Dynamics and morphology of NGC 4449 and NGC 4490

BSc Physics & Astronomy

Report Bachelor Project

Dynamics and morphology of NGC 4449 and

NGC 4490

A study done at Anton Pannekoek Observatory

by

Vincent R. Groeneveld

11617446

May 4, 2021

15 EC

Conducted between February 2020 and May 2021

Supervisor:

Drs. M.R. Sloot

Second examiner:

Prof. dr. R.A.D. Wijnands

Contents

Samenvatting

1 Introduction 1

2 Theory 1

2.1 Dynamics of galaxies . . . 1

2.2 Irregular galaxies and starburst galaxies . . . 2

3 Observations 2 3.1 Target selection . . . 2 3.2 Instruments . . . 2 3.3 Imaging . . . 3 3.4 Spectroscopy . . . 3 3.5 Spectral calibration . . . 4 4 Analysis 5 4.1 Galaxy orientation and plane of rotation . . . 5

4.2 Star forming regions . . . 6

4.3 Radial velocity . . . 6

5 Results and Discussion 7 5.1 NGC 4449 . . . 7 5.2 NGC 4490 . . . 7 5.3 Error bars . . . 8 6 Conclusion 8 6.1 Summary . . . 8 6.2 Recommendations . . . 8

Samenvatting

In dit onderzoek is er onderzoek gedaan naar de beweging van sterren in

onregelmatige sterrenstelsels. Onregelmatige sterrenstelsels zijn sterrenstelsels die

geen typische vorm hebben zoals een spiraal- of elliptisch sterrenstelsel. Om te leren

hoe deze sterrenstelsels zijn ontstaan is het belangrijk om de bewegingen in dit type

sterrenstelsel te analyseren. In dit onderzoek zijn de twee sterrenstelsels NGC 4449

en NGC 4490 geobserveerd (zie figuur 1). De waarnemingen voor dit onderzoek zijn

gedaan met de 36 cm en de 51 cm telescopen van het Anton Pannekoek

Observatorium aan het Anton Pannekoek Instituut in Amsterdam. In totaal is 14

nachten geobserveerd waarin er 50 uur aan data is verzameld.

De 51 cm telescoop is gebruikt om foto’s te maken van de twee sterrenstelsels (zie

figuur 1). Hiermee is de positie bepaald van stervormingsgebieden binnen het

sterrenstelsel. Daarna zijn spectroscopische observaties gemaakt van de

stervormingsgebieden met de 36 cm telescoop. Deze spectra hebben dominante

emissielijnen die van het geïoniseerde waterstofatomen uit de stervormingsgebieden

komen.

Met het meten van de dopplerverschuiving, van de geïoniseerde waterstof

emissielijnen, is bepaald wat de snelheid van een gebied in de radiale richting is. Met

deze snelheden is er een rotatiekromme gemaakt van de sterrenstelsels.

In de rotatiekromme van NGC 4449 geen rotatie herkend. In de rotatiekromme van

NGC 4490 zit wel een rotatie, deze is niet symmetrisch.

Figuur 1: Waargenomen sterrenstelsels NGC 4490 (links) en NGC 4449. De foto’s zijn

gecombineerde zwart-wit foto’s genomen door kleurenfilters. Hierna zijn ze in

Photoshop bewerkt om er een kleurenfoto van te maken.

Astronomy& Astrophysics ©API 2021 May 4, 2021

Dynamics and morphology of NGC 4449 and NGC 4490

Radial velocity analysis of starburst galaxies at the Anton Pannekoek

Observatory

Vincent R. Groeneveld

Student nr. 11617446 Supervisor: drs. M.R. Sloot

Second Examinator: prof. dr. R.A.D. Wijnands Conducted between 01-02-2020 and 03-05-2021 Report Bachelor Project Physics and Astronomy, size 15 EC Anton Pannekoek Institute, Faculty of Science, University of Amsterdam

Received May 4, 2021

ABSTRACT

Context. The structure of irregular galaxies is still an uncategorized and relatively unknown part of the Hubble system. Recent developments explain that the morphology of irregular galaxies is due to interactions with other galaxies during the evolution of the galaxy.

Aims.Measuring and studying the rotation curve of irregular galaxies to gain more insight into the origin of irregular galaxies and their evolution.

Methods.NGC 4449 and NGC 4490 were observed using the 14” and 20” telescope of the Anton Pannekoek Observatory at the Anton Pannekoek Institute. With self developed software in Python, the radial velocities were measured by calculating the the Doppler shift of the Hα emission line. With the positions and velocities of the star forming regions a velocity diagram was created.

Results.The rotation curve of NGC 4449 did not show a rotation, the relative radial velocities measured were within −28 km/s and

18 km/s with respect to the centre of the galaxy. NGC 4490 did show rotation which is asymmetrical. The velocities of the measured star forming regions were between −113 km/s and 72 km/s with respect to the centre of the galaxy.

Key words. starburst – dwarf – individual galaxies – NGC 4449 – NGC 4490 – galactic dynamics – spectroscopy – Doppler shift – photometry

1. Introduction

About one century ago, the existence of galaxies outside the Milky Way was still unknown. Currently we estimate there to be 2 × 1012 _{observable galaxies in the universe (Conselice et al.}

2016). We also know that the typical mass of a galaxy ranges between 108_{− 10}12_M

(Carroll & Ostlie 2017), which consists

partially out of stars. We have observed that stars in galaxies create different structures. Edwin Hubble was the first person to describe and capture different categories of galaxies due to the differences in appearance (Hubble 1926). Since we currently have a lot more knowledge about galaxies, we now use an im-proved version of this classification system described in Mihalas & Binney (1981).

There are three main categories described by the Hubble sys-tem, spiral galaxies, elliptical galaxies and irregular galaxies. When looking at a spiral galaxy, one would see that the galaxy is confined to a disk. In this disk you can not only distinguish the spiral arms, but in some cases also a central bulge or a bar. An elliptical galaxy distinguishes itself from an spiral galaxy due to the lack of structure. These types of galaxies do not have dis-tinct features apart from their shape. Irregular galaxies are clas-sified as galaxies that do not fall within the spiral and elliptical category. This means that the structure of an irregular galaxy is unique. The rather peculiar morphology most likely originates

from merging or other interactions between galaxies that hap-pened during the evolution of the galaxy.

It is hard to determine the evolutionary history and get an un-derstanding of the nature of irregular galaxies by looking at the morphology alone. Therefore this research is focused on mea-suring the rotation curve of starburst galaxies NGC 4449 and NGC 4490 using high resolution spectroscopy. The goal of this research is to find possible signatures of spiral features in the rotation curves of the galaxies.

Section 2 will discuss the theoretical background of the mor-phological and dynamical properties of galaxies. The equipment and method used for the observations are described in section 3. In section 4 it is explained how the data has been analysed. Section 5 discusses the results of this study. Section 6 contains a summary and recommendations for future research.

2. Theory

2.1. Dynamics of galaxies

The dynamical behaviour of galaxies is closely related to their Hubble type. Within galaxies all material rotates around the cen-tre of mass. For spiral galaxies all material rotates in the same direction, either in the retrograde or prograde direction of the spiral arms. In elliptical galaxies this rotation does not favor any

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direction of rotation and the orbits are not bound to a plane of rotation.

The radial velocity of areas in a galaxy are measured using the Doppler shift of a spectral line. The radial velocity, vr, is

calculated according to the Doppler shift formula vr=

λobs−λ0

λ0

c. (1)

λ0is the rest wavelength, λobsis the observed wavelength and c

is the speed of light (Carroll & Ostlie 2017).

A rotation curve shows the measured radial velocity of re-gions in the galaxy as a function of the distance from the cen-tre of the galaxy. For elliptical galaxies this curve will show the data points randomly within a velocity range, because there is no preferred direction of rotation and the areas are distributed ran-domly in the ellipsoid.

The rotational diagram for spiral galaxies contains a lot of char-acteristics of the spiral galaxy if the plane of rotation is chosen correctly. Since a spiral type galaxy has a favored direction of ro-tation, the diagram shows on one side of the centre an opposite velocity from the other side of the centre.

2.2. Irregular galaxies and starburst galaxies

Irregular galaxies are divided into two categories, Irr I are irregu-lar galaxies that have some kind of recognizable structure and Irr II are galaxies without any recognizable structure. Irr II galaxies tend to be fainter and smaller, both in size and mass, than spi-ral and elliptical galaxies (Gallagher & Hunter 1984). These Irr II galaxies are the so called dwarf irregular galaxies. The dis-tinction between irregular galaxies and dwarf irregular galaxies happens around L ∼ 108L(Sparke, & Gallagher 2007). In most

cases, the origin of these structural differences from Irr II galax-ies are interactions with other galaxgalax-ies. An example of such an interaction is a merging galaxy in which resulted in an unrecog-nisable structure of the galaxy.

Another characteristic for Irr II galaxies is their relative large fraction of young stars. This indicates that these irregular galax-ies have a high star formation rate which they maintain for a long period of time compared to non-starburst galaxies (Lehnert & Heckman 1996; Sparke, & Gallagher 2007). Unlike spiral galax-ies, star formation in irregular galaxies is typically concentrated in the centre of the galaxy. Therefore the centre of the galaxy is the brightest (Carroll & Ostlie 2017). The high star formation rate results in a high HII abundance throughout the galaxy be-cause of the ionizing radiation coming from young stars.

Irr II galaxies are often categorised as starburst galaxy due to the high HII abundance throughout its structure. The high HII abundances is a consequence of the strongly enhanced star for-mation rate within the galaxy. According to Lehnert & Heckman (1996), there is no correlation with high HII abundances and the dynamics of a galaxy. Important for this research is that the high HII abundances causes the galaxy to have a high Hα emission rate.

3. Observations

This research was executed at the Anton Pannekoek Observatory in Amsterdam, The Netherlands. All observations were carried out from the 4th _{of February 2020 until the 22}th _{of April. This}

section describes how the observations and data pre-processing were performed.

3.1. Target selection

For the targets in this research it was important that they met three criteria. The first criterion for the target selection was that the target galaxies were Hubble type irregular. The second cri-terion is that the galaxy had to have dominant and bright star forming regions throughout the entire galaxy which could be spatially resolved. This constrains the chosen target galaxies to be starburst galaxies since they have very high HII abundance throughout the entire galaxy. The last criterion was that the ob-ject was visible during the observational period during the whole night. This criterion should be fulfilled in order to increase the possible exposure time and thus set a higher upper limit for the visible magnitude of the object.

NGC 4449 is the only object that satisfied all the crite-ria mentioned above. It is classified as an irregular Magellanic dwarf galaxy, Hubble type IBm. It has an apparent magnitude of mv = 9.65 (Makarova 1999), and has an apparant size of

6.20× 4.40_{(Finlay 2003).}

Due to a lack of suitable candidates NGC 4490 was also cho-sen as a target in this research. NGC 4490 is classified as Hub-ble type SB(s)dp, which stands for barred dwarf spiral galaxy, and is also a starburst galaxy. It has a apparent magnitude of mv = 9.38 and is tidally locked with its satellite galaxy NGC

4485 (Elmegreen et al. 1998). This made it interesting to study if there were any distortion of the rotational symmetry in the sys-tem. NGC 4485 has a magnitude of mv= 11.83 and is classified

as an irregular dwarf, IB(s)mp type, galaxy. The apparent size of the entire system is 7.00× 3.50(Nilson 1973). The mentioned Hubble galaxy classifications are determined by de Vaucouleurs (1963); Ann, Seo & Ha (2015).

3.2. Instruments

The observations of this research were executed at the An-ton Pannekoek Observatory (API) located in Amsterdam, Lat: 52◦21015” N, Long: 4◦57027” S, 8m above sea level. Due to the urban location of the observatory the seeing was typically be-tween a range of 2.0” − 2.8”. The exact values during the obser-vations are shown in table 1.

The imaging and spectroscopic observations at the APO were carried out with different instrumental setups. The tele-scope was a 20” Ritchey-Chrétien (RCOS) mounted on a 10 Mi-cron GM4000QCI mount. The detector attached to the telescope during these observations was a FLI Proline 16803 CCD detec-tor. The CCD detector had an imaging field of 4096×4096 pixels with a pixel size of 9 µm. The quantum efficiency of the detector was 60%. The field of view of an image was 29.40_{× 29.4}0_{, which}

resulted in a plate scale of 0.43” pixel−1. During observations the detector was kept at a constant temperature of −30C◦_{. The}

following filters were used during the photometric observations: L, R, B, I, Hα, Hβ, [OIII] and [SII] were Chorma photometric bessel filters, and the V filter was a Baader planetarium photo-metric bessel filter.

The spectroscopic data was gathered using a 14” Meade LX200 telescope mounted on a 10 Micron GM4000QCI mount. Two different spectrographs were used, a LISA spectrograph with a 300 l/mm−1_{grating, and a Shelyak Instruments}

LHIRE-SIII spectrograph with a 600 l mm−1_{and 2400 l mm}−1_grating.

The LISA spectrograph used an ATIK ATK-314L+ CCD de-tector which was kept at a constant temperature of −20C◦. The CCD chip had 1392 × 1040 pixels of size 6.45 µ m and a quan-tum efficiency of 65%. This setup reached a spectral resolution of R = 1600. The guiding camera was an ATIK Titan camera.

Vincent R. Groeneveld: Dynamics and morphology of NGC 4449 and NGC 4490

Spectra observed using the LISA spectrograph had a domain from 4000Å to 7200Å.

The LHIRESIII spectrograph used a ZWO ASI 183 MM Pro CMOS detector, which had a gain of 111 in the data collection. The detector was cooled to a constant temperature of −20C◦. For guiding another ZWO ASI 183 MM Pro detector was used. These CMOS detecors had a 5496 × 3672 pixel chip with a pix-elsize of 2.4 µm. The quantum efficiency of these detectors was 84%. The LHIRESIII spectrograph has an interchangable grat-ing. The 2400 l mm−1, R = 17000, grating was tried in this re-search since the highest possible spectral resolution of this setup was desired. It turned out that this grating in this setup had an upper limit for objects with magnitude ∼ 5. This concluded that the Hα emission signal from the targets was not strong enough to detect with the 2400 l mm−1_{grating. Therefore the 600 l mm}−1_,

R= 4250, grating was used, which was set to a domain of 6350Å to 7250Å. It is known that the LHIRESIII spectrograph is unsta-ble for long exposures. The spectrum will shift over the CMOS detector during long exposures. How this was resolved is de-scribed in section 4.

3.3. Imaging

Imaging observations were important for this research to link ob-served star forming regions to the location on the spectrum. Also, from photometric images the plane of rotation and centre of the galaxy were determined. Prior to observations flat field calibra-tion frames were taken by aiming the slit of the dome towards east and the telescope at the zenith. The amount of flat field im-ages taken was 10 − 15 with an exposure time of 5 − 10 seconds for every filter. When the flat field sequence was finished, the telescope was focused using a semi automatic focusing sequence on a star nearby the object with an apparent magnitude range of 6 to 6.5. Slewing the telescope towards the target resulted in the target being centred in the field of view and no major corrections were needed. The software MaximDL was used for the data ac-quisition, filter sequencing and focusing.

The mount’s tracking was sufficient for images with a 60s exposure and no additional guiding was needed. The dome of the observatory had to be rotated manually every hour to prevent the dome from blocking the object. Furthermore, the focus of the camera was monitored during the night and adjusted every 3 hours. Filters used during the observational run switched on a loop throughout the observation until the imaging run was com-plete. After completing the observations, 50 bias frames and 30 dark frames were taken. Further observational details are shown in table 1. After observations the data was reduced using Max-imDL according to standard data reduction procedure for the bias, dark and flat field corrections.

3.4. Spectroscopy

The MaximDL software used in the photometric data acquisi-tion was also used for spectroscopic data acquisiacquisi-tion. Spectro-scopic observations typically started before civil twilight in or-der to obtain flat field calibration frames for the guiding camera. Calibrating the guiding images was done in order to determine the slit position more accurately. After obtaining the flat field images, the acquisition detector was focused using a neon cali-bration lamp. This process happened manually on the emission lines in the calibration spectrum closest to the Hα emission line. It was important that the telescope was focused on the slit of the spectrograph. Therefore the guiding camera was focused on

Fig. 1. This figure shows the processed image of NGC 4449. For com-bining the images from different filters the software Adobe Photoshop CS6 was used. The total exposure time for each filter can be found in table 1. In the figure the different positions and hour angle of the slit are visualised. The Hα features in the galaxy are highlighted with a teal color scheme.

the slit before the telescope is focused on the guiding camera. The telescope was focussed on the guiding camera by pointing it at a magnitude 6 star near the object. The 14” telescope had to be focused manually by moving and securing the lens in place. To get the most accurate focus the FWHM of the star was moni-tored in the process of focusing the telescope. Slewing to a target was inaccurate by approximately 3◦_{. The telescope did not have}

a guiding scope, therefore, recognising star patterns and aiming the telescope was done manually with the Stellarium software. On average this entire process took 1.5 hours. This had to be repeated due to the meridian flip, which happened between one and two o’clock, depending on the date. The position and hour angle of the slit were determined by taking 60s test exposures. With the test exposures, it was determined whether there were multiple star forming regions on the slit position. Due to the in-stability of the LHIRES spectrograph it was required to obtain 5s neon calibration frames before and after each exposure.

During the exposure, the internal tracking of the mount was not efficient due to the long exposure time. Therefore, the auto-matic guiding from the mount was assisted with manual track-ing adjustments durtrack-ing the observations. The adjustments were based on the shift observed in the continuous, 5s exposure, guid-ing camera feed. The dome was rotated after every exposure to prevent it from blocking the object. After completing the spec-troscopic observations, dark and bias frames were taken for both the acquisition camera and the guiding camera. Since the acqui-sition dark frames have the same exposure time as the spectra, the dark frames for the LHIRES spectrograph were taken during the entire day. The heat from the sun on the dome in the after-noon generated heat inside the dome. This caused that the acqui-sition camera could not reach a constant temperature of −20C· while taking dark frames. The details of the spectroscopic obser-vations are shown in table 2.

After observations dark and bias corrections were applied using MaximDL. The amp glow from the Sony IMX183 chip

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Table 1. Journal of the imaging observations. Not all observations were used for the analysis in section 4.1 and 4.2 since the image quality varied due to the weather conditions.

Object Date (2020) L R V B I Hα Hβ [OIII] [SII] Seeing # Flat field # Dark frames

NGC 4449 7 February 30 15 15 20 - 30 - - - 3.8"1 10 30 26 March 20 - - - 14 20 20 20 - 2.2" 15 30 15 April 20 15 15 15 10 20 20 10 35 2.6" 15 30 NGC 4490 4 February 6 3 2 3 - 6 - - - 2.5" 10 30 27 February 40 27 26 26 - 40 - - - > 4.0"1 _{10 30} 16 March 17 17 17 - - 17 - - - 3.1"1 15 30 26 March 32 5 5 - 28 32 32 32 - 2.0" 15 30 15 April 33 25 25 25 15 - - - 35 2.4" 15 30

Notes.(1)_{Images not used for the final image due to the seeing being higher than 3.0".}

Table 2. Journal of spectroscopic observations. The LISA spectrograph had a 300 l/mm−1_{grating, the LHIRES 600 l/mm}−1_{. Both are described in}

section 3.2. The exposure time for each spectrum with the LISA spectrograph was 900 s, with the LHIRES spectrograph this was 2000 s. The slit positions are shown in figure 1 and 2.

Object Date (2020) spectrograph Slit Position # Images

NGC 4449 21 March LISA 1 11 22 March LISA 3 13 22 March LISA 4 11 19 April LHIRES 1 4 20 April LHIRES 1 3 20 April LHIRES 2 5 NGC 4490 16 March LISA 1 6 16 March LISA 2 6 16 March LISA 3 4 21 April LHIRES 1 7 22 April LHIRES 2 8

on the acquisition camera for the LHIRES spectrograph was re-moved by these corrections. The fluctuating temperatures of the sensor in the dark frames was not taken into a count because there would not be more than two dark frames left for the cali-brations. A Gaussian blur of 1.5 pixels was applied on the spectra in order to reduce the pixel-to-pixel variations while maintaining spectral features.

3.5. Spectral calibration

The wavelength calibration for the LISA and LHIRES spectro-graph were performed similarly to standard calibration proce-dures. The position of the calibration lines were determined with Gaussian fits. However there were two differences where the cal-ibration diverges from standard procedures.

In case of the LISA spectrograph the spectra was calibrated using light polution features seen in figure 3. The sources of the emission lines were identified as mercury (Hg) and barium (Ba), emitted by the metal halide street lighting surrounding the ob-servatory, table 3. These light pollution features were advanta-geous for determining the position of features in the spectrum because these features are similarly effected by the instability of the spectrograph as the features coming from the objects. For the LHIRES spectrograph the spectrum shifted during a 2000s exposure by 0.3σ of the calibration peak.

During the calibration of the LISA spectra it was concluded that the error bars with the current resolution would become & 100 km/s. This resolution is not high enough to measure a significant Doppler shift. Therefore the data from the LISA spec-trograph was not included in this research.

The result of the spectral calibration for the LHIRES spectra can be seen in figure 4. The residuals of the second order poly-nomial fit typically had values between −0.2 Å and 0.2 Å while the first order residuals had values between −0.8 Å and 0.6 Å. The wavelengths therefore were calculated using a second order polynomial dispersion function. The dispersion formula can be written as

λcalib= k12· pixelcalib+ k2· pixelcalib+ k3, (2)

with k1, k2 and k3 the dispersion parameters and pixelcalib the

centre of the calibration line. The k1and k2parameter were used

for calculating the wavelength of an observed emission line with the formula

λA= DA(∆pixelcalib−A, λcalib)

= k12·∆pixelcalib−A+ k2·∆pixelcalib−A+ λcalib. (3)

This formula calculates the wavelength of an emission line us-ing the difference in pixels between a calibration line and the observed emission line,∆pixelcalib−A.

The dispersion parameters were calculated for each star forming region individually, because it can not be assumed that the dispersion is the same across the spectrum. Because the LHIRES spectra shifted during the exposure, the radial veloc-ity was calculated with both calibration frames before and after the exposure. This implicated that both the neon spectrum be-fore and after were calibrated. The neon emission lines used to calculate the dispersion function are shown in table 4.

Vincent R. Groeneveld: Dynamics and morphology of NGC 4449 and NGC 4490

Fig. 2. This figure shows the processed image of NGC 4490. For com-bining the images from different filters the software Adobe Photoshop CS6 was used. The total exposure time for each filter can be found in table 1. In the figure the different positions and hour angle of the slit are visualised. The Hα features in the galaxy are highlighted with a red color scheme.

Fig. 3. Example of a spectrum taken with the LISA spectrograph. The figure shows emission features from street lights surrounding the Anton Pannekoek Observatory, indicated with red markers. The Hα emission line is indicated with a green marker. Other features in the spectrum are from the urban environment of the Anton Pannekoek Observatory.

4. Analysis

4.1. Galaxy orientation and plane of rotation

The centre of the galaxy was determined in order to set the ref-erence point for the relative velocities and the position of the assumed plane of rotation. The centre was determined using the contour plots shown in figure 5, in which the centre of the high-est contour was regarded as centre of the galaxy.

Furthermore, the orientation of the plane of rotation was de-termined. The positions of the star forming regions were pro-jected on this plane of rotation to determine the radius from the centre. The assumptions for the plane of rotation were that the plane should go through the centre of the galaxy and the plane

Fig. 4. The top of this figure shows the position of the neon calibration peaks in the spectrum, on the x-axis, with respect to their wavelengths, on the y-axis. The bottom part of the figure is the residual of the first order polynomial, shown in yellow, and the second order polynomial fit, shown in blue. Because of the significant better fit for the second order polynomial, it was decided to use a second order polynomial to calculate the dispersion of the spectrum.

Table 3. Strong emission lines from streetlights surrounding the Science Park campus of the University of Amsterdam. These emission features were used for the calibration of LISA spectra. (Burns et al. 1950; Karls-son & Litzén 1999).

Transition Wavelength Å Hg I 4046.563 Hg I 4358.328 Hg I 5460.735 Ba I 6110.783

Table 4. Strong neon emission lines used for the calibration of the LHIRES spectra (Saloman & Sansonetti 2004).

Transition Wavelength Å Ne I 6382.9917 Ne I 6402.248 Ne I 6506.5281 Ne I 6532.8822 Ne I 6598.9529 Ne I 6678.2762 Ne I 6717.0430 Ne I 6929.4673 Ne I 7032.4131 Ne I 7173.9381 Ne I 7245.1666

would be oriented along the long axis of the galaxy due to the inclination of the galaxy. In figure 5 the determined plane of ro-tation is shown, which satisfied both assumptions.

The error introduced by the galactic centre determination was partially responsible for the error on the distance towards the centre. The selection process induced an error of approximately 6 pixels, 2.6”. The orientation of the plane of rotation had an er-ror of approximately 5◦. This was determined by testing several other orientations for the plane of rotation and by looking at the spread of the data in the rotation curve. This induced an error in the positions of 0.4%.

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Fig. 5. This figure shows the L filter image of NGC 4449. The total exposure time was 46 minutes. In blue there are 7 contours on a loga-rithmic scale between a 6σ and the peak value. The green line in this plot is the determined plane of rotation. The red cross is the determined centre of the galaxy.

4.2. Star forming regions

The star forming regions used in this study were selected based on properties that were reduced from the spectrum. Figure 7 shows an example of a spectrum with Hα emission lines. The first selection threshold was a threshold for the width of the star forming region. The maximum width of the star forming region in the spectrum was determined as 20 pixels wide. In figure 7 this implies that every selected star forming region was not big-ger than 20 pixels in the vertical direction. This threshold was used to justify the assumption that the dispersion of the spec-trum within a star forming region is constant. The second selec-tion threshold was based on the brightness of the star forming region. It was determined that the averaged Hα peak is not al-lowed to have a signal to noise ratio lower than 17 in order to fit a Gaussian function on the Hα peak.

The selected star forming regions in the spectrum were matched with a position in the stacked Hα image, figure 6. This position was determined by overlaying the contour plots from the stacked Hα image with the guiding images of the spectro-graph, which contain the slit positions. An example of matched star forming regions is shown in figure 6 and 7.

The area selection in the spectrum and in the image had an error of approximately 6 pixels in the image, which is 2.6”. This is the same error induced by the centre determination in section 4.1, which is dominated by the position determination of regions in the Hα image.

4.3. Radial velocity

In order to find the radial velocities of different star forming regions within a galaxy, the Doppler shift of the Hα emission peaks in the spectral data was examined. The centre of the

emis-Fig. 6. This is the Hα image of NGC 4449. Some star forming regions are indicated with the blue circles and their letters. The red rectangle is the position of the slit. The green line is the determined plane of rotation as indicated in figure 5.

Fig. 7. The left side the figure shows the spectrum of star forming areas. The white features are Hα emission from star forming regions. The red line indicates the position of a calibration line in a calibration frame. The blue lines indicate the wavelength differences between the calibra-tion line and the wavelength of the signal for areas A and B. The right side of the figure shows a vertical plot of the signal. The letters indicate the regions as seen in figure 6.

sion line was determined by fitting a Gaussian distribution to the data.

Because of the shift in the spectrum at long exposure time, it was not possible to measure an exact absolute radial velocity. However, since one spectrum contained multiple star forming regions, the spectra did contain information about the relative

Vincent R. Groeneveld: Dynamics and morphology of NGC 4449 and NGC 4490

Fig. 8. This is the measured rotation curve of galaxy NGC 4449. The observed relative radial velocities were between −28 km/s and 18 km/s. The error bars had a size of around 14 km/s. The data shown in this fig-ure was collected with two different slit orientations. Both slit positions observed the centre of the galaxy, which was the reference point for the relative radial velocities. Therefore both positions were compared and represented in the same figure.

radial velocities with the formula vr(A−B)= vrA− vrB=∆λ

A−B

λ0

c. (4)

∆λA−B is the difference between measured Hα wavelength of

area A and area B.

Combining formula 3 and 4, the relative radial velocities were measured for every star forming region with respect to the centre. The resulting velocities were averaged over all exposures with their weight according to the signal to noise ratio of the Hα peak. The slit of the spectrograph was oriented in multiple di ffer-ent oriffer-entations for both galaxies as shown in figure 1 and figure 2. Areas on different slit orientations could only be compared if both positions measured at least one shared star forming region. The error on the relative radial velocity was induced by mul-tiple causes. The most dominant error was due to the centre de-termination of the Hα peak. This depended on the brightness of the star forming region and the Gaussian fit through the data. Another error was due to the instability of the spectrograph. The shift of the spectrum during a single 2000s exposure was gener-ally 0.5 Å. The second most important error was caused by the dispersion parameters. This was mostly due to the residuals as seen in figure 7. The final error was estimated as the outermost measured radial velocities relative to the centre of the galaxy.

5. Results and Discussion

5.1. NGC 4449

The result for NGC 4449 is illustrated in figure 8. The projection of the measured star forming regions on the plane of rotation range between 120 kpc from the centre. The relative velocities are within −28 km/h and 18 km/s with respect to the centre. Both slit positions were observed going through the centre, therefore they are directly comparable with each other. No significant dif-ference was found between the velocity distribution on the right side of the centre and on the left side of the centre.

The causes for not observing a difference in velocity distribu-tion, and thus a rotadistribu-tion, can both be instrumental and physical. There are two physical causes for not observing symmetric ro-tation. One of them is the inclination of the galaxy. Due to the

Fig. 9. This is the measured rotation curve of NGC 4490. The values of the relative radial velocities ranged from 72 km/s till −113 km/s, with typical error bars of 15 km/s. The two slit positions are represented in different colors since they did not have a star forming region in common, figure ??. Slit position 2 was shifted vertically down by 50 km/s because that visually overlapped more with position 1. The green data point on the far left side of the graph represents satellite galaxy NGC 4485.

inclination of the system, the projection of rotation observed in this study is possibly dominated by local variations instead of ro-tations on galactic scale. This will possibly result into the chaotic behavior as seen in the rotational diagram.

Secondly, it is thought that irregular galaxies are disturbed by interactions with other galaxies in their evolution. There-fore, the assumption that the system should behave like a spiral galaxy could be incorrect. For NGC 4449 this has been studied by Martinez-Delgado et al. (2012). In this paper they suggest that there are multiple counter rotating planes in the galaxy which is the result of a galactic merger in the past of NGC 4449. In order to confirm this, further research on NGC 4449 should be able to distinguish multiple independent rotation curves.

The instrumental causes are the size of the error bars and the lack of data points across the galaxy. To improve the resolution of the data points a more stable spectrograph should be used. For the Anton Pannekoek Observatory this means that there can not be a significant improvement in the resolution using the cur-rently available instruments. The lack of data across the galaxy can be improved by using more exposure time in order to get more signal. This enables the fainter areas within the galaxy to become observable with the spectrograph and the brighter region to be split up into smaller areas. This will increase the amount of measured star forming regions with potentially smaller error bars.

5.2. NGC 4490

The rotational diagram for NGC 4490 is shown in figure 9. The projection of the measured star forming regions on the plane of rotation are within 280 kpc with respect to the determined cen-tre. The relative velocities are between −113 km/h and 72 km/s, which is in agreement with previous observations by Pearson et al. (2018). The observed slit positions did not have a star form-ing region in common. Therefore they are plotted in independent from each other by using different colours. Position 2 is shifted down by 50 km/h. The left side of the diagram has a positive radial velocity compared to the centre, while the right side has a negative radial velocity compared to the centre. This suggests a rotation in the galaxy.

Report bachelor Project University of Amsterdam

Although there is a rotation visible in the galaxy, the data was unable to suggest a distinct difference between the spiral arms suggested due to the interactions between NGC 4490 and NGC 4485 (Elmegreen et al. 1998). To measure these di ffer-ences, more star forming regions should be measured throughout the galaxy, especially in the tidally locked spiral arm. A sug-gestion to observe more star forming region is to have a longer integrated exposure time by stacking the spectra. By implement-ing this improvement, fainter star formimplement-ing regions will become measurable. This way there are more areas distributed across the whole galaxy.

Another suggestion would be to observe more slit positions. Important for these positions is that they have at least one star forming region in common with another slit position. This way all observed areas can be compared.

5.3. Error bars

The sizes of the observed radial velocities were dominated by the spread in the measured is explained in section 4.3. Due to the shifted spectrum, the highest and lowest radial velocities were taken to be the size of the error bars. Only relative velocities were measured in this research. Consequently, the determined velocity errors is the combined error of the central region and the star forming region.

The horizontal error on the data was dominated by the in-accuracy of manually determining the position of each area on the image and selecting each area on the spectrum. The error in this process was chosen to be 6 pixels for both errors combined, which led to 2.5 kpc in figure 8 and 5.6 kpc in figure 9.

6. Conclusion

6.1. Summary

The objective of this research was to measure and describe the dynamics of the starburst galaxies NGC 4449 and NGC 4490. This was done by measuring the Doppler shift of Hα emission lines using spectroscopic data taken at the Anton Pannekoek Ob-servatory in Amsterdam. It is found in this research that galaxy NGC 4490 has a spiral like rotation. This research was not able to determine a rotation in galaxy NGC 4449.

6.2. Recommendations

In this research there was no stacking of the spectroscopic data due to the instability of the LHIRES spectrograph. However, stacking the data should be possible and is highly recommended for further research. If similar observations are performed at the Anton Pannekoek Observatory, it is strongly suggested to stack the spectroscopic observations to decrease the noise of the ob-servations and get more detailed spectra.

By stacking one will be able to observe a spectrum which can be analysed continuously over the whole slit position due to the higher signal to noise ratio. This will result into a more accurate rotation curve.

Due to the geographical location of the Anton Pannekoek Observatory, it is not recommended to measure the relative in-ternal velocities smaller than 20 km/h of objects with a higher visible magnitude than 10 with the current instruments.

For further similar research it would be very interesting to not only measure Hα emission lines, but also [OIII], [SII] and Hβ emission lines. These emission lines are also present in the spectrum and would increase the accuracy of the measurements.

AcknowledgementsI would like to thank drs. M.R. Sloot for all the moments of joy and supporting me in all my challenges. Also D. Chandrikasingh for her support and motivating me throughout this process.

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