Tom Hughes (舒大同) | Research Fellow in Extragalactic Astrophysics

Feathers, a bowling ball and a vacuum chamber

Posted on 05/11/2014 by Tom

Check out these videos of Galileo’s infamous thought experiment in action.

Most people are taught about this experiment in school science lessons. Two objects, one heavier than the other, are connected by string and then dropped from a height. The assumption that heavier objects fall faster than lighter ones predicts the string should become tight as the heavier object tries to pull the lighter object downwards faster. Of course, the combined total weight is means the whole system should actually fall faster, leading to a contradiction. The assumption is proved false, meaning that heavy and light objects should fall at the same rate of acceleration, independent of their mass.

Trying the experiment on Earth, traditionally taking objects such as a bowling ball and feather for the obvious difference in mass, it appears that heavier objects fall faster than lighter ones. However, the shape and surface area of the feather means it encounters much more air resistance during the fall than the bowling ball, slowing its acceleration. To really see that objects accelerate independent of their mass, we need to perform the experiment in a vacuum (or near-vacuum where air resistance is negligible), as shown in the video below by Brian Cox in BBC 2’s Human Universe program.

Although the above video features an interesting way to demonstrate the experiment, and is deservedly doing the rounds on social media today, I still find the most beautiful and inspiring example to be taken from the last moon walk of the Apollo 15 mission, where Cmdr. David Scott simultaneously drops a feather and a hammer. Not only is it a nice demonstration of physics, the fact that people are walking on the moon is a great testament to the incredible achievements of scientists and engineers.

Makes me think: isn’t it about time already that we got ourselves to Mars?

Posted in Science | Tagged Galileo, Moon, physics | Comments Off

Stacking optical spectra of galaxies

Posted on 30/10/2014 by Tom

Here’s a short animation showing the effects of stacking up spectra.

Measuring the intensities and equivalent widths of emission lines can be tricky in galaxy spectra where the signal-to-noise (S/N) is low, mostly due to difficulty in determining the baseline continuum. The optical spectrum below is an example of one of the worst cases (i.e. not representative of our dataset!) where some emission lines are measurable but the continuum is ill-defined due to the high noise.

In order to double check that no systematic trends or bias are introduced in measurements for low S/N observations of a sample of galaxies, it is helpful to stack and combine all the spectra together to create an average spectrum. We can then see how measured spectral properties from the average spectra compare to the mean properties from the individual spectra.

One of the key steps for properly stacking spectra is to resample each individual spectrum to a common wavelength scale, making sure to conserve the total flux. Rather than re-invent the wheel, after some googling I found this article on the AstroBetter site. Using their example code snippet for using pysynphot to resample a 1D spectrum, given as

from pysynphot import observation
from pysynphot import spectrum

def rebin_spec(wave, specin, wnew):
    spec=spectrum.ArraySourceSpectrum(wave=wave,flux=specin)
    f=np.ones(len(wave))
    filt=spectrum.ArraySpectralElement(wave,f,waveunits='angstrom')
    obs=observation.Observation(spec,filt,binset=wnew,force='taper')

    return obs.binflux

seemed to work straight out of the box with no additional tweaking necessary. I included this code in a python script that inputs each spectra, resamples them, then takes the median flux in each wavelength bin. It didn’t take much extra effort to export the stacked spectrum as each new observation was added to the calculation, and using ImageMagick to create an animated GIF with the terminal command

~> convert -delay 10 -loop 0 *.png stacking.gif

produced the GIF below. It nicely demonstrates how stacking more and more spectra increases the S/N ratio. This is particularly evident at the blue end of the spectrum (wavelengths < 4000 angstroms), where the H8, H9 and H10 Balmer absorption features are initially hidden under the noise and require many spectra to be stacked (N > 70) before they are easily identifiable. You’ll also notice that the continuum wiggles around a bit before settling down at a stable average continuum, which is mostly due to uncertainties in the shape of the flux calibration solution.

Finally, you can see the full size GIF by clicking on the image.

Posted in Code, Data Analysis | Tagged python, spectroscopy | Comments Off

Role of hydrogen in the Evolution of Galaxies

Posted on 27/09/2014 by Tom

I recently returned from an amazing trip to Malaysia.

The conference I attended in Kuching covered many different aspects of hydrogen’s role in galaxy evolution, from its link to star formation and chemical evolution, to the impact of accretion, outflows and environmental effects, and also the evolution of hydrogen in the broader cosmological context.

There were many interesting talks, with a programme schedule that gave plenty of time and opportunity for discussions and introductions. I really enjoyed the review talks, particularly a talk given by Naomi McClure-Griffiths on the HI gas in the circum-galactic medium (a topic I had not previously studied in great detail), and Karin Sandstrom’s comprehensive review of the Kennicutt-Schmidt law.

My contribution was a talk on my work on the link between the gas content and the stellar mass – metallicity relation, where I found the HI gas fraction strongly correlated with both the metallicity and the scatter in the mass-metallicity relation whereby, at fixed stellar mass, galaxies with lower gas fractions typically also have higher oxygen abundances. Statistically the stellar-mass metallicity relation is mostly invariant between environments, suggesting that internal processes play a key role in governing the chemical evolution of galaxies. This work was completed during my time at KIAA and published in A&A last year. Click the image below (or here) to grab a PDF copy of my talk.

After the conference, I took some vacation (since my latest NGC 891 paper was recently submitted) and flew from Kuching up to Kota Kinabalu to explore more of Borneo, visiting the Sepilok Orangutan Rehabilitation Centre, the Kinabatangan River and the Guomantang Caves. The range of wildlife we saw was amazing – many different species of ape and monkey, including a western tarsier. I thoroughly recommend Sabah as a holiday destination.

Posted in Research, Travel | Tagged conference, gas, hydrogen, Kuching | Comments Off

Imaging the Moon with an iPhone

Posted on 11/09/2014 by Tom

…attached to a 12-inch diameter telescope!

Studying the night sky has been a passion since long before heading to university to study astrophysics. I’ve collected a number of telescopes over the years, including a 3-inch Cassegrain,
8-inch Dobsonian, and some smaller scopes for experimentation.

However, my most prized possession is my 12-inch Meade LX90GPS pictured on the left. It cost a small fortune, but is an absolute beast that allows observing deep sky objects with relative ease compared to my smaller scopes.

Sadly, the sheer size of the thing means its totally impractical for me to easily move around whilst working abroad. Factoring in the heavy tripod, a case of eyepieces/leads/filters, and some other less-useful accessories, it quickly becomes a huge headache. The overlapping region of the Venn diagram depicting “At Home In UK”, “Free Time”, and “Clear Skies” is extremely small, leading to a highly limited number of days when I can go observing with the 12-inch.

Spending the weekend with friends in a countryside holiday cottage in the UK’s Peak District gave the perfect opportunity to return to some amateur astronomy with this telescope. Luckily, the usual British (i.e. damp, grey, and overcast) weather cleared for one day and night and we were able to observe some stuff.

We first turned the scope to the Moon, which is currently at perigee and may appear 14% larger and 30% brighter – the so-called Supermoon phenomenon. With a 26 mm Plossl eyepiece on the 12-inch, the entire surface of the Moon does not fit in the field of view. However, the level of detail available due to the magnification is amazing. The Moon was not quite full, giving the craters towards the edge a three-dimensional appearance that contrasted with the flatter plains across the disc. We zoomed in using 9 mm and 6 mm eyepieces, studying individual craters.

My friends wanted to take some photos to capture these beautiful views. In the past, I’ve wasted many hours trying astrophotography. I could never afford a digital SLR camera, and instead tried a variety of cheaper alternatives including modified webcams and the Meade Planetary Imager, with limited success. We tried piggybacking a Sony compact digital camera, but we couldn’t seem to focus the image on the camera CCD.

Interestingly, our iPhone 4 and 4S models were not only able to quickly focus on the image at the eyepiece, but also adjust the contrast to avoid saturation from the bright moon. Some of our better results are shown below – I find they seem to nicely capture the curvature / spherical nature of the Moon, which so often appears just as a flat circle in the sky. The images are mostly centered around the Tycho Crater, Mare Nubium (Sea of Clouds) and Mare Humorium (Sea of Moisture). Amazing what you can do with your mobile phone these days.

After a few hours, I spent some time locating and observing the Hercules globular cluster (M13), which appeared as a fuzzy blob, and the Andromeda Galaxy (M31), which appeared as a larger fuzzy blob. Afterwards, however, the observing was cut short – I had forgotten the dew shield to protect the telescope from moisture and the lens was fogging up with condensation. D’oh!

Click on each image below to see the full size version.

Posted in Astronomy | Tagged iPhone, Moon, observing, telescope | Comments Off

Galaxies in 3D across the Universe

Posted on 08/07/2014 by Tom

Nearby galaxy studies are increasingly shifting from using global, integrated measurements to spatially-resolved observations available thanks to technological improvements in IFU 3D-spectroscopy and radio interferometry. We’re into the second day of the IAU Symposium 309, Galaxies in 3D across the Universe, which has been discussing the efforts to exploit these advancements and map a plethora of galaxy properties for a large range and quantity of nearby galaxies, and the push to the higher redshift Universe. It’s been very interesting so far, with a nice program of talks, and many beautiful maps of spectroscopically-derived properties. Check out, for example, the lovely composite image from the CALIFA team.

It’s my first time I’m presenting two posters at a conference. One is presenting the analysis of the Herschel FIR spectroscopy of NGC 891, with the paper nearing completion for submission to the journal. The other paper is advertising ongoing efforts to derive metallicity gradients for galaxies in the Herschel Reference Survey, using VLT/FORS2 observations. I’m part way through the data reduction and hope to make significant progress by the fall.

You can get a pdf copy of the poster by clicking the image below.

Posted in Research | Tagged metallicity, observations, spectroscopy | Comments Off

An emerging sacrifice

Posted on 30/06/2014 by Tom

Time is flying by, and I now find myself settled into my current position.

I’m enjoying my research, working with the UGent group, and living in Ghent (the beer is truly excellent). In short, these days I really cannot complain about anything significant.

However, there is one issue I am starting to notice after now living abroad for over three years. Prior to leaving the UK to pursue a career in research astronomy, I had many good friendships and relationships not just with fellow astronomers and students, but also those with whom I attended primary and high school. Moving to China, I naturally accepted that due to distance and the cost of travel, meeting up with friends and family would be a sacrifice and, though I obviously missed them, this sacrifice was easily rationalized by both my desire to continue in astronomy and the idea that the fellowship was only short-term in the grand scheme of things.

Moving back to Europe, I envisaged I could continue all these friendships from where we left them. After all, the Eurostar is just around the corner and my hometown is now a mere 5 hour / €60 journey rather than the 14 hour / £700 flight from Beijing. Such ideas have turned out to be very naive, on one hand because I am busier than ever whilst, on the other hand, peoples lives back home have moved on without me. Via Facebook, I see old friends getting married, buying houses, having babies and, although of course I am more than happy to see them getting on well with life, I still feel a great sadness that I am no longer a part of their lives.

One can argue that this isn’t just a problem of living abroad, that life is naturally made of meetings and partings as people make their futures, and that so-called social media gives an artificial window into others lives. None-the-less, I imagine the decision to live abroad undoubtedly accelerates the process, as I can no longer easily attend parties, weddings or reunions.

There is, of course, an advantage in all this. I’ve just returned from spending a long weekend in the UK, going to a stag party and meeting up with a couple of old friends. All those missed events make those few opportunities to spend time with friends and family all the more special, and the effort to seize those opportunities all the more worthwhile.

So, as I start considering where the next postdoc position may take me, I hope that the long term goal of the building an international career will be worth the necessary short-term sacrifices.

Posted in Postdoc Life | Tagged career, postdoc, social | Leave a comment

The SPICA Mission

Posted on 25/05/2014 by Tom

SPICA will be proposed as the next generation far-infrared space mission.

After the success of the Herschel Space Observatory, particularly from the huge leaps in knowledge gained from the PACS and SPIRE spectroscopic observations, the next space mission should seek to far surpass the Herschel capabilities and offer significant advancements that will drive astronomical research in a future landscape already being shaped by next generation facilities such as JWST and ALMA. SPICA will provide such advancements, performing large-scale far-infrared spectroscopic mapping surveys of a large range and quantity of astronomical sources from protoplanetary discs and exoplanets to the interstellar medium of nearby galaxies and the higher redshift Universe. The SAFARI instrument, a far-infrared imaging FTS-spectrometer, is designed to cover the 34 to 210 micron waveband at unprecedented resolution using the cryogenically cooled (< 6 K) ~3.2 m space telescope. See the SPICA/SAFARI factsheet below.

Given my previous research in studying the interplay between star formation and chemical evolution in nearby galaxies, and my recent enjoyment in using Herschel FIR spectroscopy to investigate the physical characteristics of the interstellar gas in the star-forming disc of NGC 891 (and also learning from other similar analyses of very nearby galaxies), I’m of course very excited by the ongoing development and future research opportunities of the SAFARI instrument for SPICA. In the context of the ISM of very nearby galaxies, having homogeneous spectroscopy with higher spatial and spectral resolution and greater spatial coverage for larger samples will offer us the chance to move beyond individual resolved studies of the physical conditions in the ISM and take a more statistical approach. However, this is just one small part of the science case for the SPICA proposal; there are many interesting things we can learn with such a facility. You can see a full range of the science discussed at the recent SPICA Science Workshop by checking out the talk slides attached to the online programme.

Finally, I’d just like to mention that I was very grateful for the opportunity to speak about my recent work on NGC 891, using this galaxy to highlight the present difficulties and limitations in using FIR fine-structure lines to probe the gas properties.

Grab a pdf copy of the factsheet by clicking the image.

Posted in Astronomy | Tagged factsheet, SAFARI, SPICA | Leave a comment

Gas properties from Herschel FIR spectroscopy

Posted on 20/05/2014 by Tom

I’ve had a great time studying how far-infrared (FIR) lines probe interstellar gas properties.

This week, I attended the 69th Dutch Astronomy Conference, held in Noordwijkerhout, The Netherlands. I presented a poster based on the latest work from my next paper in preparation, entitled “Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy” (yes, a second paper on 891), which I share with you below.

I’ve been getting my hands dirty working with FIR spectroscopy from Herschel observations, investigating the heating and cooling of the interstellar gas in the edge-on galaxy NGC 891. I examine these processes on sub-kiloparsec scales using emission maps of the [CII] 158, [OI] 63 and 145, [OIII] 88, and [NII] 122 and 205 micron FIR fine-structure lines obtained with the Herschel PACS and SPIRE instruments. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a photo dissociation region model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet radiation field, G. For the first time in NGC 891, following recent studies attempted for other galaxies like M51 and Centaurus A, I perform a pixel-by-pixel analysis to map the physical characteristics of the gas, resolving details on physical scales of 0.6 kpc.

So far, my analysis reveals that the majority of the PDRs in NGC 891’s disc have hydrogen densities and experience FUV radiation field strengths that are similar to the physical conditions of the gas in the spiral arm and inter-arm regions of the M51 galaxy. We estimate the FUV radiation field is stronger on average in the far north-eastern side compared to the rest of the disc, coinciding with the above-average star formation rate surface densities and gas-to-dust ratios observed in this region, yet a conclusive interpretation remains difficult due to observational uncertainties. The main problem I’m having at the moment is disentangling optical depth effects with the [OI] 63 micron line, which becomes optically thick faster than the [CII] 158 line, and so will likely manifest effects as we integrate along the line-of-sight through the disc (a recurring problem with interpreting edge-on galaxies). Check out the poster below for more details.

Grab a copy of the pdf by clicking the image.

Posted in Astronomy, Research | Tagged gas, Herschel, NGC 891, spectroscopy | Leave a comment

IC 2531 imaged with Faulkes Telescope

Posted on 15/03/2014 by Tom

New data taken by schools will help us better understand dust in galaxies.

Edge-on spiral galaxies like IC 2531 are used by astronomers to study a lot of interesting properties as they oﬀer us the best opportunity to determine the radial and vertical structure of the various galaxy components. The dark black bands, revealing lanes of dust obscuring the light coming from stars, allow us to understand the distribution of both the dust and stellar content. Edge-ons therefore provide an important contribution to our knowledge of the dust structure.

Because we lacked optical images of IC 2531, I proposed this galaxy as an observing target for the Faulkes Telescope Project, which provides telescope access to schools for education and outreach purposes. Our target was observed with Faulkes Telescope South by numerous school teachers and their students over the course of one month. They obtained many optical (B, V and R band) images each with typical exposure times of ~200 seconds. Then, I cleaned and combined all these individual observations to produce the final colour image below. Gert De Geyter, a colleague at the UGent Astronomical Observatory, has already begun using the data to model the stellar and dust distributions (via oligochromatic radiative transfer fitting).

See the full size version by clicking the image.

Image credit: T. M. Hughes (UGent) & F. Lewis (LCOGTN).

Posted in Astronomy | Tagged astronomy, IC 2531, observations | Leave a comment

Tom Hughes, Chartered Physicist

Posted on 28/01/2014 by Tom

I’m very pleased to be awarded Chartered Physicist status by the Institute of Physics!

Chartered Physicist (CPhys) is a professional qualification awarded by the Institute of Physics (IOP). To paraphrase the IOP webpage, CPhys represents the highest standards of professionalism, up-to-date expertise, quality and safety, and holders have demonstrated the capacity to undertake independent practice and exercise leadership. Chartered Physicists have attained an integrated masters degree in physics, acquired a breadth of physics related competence and have exercised significant levels of responsibility for a sustained period of time. The title also denotes commitment to keep pace with advancing knowledge and with the increasing expectations and requirements for which any profession must take responsibility.

Roughly twelve years ago, I was mid-way through my A-Level studies. I had chosen to study physics, maths and chemistry, which were the prerequisites for studying physics and astronomy at university. I had my heart set on studying these topics. Unfortunately, we had a terrible physics teacher. I won’t name and shame – it’s been too long and there is no point.

This teacher managed to first dissuade two out of our small class of five students from taking the subject, then apparently decided to give up on teaching the rest of us. Some lessons would be wasted by letting us freely roam the internet, but the best, most memorable lessons were spent with each student alternatively reading out loud paragraphs from the textbooks. The teacher couldn’t be bothered learning all of our names, so would just number us 1 to 3, and call us out by number to start and stop reading. It made me really angry and frustrated at the time – it was clearly not teaching us anything. We complained and complained, but it was very difficult for the school to take any action in removing/replacing the teacher. Thankfully, I had the support of some fantastic teachers at the school, who gave us extra lessons, and help and advice when making my our own way through the syllabus.

I dredge up this story of this particular teacher because he compensated his poor quality teaching with a heavy reliance on the free educational material and posters available from the IOP. In hindsight, I realise that he introduced me to the IOP’s services and professional qualifications, which surprisingly had a positive influence on my development. I kept in mind these qualifications and at each stage in my career – from undergraduate to postgrad to postdoc – checked to see how many of the requirements I had achieved and whether or not I was working at the required level. Finally, I reached a point where I could confidently apply for Chartered Physicist status.

Certainly, the 18-year-old me did not fully appreciate the amount of effort over the past decade necessary to meet the requirements of the qualification. Most effort involved many, many late nights of struggling to understand and solve physics problems. So I’m very happy to finally attain chartered status. I have had a lot of help and support from many people along the way: family, friends and peers, teachers, supervisors and colleagues, and I thank you all – you know who you are!

Yet now that I’ve reached this goal, it doesn’t end here: the right to remain a chartered physicist has to be justified every three years by showing continued professional development. So – back to work now!

Posted in Astronomy, Postdoc Life | Tagged career, chartership, IOP | Leave a comment

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Address: Instituto de Física y Astronomía
         Universidad de Valparaíso
         Av. Gran Bretaña 1111
         Valparaíso, Chile

Telephone: +56 9 9901 3258
FAX: +56 32 2508230
Skype: txhughes
E-mail: thomas.hughes (at) uv.cl

TomHughes Research Fellow in Extragalactic Astrophysics