AGA0414 Near Infrared Astronomy Prof. Alessandro Ederoclite What is near infrared? “Infra” red

Infrared radiation was discovered by William Herschel while playing with a prism. He noticed that he could disperse sunlight and measure a temperature at the various colours and he could detect some heat beyond the red… that was the first ever detection of infrared (and how infrared got its name).

Normally, we shorten “infrared” by “IR” What is IR

We have commented that CCDs are capable to detect light between 3000 and 10,000Å.

I normally use 10,000Å as the limit between optical and infrared.

But this is not carved in stone (some people start calling “infrared” things as blue as 8,000Å). What IR is

If the beginning of the IR range is nuclear, the end of it is even more fuzzy.

In general, I find this to be an ok definition:

Near Infrared 1μm - 5μm

Mid infrared 5μm - 50μm

Far infrared 50μm - 300μm

We will focus on NIR 2MASS

2 Micron All Sky Survey

This is a project which redifined near infrared astronomy.

It was carried in the 1990s with two identical telescopes in both hemispheres and provided a homogeneous survey of the sky in NIR.

It observed in three bands: J, H and Ks. Why near infrared? Why NIR

This is the realm of cold and/or faraway objects: Remember the Wien displacement law?

- Cold An object at 5,000K emits most of its light at 6000Å - Brown dwarfs - An object at 1000K emits at 30,000Å (3μm) - Protoplanetary disks - Far away You emit at 100μm - at z ~ 1 - Galaxies at z > 6 Remember the definition of ?

z = Δ λ / λ = v / c

At z = 1, Hα (rest frame 6563Å) is already at 13,126 Å ! https://en.wikipedia.org/wiki/Brown_dwarf Brown dwarfs

Brown dwarfs are self gravitating objects unable to sustain thermonuclear reactions in their interior.

They are somewhat in between a cool and a giant .

The first brown dwarf (Teide 1) was discovered with a 1.5m telescope in 1995 (Rebolo et al. 1995).

https://jgagneastro.com/phd-thesis/ Brown dwarfs

Here is a sequence of spectra of white dwarfs. Note the various molecular bands. Circumstellar discs

As we commented, a disc is a convolution of black bodies at different temperatures.

What we see here is the spectral energy distribution of a star (the “blue” component) and an extra (red) component which can be fitted with the SED of a cool disc.

(Hillebrant et al. 1992) High angular resolution

The “movies” of the orbiting the Galactic centre or the direct images of extrasolar are also made in NIR.

NIR is the best wavelength regime for adaptive optics.

We will speak about this in a specific lecture on high angular resolution. Far away objects

Lines shift to the red because of the expansion of the Universe.

This image (stack of from SDSS) show that typical optical lines get out of the optical range already at z ~ 1 The specific rate

In extragalactic astrophysics, one of the critical numbers of the evolution of a is how many stars are formed in a galaxy.

You may want to have this number divided by the mass of the galaxy in order to have a fair comparison.

Madau (1998) was the first to do this. For some time, it was called “the Madau plot”.

The specific star formation rate peaks at z ~ 1 - 2 !

Ideally observed in NIR because we can use all the tracers (lines and line ratios) which we know from the local universe. These are NIR spectra obtained with the in a project called 3D-HST.

Note that the spectrum to the left is basically a common optical spectrum… only shifted to the NIR! Dust

Dust emits mostly in the infrared. This is M31 (Andromeda’s Galaxy) in UV (GALEX), optical (OAJ) and NIR (Herschel). Dust

Here is a composition of spectra of galaxies from Galliano (2004).

“ULIRG” is “Ultra ”

Note how the dust bump (on the right) becomes weaker as the galaxy changes. Galaxies “far far away”

The furthest galaxies can be detected by the detection of the lyman alpha line (rest frame 1216Å) into the infrared.

All emission bluer than Lyman alpha is absorbed, so this is the ultimate tracer of far away objects.

This example is from Iye et al. (2006) CR7

The “COSMOS Redshift 7” (CR7) galaxy was discovered using Lyman alpha emission in the near infrared by Sobral et al. (2015). How do we observe in NIR? Suggested readings

SofI manual http://www.eso.org/sci/facilities/lasilla/instruments/sofi/doc/manual.html

Lecture on NIR detectors at Gemini (most of the following slides are “borrowed” from this lecture) https://www.noao.edu/meetings/gdw/files/Joyce_IR.pdf

IR spectroscopic observations in astronomy https://www.theochem.ru.nl/summerschool2012/lectures/cuijk_aboogert_lect1.pdf CCDs won’t work

Remember how we said that CCDs don’t detect IR photons?

How do we solve this?

We use “infrared arrays”.

From an electronic point of view, they look like CMOS (each pixel has its own electronics).

Most used detector is the Hawaii (which, nowadays, is a rather large 4k x 4k based on a HgCdTe) The main characteristics of NIR instruments

As we will see in the following slide, NIR background dominates your images.

The primary concern in a NIR instrument is to mitigate the thermal emission.

NIR instruments are normally in a vessel in vacuum cooled with 4He (3K).

In the picture, I am next to SofI (Son of ISAAC) on the A side of the in La Silla (I was 2nd instrument scientist of SofI). The background

The problem in IR is that the background is as bright (often brighter) than your signal.

The background is due to the emission of the atmosphere (in fact, a lot of it is the air around your telescope and the telescope and the instrument themselves!)

How do you fix it?

You dither.

NIR spectroscopy

In spectroscopy you use “on-off” patterns like the, so called, ABBA pattern.

You switch your target between two positions (“A” and “B”) along your slit.

Here is an example with GNIRS. This is the result of subtracting a spectrum in “A” position from a spectrum in “B” position.

Isn’t it beautiful? (correct answer is “yes”) Exercise

Go to the exposure time calculator of SofI http://www.eso.org/sci/facilities/lasilla/instruments/sofi.html http://www.eso.org/observing/etc/

Try to get the exposure time to observe a mag 15 star both in photometry and in spectroscopy. http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=SOFI+INS.MODE=swimaging http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=SOFI+INS.MODE=swspectr