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7/27/2019 Sumit ASI Talk

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Search for Star formation trigger in Wolf-Rayet

Galaxies

Authors: Sumit Jaiswal and Amitesh Omar

Data Used: HI 21-cm line observations using GMRT and Hα narrowband

observations using 1.3m-Devasthal Telescope

Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital

Star formation in WR-Galaxies ASI-2013

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Wolf-Rayet Galaxies ?

The Wolf-Rayet (WR) galaxies are a special type of star-burst galaxies

where an episode of massive star formation is only a few Myr old.

A subset of HII galaxies, whose integrated spectra show broad

emission lines (mainly HeII 4686 and CIV 5808; also N and O)

attributed to the presence of WR stars.

The strengths of emission lines indicate a substantial population (102 -

105) of WR stars in WR galaxies.

Figure: Typical WR spectra

The most massive O-type stars ( M ≥ 25 M ⊙) undergo WR phase at nearly a few Myr after their birth,

spending only a short time (t WR ≤ 1 Myr) in this phase before their supernova explosion.

WR galaxies therefore offer an opportunity to study phenomenon associated with the very early phases or

triggering mechanisms of star-formation in galaxies.


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Cause of WR phase of galaxy

Conditions for a galaxy to be in WR phase:

Sufficient raw material for star formation (i.e. gas & dust).Low metallicity (i.e. IMF is shifted towards massive stars end).

Triggering mechanism for star formation?

Tidal Interaction as the Triggering Mechanism:

Most galaxies are not isolated, instead found in groups.

Interaction between galaxies determine the morphology and

evolutionary stages of galaxies.

There are basic interconnections between interactions, starbursts, andmorphology.

Galaxy-Galaxy Tidal Interaction is therefore considered to be the

fundamental triggering mechanism for starburst in galaxies.


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Motivation & Sample Selection

The WR features can be found in almost all morphological class of galaxies. The

minimum requirement for all WR galaxies is ongoing or recent (<

10 Myr) massivestar formation.

The most extensive catalogue having 570 WR galaxies was published by

Brinchmann et al. (2008) using SDSS (Sloan Digital Sky Survey) data release 6.

Most WR galaxies are found to be interacting. However, a small subset of WR

galaxies does not show any tidal feature in their optical images.

To verify tidal interaction mechanism for starburst in apparently isolated galaxies, It

is believed that interactions are also possible with faint dwarf galaxies or intergalactic

HI clouds.

We want to trace such a possibility of interaction in these galaxies (within 25 Mpc

distance) by imaging two Hydrogen lines — HI 21-cm line and Hα 656.3-nm line.


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Observations

HI 21-cm Radio line observation

As atomic hydrogen gas i.e. HI envelop is much bigger than the

optical envelop, HI line is the best tracer for tidal interactions in

galaxies.

The short baselines of GMRT are very sensitive to detect the

tidal tail diffuse HI emission from these galaxies.

Hα 656.3-nm Optical line observation

As Hα emission is a very good tracer of HII regions, the

Hα images therefore show how star formation regions are

distributed within their host galaxies with respect to the

environmental effects.

Nearby galaxies within 25 Mpc distance can be mapped using

Hα filter at 1.3m-Devasthal Telescope.


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Interaction Feature in Mrk 996 (from HI observation)

The HI channel images clearly shows the HI emission due to rotating galactic disk along with a weak

HI emission extending over several channels. This extended weak HI emission can not be attributed to the

galaxy rotation. This emission probably may be because of the tidal tail or small faint companion. We have

therefore kinematically traced the tidal tail feature in the galaxy Mrk 996 which should be the cause of its

WR-phase. The faint companion (cause of the tidal interaction) may be along the line of sight of Mrk 996.

-06 18

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1705.6 KM/S 1698.7 KM/S 1691.7 KM/S 1684.8 KM/S 1677.8 KM/S 1670.9 KM/S

-06 18

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1663.9 KM/S 1657.0 KM/S 1 65 0. 0 K M/ S 1 64 3. 1 K M/ S 1 63 6. 1 K M/ S 1 62 9. 2 K M/ S

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1622.2 KM/S 1615.3 KM/S 1608.4 KM/S 1601.4 KM/S 1594.5 KM/S 1587.5 KM/S

D E C L I N A T I O N

( J 2 0 0 0 )

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1 58 0. 6 K M/ S 1 573 .6 KM /S 1 56 6. 7 K M/ S 1 55 9. 8 K M/ S 1 55 2. 8 K M/ S 1545.9 KM/S

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1538.9 KM/S 1 532 .0 KM /S 1 52 5. 1 K M/ S 1 51 8. 1 K M/ S 1 51 1. 2 K M/ S 1504.2 KM/S

01 27 45 40 35 30

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1497.3 KM/S 1490.4 KM/S

RIGHT ASCENSION (J2000)01 27 45 40 35 30

1483.4 KM/S 1476.5 KM/S

01 27 45 40 35 30

1469.5 KM/S 1462.6 KM/S

Figure: Channel images showing HI emission flux contours overlying on the gray

scale optical R-band image of Mrk 996. The contours representing HI emission flux

are drawn at 3, 4.5, 6, 9 and 12 mJy/Beam.Star formation in WR-Galaxies ASI-2013

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Interaction Feature in Mrk 996 (from Hα observation)

The Hα emission is seen in a disk shape, misaligned by nearly 40◦ from its old stellar disk. Tidal interactions

and minor mergers between galaxies can cause gas in the outer region to loose angular momentum and fall

towards the centre of galaxy. This fresh supply of gas can be effectively converted to stars causing nuclear

starburst. As the new gas can come with different angular momentum, it is not necessary that orbits of fresh

gas are aligned with the old stellar disk. Therefore, the tidal interaction is likely to be responsible for the WR

phase of the galaxy.

Figure: The Hα iso-intensity contours (thick line) overlying on SDSS r-band

iso-intensity contours (thin line); contour levels are in logarithmic scale.Star formation in WR-Galaxies ASI-2013

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Results

The tidal interaction feature (previously not known) of the galaxy

Mrk 996 has been kinematically traced in HI .

The misalignment of the Hα and r- disks shows that tidal interaction is

likely to be responsible for the WR phase of the galaxy.

If tidal features in HI are not detected for our sample galaxies, it will

indicate a strong consideration of other possible mechanisms of star

formation trigger in galaxies. Such a study is unique and extremely

beneficial in understanding process of star formation through tidal

interactions or due to other mechanisms.

Thanks for your attention ...


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