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Cannibaliza?on of Dwarf Galaxies by the Milky Way Distance to the Leading Arm of the Magellanic System
Jacqueline Antwi-‐Danso1 and Kat Barger1,2 1Department of Physics & Astronomy, Texas Chris?an University, Fort Worth, TX 76129, USA
2Department of Physics, University of Notre Dame, IN 46556, USA
Jacqueline Antwi-‐Danso Department of Physics and Astronomy, Texas Chris?an University ✉︎ Email: [email protected] ☞ Website: www.tcu.digica?on.com/jacqueline_antwidanso ☏ Phone: (817)-‐703-‐2418
Contact 1. Fox et al. 2014 2. Reynolds et al. 1998 3. Haffner et al. 2003 4. Caset-‐Dinescu et al. 2014 5. Fox et al. 2005 6. Bland-‐Hawthorn et al. 1999, 2001 7. Haardt & Madau 2001
References
Background Since the strength of the Hα emission is directly propor?onal to the rate of recombina?on, Hα emission traces all the warm ionized gas in the Leading Arm. Figure 4 shows the Fox et al. 2005 model for the ionizing flux of the MW combined with the Haardt & Madau 2001 model for the EGB as a func?on of distance from the Galac?c center. The ionizing flux is most intense at the center because that is where most star forma?on takes place. This plot is two dimensional for illustra?on purposes, but is adequate for describing this region in three-‐dimensional space because the ionizing flux contribu?ons from the MW and EGB are symmetric. We took the calculated Hα intensi?es and their corresponding sta?s?cal errors and calculated the ionizing flux needed to reproduce the Hα emission through photoioniza?on alone using Equa?on 1. We then compared this value with MW and EGB models of the ionizing flux in Figure to determine the distance that best matched the an?cipated flux at the Galac?c longitude and la?tude of our observa?ons. From this, our best es?mate for the distance to the Leading Arm at the loca?on of our observa?ons is 18.1 kpc from the Sun. This represents a minimum value for what the distance could be, as we are ignoring the ionizing contribu?on of the Magellanic Clouds and sources of collisional ioniza?on. Therefore, we find that this part of the LA is ≥ 14.8 kpc from the Sun at a Galac?c height of ≥ 9 kpc. Casse?-‐Dinescu et al. 2014 find the O6V star formed in the LA to be 40 kpc4 from the Sun. Using the reported coordinates of the star, we found it to be at a Galac?c height of -‐10 kpc; hence our study is consistent with their results. ObservaHons and ReducHon
To determine the Hα intensity, we modeled the emission as a Gaussian convolved with the WHAM instrument profile. Since the LA Hα emission is faint and diffuse, this behooves us to employ sta?s?cal methods that ensure that our uncertain?es are reported to a high degree of accuracy. Of the seven sightlines, four exceed a 3σ detec?on. We determined the best fit by minimizing the χ2 with the goodness-‐of-‐fit technique, using the IDL MPFIT rou?nes. To calculate the systema?c uncertain?es in our intensity values, we created a three-‐dimensional best χ2±1 volume and found the loca?on of the minimum value in the volume, which corresponds to the loca?on of the best χ2 value for the data set. Using this loca?on as an index, we created a similar intensity volume and calculated the differences between the value at that loca?on in the volume and the maximum and minimum values in the volume. These represent the upper and lower systema?c uncertain?es respec?vely.
Error Analysis
The Na?onal Science Founda?on supported WHAM through AST 1108911 and the TCU REU through grant number NSF PHY-‐1358770. K. A. Barger is further supported through NSF Astronomy and Astrophysical Postdoctoral Fellowship award AST 1203059.
Acknowledgements
Results
Fig 1. Map of the 21cm HI emission in the Magellanic System noHng distances above and below the GalacHc plane of our observed region and the O6V star. Photo credit: Nidever, et al. 2010, NRAO/AUI/NSF and Meilinger, Leiden-‐ArgenHne-‐Bonn Survey, Parkes Observatory, Westerbork Observatory, Arecibo Observatory.
Fig 2. – LocaHons of Hα observaHons in the LA. DetecHons are open circles and non-‐detecHons are cross-‐filled circles. The seven WHAM observaHons presented in this study lie within the black rectangle and are labeled (a-‐g). LocaHons with HST/COS observaHons by Fox et al. 2014 are marked with orange circles. The star marks the locaHon of an O6V star observed by Case_-‐Dinescu et al. 2014.
Fig 4. – Ionizing radiaHon field of the Galaxy5,6 and the EGB7. The plot shows a 400 x 400 kpc slice through the center of the Galaxy. The numbers on the contour lines give the logarithm of the flux in units of photons cm-‐2 s-‐1. The ionizing contribuHon of the EGB has a constant value of 3.22 x 104 photons cm-‐2 s-‐1 (Haardt & Madau 2001, Bland-‐Hawthorn et al. 1991, 2001) .
10
15
20
25
88 83 78
g
a
b
f
c d
e
Magellanic Stream Longitude (Degrees)
Ma
ge
lla
nic
Str
ea
m L
atitu
de
(D
eg
re
es)
30
-20
20
100 70 40 10 -20
Magellanic Stream Longitude (Degrees)
Ma
ge
lla
nic
Str
ea
m L
atitu
de
(D
eg
re
es)
Log(HI Column Density / cm-2)
17.6 17.8 18.0 18.2 18.4
Leading Arm
Magellanic Stream
LMC
SMC
Milky Way
We collected our observa?ons using the Wisconsin Hα Mapper Telescope (WHAM) at the Cerro Tololo Inter-‐American Observatory (CTIO) in Chile. It is currently the best instrument available for detec?ng faint emission from diffuse ionized sources due its high-‐throughput duel Fabry-‐Perόt spectrometer combined with a 0.6-‐meter objec?ve lens2. We made seven pointed Hα observa?ons posi?oned at 75 – 90° from the LMC and SMC and reduced the data with the same On-‐Off target procedure used in Barger et al. 2015 and the standard WHAM pipeline3. We ignore collisional sources of ioniza?on and photoioniza?on from the Magellanic Clouds (MCs) and propose that if the MW and EGB cause most of the photoioniza?on, then the rate of ioniza?on of the gas is propor?onal to the rate of recombina?on of the protons and electrons. The strength of the Hα emission will then be propor?onal to the ionizing flux because the Hα transi?on line arises from recombina?on. Since our Universe is made of mostly hydrogen, the number of “free” electrons will be approximately equal to that of ionized hydrogen atoms, hence: We assumed that the gas is op?cally thick and that Te=104 K since Hα emission peaks at this temperature.
Galaxies form stars from clouds of gas and dust. In the absence of a source of replenishment, large galaxies – like the Milky Way (MW) – deplete their gas reservoirs in a few billion years, far shorter than their life?mes. Chemical evolu?on models suggest that the Galaxy has survived by accre?ng low metallicity gas from the intergalac?c medium and dwarf galaxies. Tidal interac?ons between two dwarf galaxies near the MW, the Large and Small Magellanic Clouds (LMC and SMC), have caused large quan??es of gas to be flung into the halo of the MW. Much of this ?dal debris, known as the Magellanic System (Msys), is currently headed towards the disk of the MW, spearheaded by the Leading Arm (LA). Es?mates for the amount of gas contained in the Msys suggest that it could raise the present star forma?on rate (SFR) of the Galaxy1. Unfortunately, its posi?on predisposes it to ionizing radia?on from the MW halo and extragalac?c background (EGB), hindering gas accre?on. The LA, however, appears to have already survived the trip to the disk as its morphology indicates interac?on with the interstellar medium (ISM) of the Galaxy. The exact amount of gas that this structure contains is uncertain because of weak constrains in its distance. We made an emission-‐line study of the Hα ioniza?on in the LA and compared the observed with models of the an?cipated ionizing flux from the MW and EGB. Based on this, our current best es?mate for the distance to the LA at 80° from the LMC is d ≤ 14.8 kpc from the Sun.
Fig 3. – Data fi_ng using modified IDL MPFIT rouHnes. The blue line is the “best guess” fit of the data set. The fit with the best 𝜒2 and its corresponding background are ploged in purple.
LSR Velocity (km s-‐1)
Intensity
(mR/km
s-‐1 )
�LC = 2.1⇥ 10
5
✓IH↵
0.1R
◆✓Te
10
4K
◆0.094
photons cm
�2s
�1.
Observed Region
+6.3
-‐3.3
∼10 kpc
≥ 9 kpc
(1)
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