first stars?
  • When did the first galaxies assemble and what were their properties?
  • What kinds of stars are galaxies made of?
  • How many generations of stars do galaxies host and when did they form?
  • What is the star formation history of the Universe?
  • When and how did galaxies as we see them today form?
  • How did galaxies evolve through time?

The Era of Re-ionisation

  • When did it happen?
  • What caused it (stars, galaxies, ...)?

Methods

There are different methods of observations to look for the answers of the open questions above:

  • large area surveys to study the galaxy formation and evolution and to look for rare objects like very high-redshift quasars or brown dwarfs,
  • deep surveys to study the first galaxies and the faint end of the galaxy population,
  • multi-band imaging in NIR or NIR spectroscopy to study the first galaxies/stars and the era of re-ionisation,
  • narrow-band designated redshift surveys,
  • follow-up observations of Gamma-ray Bursts (GRBs) to locate and study the host galaxies and probe the otherwise undetected galaxy populations and high-redshift galaxies

Current examples in the near-IR:
VISTA surveys like UltraVISTA, VIDEO, VHS, VIKING; UKIDSS surveys like LAS, DXS, UDS

High-redshift galaxy observations are essential for these two fundamental astrophysics subjects: galaxy formation and evolution, and the physics of the early Universe.

Updates: 14.05.2012 (Ayse, Tolga)

AGN

The innermost regions of some galaxies output high amounts of energy. These objects are classified as active galactic nuclei (AGN). The activity is believed to result from the infall of material onto the central supermassive black hole via a sub-pc scale accretion disc. The observations of AGN at infrared wavelengths can be aimed at:

  • Identification of AGN among normal galaxies by looking for a power-law in the spectrum (Stern et. al. 2005)
  • Understanding the nature of the pc-sized dusty torus which is believed to play a key role in AGN classification (Elitzur & Shlosman 1988)
  • The AGN-starburst connection

References

Elitzur, M. and Shlosman, I., ApJL, 648, 101, (2006).
Stern et. al., ApJ, 631, 163, (2005).

What can be done with DAG

Thanks to the nuStar and Spectrum-Roentgen Gamma satellites and continuous FERMI monitoring, when DAG is established there will be thousands of newly discovered AGN.

DAG can contribute to observations of these sources in a number of ways.

  • Multiwavelength imaging : Thanks to the planned two Nasmyth foci, DAG can perform quasi-simultaneous optical - NIR imaging of a number selected AGN. Such observations, when combined with high (X-rays to TeV) and low (mm - radio) energy data may help revealing the nature of the accretion and structure of the jet in these systems. For such a purpose only two standard imagers with high QE in optical and NIR can be enough. Since the variability timescales are long in these systems timing resolution is not important. Due to the improved spatial resolution of high energy instruments, coordinates of newly discovered sources will be accurate enough. Therefore the field of view of the imagers is not really important for these observations (field of view of several arcminutes should be enough to have a comparison star fit in the image).
  • Spectroscopy : First of all will be necessary for the determination of the redshift of a newly discovered AGN. Also can be used in long term monitoring programs to obtain information on the time evolution specific line features. For this purpose a TFOSC or EFOSC type high S/N instrument would be enough with relatively moderate resolution.
  • Polarization measurements : For relatively brighter AGNs (they are typically bright enough) polarization measurements obtained at several bands and the time evolution of these measurements may provide important knowledge about the nature of the relativistic jet when combined especially with Gamma-ray flux variations.

All of the above mentioned methods generally need relatively long observation campaigns that last several years, where individual observations are obtained with a frequency of maybe 1 or 2 per night per source. This requires an observatory policy that allows obtaining regular monitoring observations of selected sources during a given night although the night was given to a different project. Since the individual AGN observations will be short it should not be a problem for the observer but this needs to be planned and organized in advance. Also at robotic observations (at least to some degree) will be very important for these regular monitoring observations.

Also monitoring of a number of AGN with high energy telescopes open the chance for a number of ToO observations in the optical and NIR. DAG observatory policy should also be designed to allow these kinds of ToO observations.

Finally a lot of AGN that will be discovered in the future will be highly obscured. Therefore opposite to the existing population of currently known AGN, these systems will be significantly dimmer. So these newly discovered sources will often be more suitable targets for DAG instead of smaller telescopes that are usually used these days.

Updates: 14.05.2012 (Korhan)

Lyman break galaxies (LBGs)

Lyman break galaxies facilitated for the first time observational estimates of the star-formation history of the Universe out to high redshifts (Madau et al. 1996; Steidel et al. 1999). Lyman-break galaxies are star-forming galaxies at high redshift that are selected using the differing appearance of the galaxy in several imaging filters due to the position of the Lyman limit. These star-forming galaxies at redshifts z>2.5 will be very faint or absent in the U-filter since for z>2.5 this filter is sensitive to flux from the blue side of the Lyman-limit in the restframe of the galaxy. Lyman-limit shifts to greater wavelenghts due to the expansion of the universe. LBGs can be selected from deep broad-band photometry using the Lyman-break, or "dropout", technique pioneered by Steidel, Pettini & Hamilton (1995). The technique has primarily been used to select galaxies at redshifts of z = 3–4 using ultraviolet and optical filters, but progress in infrared astronomy has allowed the use of this technique at higher redshifts using infrared filters (Curtis-Lake et al. 2012; Bielby et al. 2012).


Fig-3.1-1: A new technique (Lyman-Break technique), in combination with the powerful light--gathering power of the new generation of ground--based optical/infrared telescopes, is allowing one to map out the structure traced by galaxies on large scales, when the universe was only 10% of its current age.The images were taken through red, green, and ultraviolet filters specially designed for finding high redshift galaxies. The object at the center of the circle is clearly present in both the red and the green image, but disappears in the UV image.

Updates: 14.05.2012 (Aybüke)

Dedicated Projects/Examples

Spectrographic Areal Unit for Research on Optical Nebulae - SAURON
SAURON is a panoramic integral-field spectrograph using a lenslet array based on the TIGER principle (Bacon et al. 1995). SAURON uses a lens array to obtain two-dimensional spectroscopy with complete spatial coverage over a field of 33″x41″ in low-resolution mode (0.94" lenslets) and of 9″x11″ in high-resolution mode (0.26″ lenslets). The spectra cover 4800 Å to 5400 Å with a resolution of 3 Å (σ=75 km/s). It is a dedicated instrument that was mounted on the 4.2-metre William Herschel Telescope (WHT) on La Palma. It was used to measure the kinematics and line strength distribution for a representative sample of 72 nearby early-type galaxies (ellipticals, lenticulars, and Sa bulges, in clusters and in the field). The main goal is to understand the formation and evolution of elliptical and lenticular galaxies and of spiral bulges from 3D observations.
Near-infrared diagnosis of stellar populations
Near-IR would provide an excellent opportunity to conduct studies like SAURON without being effected by dust obscuration. The NIR data is very valuable source of information both for obscured galaxies like starbursts and for early type galaxies whose NIR rest-frame light is dominated by a single component of the stellar population: cool giant stars. Moreover, the NIR colours and indices are more nearly mass-weighted, i.e. the NIR mass-to-light ratio is closer to one (e.g. Worthey 1994, ApJS, 95, 107). Recent studies have shown that the NIR diagnostic of stellar populations is a powerful tool in braking some of the current diagnostic degeneracies, when coupled with novel stellar population models (e.g. Maraston 2005, MNRAS, 362, 799). The NIR observations of normal galaxies of different morphological types from elliptical to spirals is also showed to be reproduced by stellar population models (Mannucci et al. 2001, MNRAS, 326, 745).
Mid-IR observations are also very important to construct a wider spectral energy distribution (SED) of high-redshift galaxies as the rest-frame near-IR is the key to determine the mass of the galaxy and the unobscured star formation, and to study the older stellar populations.

References

Bielby et al. 2012
Curtis-Lake et al. 2012
Madau et al. 1996
Mannucci et al. 2001, MNRAS, 326, 745
Maraston 2005, MNRAS, 362, 799
Steidel et al. 1999
Steidel, Pettini & Hamilton 1995
Worthey 1994, ApJS, 95, 107

What can be done with DAG
  • Northern Hemisphere equivalents of VISTA surveys like UltraVISTA, VIDEO, VHS. However repetition of UKIDSS surveys must be avoided.
  • DAG can do surveys to find the Lyman-Break Galaxies in the near infrared. Detailed observations of the known ones can also be done to study the galaxy properties like mass, star-formation and metallicity.
  • Follow-up observations of Gamma-ray Bursts (GRBs) to locate and study the host galaxies of the high-redshift GRBs:
    This requires either a multi-band imager like GROND, at least covering r,z,J,H,K bands, or a spectrograph with a large wavelength coverage like XSHOOTER. An Integral Field Unit (IFU) spectrograph with a FoV of at least 5"x5" to cover the Swift XRT error circle would overcome the problem of locating the GRB for a spectroscopic observation.
  • A complementary project to SAURON to study the galaxy properties in the near-IR without the effects of dust obscuration. The stellar population studies have already started to extend in the near-IR and there are some promising spectral features that can be used to study the galaxy properties (see the above text).
  • Mid-IR observations are also very important to construct a wider spectral energy distribution (SED) of high-redshift galaxies as the rest-frame near-IR is the key to determine the mass of the galaxy and the unobscured star formation, and to study the older stellar populations.

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