Grey Bird Explorations

After a lengthly hiatus I am back on line in the midst of the pandemic and related chaos. On Thursday, 8 October 2020, I presented an invited webinar concerning study of the Milky Way Galaxy structure and evolution at LIneA – Interinstitutional Laboratory of e-Astronomy, based in Rio de Janeiro, Brazil. This was live on Zoom and also streamed via YouTube. Here is a PDF copy of the slide deck.

An edited version of the automatically generated transcript follows below.

Moderator: Good afternoon everyone.
Peregrine? Could you share your screen? And good morning.

Peregrine:

Good afternoon. This is Peregrine McGehee coming to you live from Santa Clarita, California.

Moderator:

Thank you everyone, I will just make a short introduction alright.

Hello everyone, welcome to another talk in our webinar series. I would like to remember that this webinar is also being streamed live on YouTube. Tou can find the link on our website or access it on YouTube channel LineaDivulgação.

Today we have Peregrine McGehee from the College of the Canyons presenting the talk entitled Mapping the Milky Way and Beyond. And Peregrine thank you very much for being with us so the floor is yours .

Peregrine:

Introduction

All right and first of all I want to thank you very much for the invite to give a webinar. I have not yet walked in the streets of Brazil so this is my closest chance so far!

So thank you for the opportunity and so for today I am going to be talking about one of the four main science themes that will be explored in the Legacy Survey of Space and Time that will be
undertaken by the Rubin Observatory currently under construction on Cerro Pachon in Chile.

To begin with this is not an image from Cerro Pachon. This is actually somewhat to the north. This is a marvelous work of astrophotography showing extinct volcanoes in the Atacama desert in northern Chile – a nice view of the Milky Way and our two satellite galaxies, the Large and Small Magellanic Clouds.

So this is basically setting the stage for some of the science and some of the explorations that we would want to do as part of the LSST.

That being said let me give you just very quick introduction to myself. So I am presently with the Earth and Space Sciences department at College of the Canyons in Santa Clarita, California – which is about 60 kilometers to the northwest of downtown Los Angeles. It is a community college and one of the major ones in the area.

My own history: in the dawn of time actually I was a control systems engineer working on numerous ground-based telescopes and particle accelerators. In the context of working with the Sloan Digital Sky Survey I was convinced to go back to school and pursue my doctorate in Astronomy which I completed 15 years ago at New Mexico State University in Las Cruces. So I am now here! That is just a short version of my background.

Key Science Themes

So what I want to get into today is that I want to give you an introduction to the Legacy Survey of Space and Time and the Vera Rubin Observatory. I think many of you are probably familiar with it already but I want to get a little bit more nito one of the four main science themes. So as you know the Rubin Observatory is well under construction on the summit of Cerro Pachon just right down the ridge from SOAR and from Gemini South.

Within the LSST, which is now the survey name, there are four main science themes. One of course is answering the obvious deep cosmological questions of understanding dark matter and dark energy. The second theme is identifying potentially hazardous asteroids and also looking far out into the Kuiper Belt and beyond, that is, looking at the original construction material for the solar system. The third is looking at the transient or the variable sky, noting changes in brightness, changes in position. This is one of the themes that LSST is going to be groundbreaking in because we are going to map the sky over and over again for a decade. We will see things that we have never expected before!

But coming into the final theme of the four main science themes is the formation and structure of the Milky Way and this is going to be the focus of my presentation today.

So one of the science collaborations within LSST is the Stars, Milky Way, and Local Volume. Historically there were two separate science collaborations, one dealing with stellar populations, the
second dealing with galactic structure. At some point the realization came to all that
many of the concerns and many of the themes between these two science science collaborations were shared and so they merged together. So we’re now the Stars, Milky Way, and Local Volume collaboration looking at the past and the present of the Milky Way Galaxy and our neighbors.

We are going to be looking at how we are structuring the working groups, what the status of the science collaboration is, and particularly what are the things that we might be able to do especially as we’re coming into the post-Gaia era.

In part one, which is an introduction to the observatory and to the collaborations as a whole, a number of slides that I will be adding in here actually came from presentations made at the August Project and Community Workshop – so if you were there they may look a little familiar!

The Vera Rubin Observatory.

But what is the Vera Rubin observatory? Well, the survey telescope is now christened the Simonyi Survey Telescope. This is an 8.4 meter diameter reflector with an extremely wide field of view.
In three nights we can observe the entire sky but of course that is just in one filter. We will be using six different filters. This are the standard ugriz like we saw in SDSS plus a near infrared y band. As I said it is located on the ridge of Cerro Pachon
and it’s just about a hundred kilometers inland from La Serena so it’s basically the next mountain over from Cerro Tololo.

Construction is well underway – it is great to see that the observatory s now looking much more like it’s architectural renderings and here is a close-up of the optical support structure.
The original schedule (pre-COVID) was for operations start in October of 2022 so just two years hence. We will see how that schedule has to get modified due to the events in the world but
this is a telescope project that is well underway and well on its way to completion and start of operations.

The heart of the observatory. Unlike many other astronomical facilities the Rubin observatory only has one instrument and this instrument is an imager, a 3.2 gigapixel CCD device, constructed at SLAC in Stanford, California. Here is one of its early images – a test image done in the lab of a head of broccoli! Of course this is nowhere near the full size of the image, remember it’s 3.2 gigapixels in size so f you could zoom into it the detail is is absolutely incredible.

The Science Collaborations

I want to talk about the context of the work on the Milky Way and the Local Volume and the construction funding from the National Science Foundation and the U. S. Department of Energ that is for construction and for operations. The science is a completely separate effort. The collaborations are self-managed and self-governed and there are eight of them.

Starting on the top TVS is transients and the variable sky. SMWLV of course is the Stars, Milky Way, and Local Volume. DESC is the dark energy survey. SSSC for solar system objects. There is a separate science collaboration for galaxies and yet another one for active galactic nuclei. A seventh one dealing with strong lensing and finally one dealing with information science. This last one is more of a technology based working group as opposed to having a specific science theme.

At least as of August [2020] there were over 1500 members in these eight different science collaborations. Within Stars, Milky Way, and Local Volume it was at the time it was just shy of 200 and we sort of self-organized into a number of working groups that I will get into. These science collaborations are in various different states of organization and formality with actually the dark energy science collaboration being the most formally structured. But you can see that there is
quite a diversity in terms of the subjects that we are talking about and the relative number of participants so far in these working groups.

We are spread across the entire world: five continents and 20 countries. Here in the United States where I am speaking from you can see the breakdown by state. When we had the Project and Community Workshop in August traditionally we would have convened at a physical location in Tucson Arizona taking advantage of the hot August Weather and with several hundred people at a large hotel complex. But because we had to go virtual we ended up with over 700 participants in virtually every time zone on the planet. Although we missed the face-to-face interaction I think because we had so many people in an online venue it was a very successful week!

Talking about science here we are at the Stars, Milky Way, and Local Volume collaboration. We are going to be able to study the structure in the Milky Way, we are going to resolve stellar populations, and get information on 17 billion stars. Of course the dark energy science collaboration is going to be working with dark energy and dark matter. With the variable sky and AGN collaborations we are looking at the transient and variable universe, and finally through the solar system science collaboration effort we will be able to produce an extremely complete inventory of the solar system. So no matter what your favorite branch of astronomy is chances are through that through the LSST survey at the Rubin observatory we are going to be able to address that.

The Stars, Milky Way, and Local Volume science collaboration

So that established context and now I want to get into the nature and the structure of our particular science collaboration. To begin with in order to study the structure and evolution of Milky Way we have to look across the entire Galactic sky including fields that many of the other science collaborations do not have a strong reason to work in. What I am talking about are the crowded fields like in the disk, within the visible Milky Way itself, in the Galactic bulge and also in like the crowded fields in the Large and Small Magellanic Clouds.

There is often an impression that many of our science cases, looking at Galactic structure and evolution, are focused on looking at static images and perhaps leveraging off the deep co-adds across the survey. Bur here as well as we will discover there are many explorations that make use of time domain imaging, both for time domain photometry and also for
astrometry watching the motions of objects over time.

I am going to list some of the science that has been proposed. As you know this is a survey – as we found out for example by working at the Sloan Digital Sky Survey even though a survey may have been designed with a particular set of science cases in mind there are so many other investigations that you can do with this incredible data.

The broad scale science drivers for Stars, Milky Way, and Local Volume are first of all we want to understand the assembly history and structure of the Milky Way and of the local volume members.
Also we want to be able to understand the fundamental properties of stars within several hundred parsecs of the sun so getting all the way down into the brown dwarf and maybe even the free floating planet regime.

Now I want to talk to talk about some of the example science cases that deal with the time domain. Obviously in time domain photometry we are looking at stars and many stars are variable. We think we will be able to get multi-color light curves to an accuracy of two percent or better or at least 50 million variable stars. We will be looking at studies of cataclysmic variables like dwarf novae systems, eclipsing binaries, and we are going to find all kinds of rare types of variables just because we are looking at the sky again and again in multiple colors.

We will be able to grab large samples of eclipsing binary stars. As you remember eclipsing binaries are great because you know the system is edge-on and so when you are going pull your standard Kepler’s Third Law trick nd apply it to the period and the orbital size of the binary system. You basically will know the mass because it’s edge-on.

Besides looking at time domain photometry we can look at time domain astrometry. We will find all kinds of rare faint and high proper motion objects. We will look at the faint end of the stellar mass function and we will find, you know, the nearby brown dwarfs with high proper motions, and we will find free floating planet candidates. We will be able to use trigonometric parallaxes both with the LSST data itself and as I will discuss further on by combining and leveraging off the precise data that’s come out of the Gaia satellite. We will be able to do a complete census of the solar neighborhood out to 100 parsecs based on trigonometric parallax measurements for very faint objects like down to a r band magnitude of 17.

Time domain studies aside when we do static science which is also going to leverage off of the ability to do very deep co-adds especially from 10 years of survey data in multi-bands. We will be able to hunt for more of our dwarf satellites not only for the Milky Way but across the local group like more around M31 and M33. We will also be able to find intergalactic stars.

By using color combinations like the z minus the near-infared y color we will be able to identify very low mass subdwarfs and distinguish them from he higher metallicity disk dwarfs. We be able to study white dwarfs and look at their luminosity function and we will be able to improve three-dimensional studies of interstellar extinction which is of course a byproduct of the interstellar dust.

There is more – we will be able to look at the chemistry and proper motion of mid mass, like solar mass, main sequence stars out to 100 kiloparsecs. We will be able to create deep color-magnitude diagrams or about 50 percent of the known global clusters. We will be able to look at at streams like from the Sagittarius dwarf and look at the chemical composition, the motions, the spatial profile of these field of streams entities. We will also be able to look at the halo and local group out to 400, 000 parsecs using maps of RR Lyrae stars which of course would be identified by the time domain photometry . There is a lot we are going to be able to do with understanding the nature, the structure, and the evolution of our galaxy.

There is a lot of work here so when we want to talk about the collaboration we want to talk about our structure and present leadership. Right now we have a single tiered structure so all everybody who is a member of the science collaboration has the same rights and privileges. Of course as some of you are aware once we have commissioning and operations data in hand there are issues with data rights that are being resolved.

Right now we have about 180 members and nominally each of our co-chairs serve for three years. I started just over a year ago and Will Clarkson at the University of Michigan-Dearborn started this year so we would think that probably next year we would hold an election. Individual could basically self-nominate, we will do a polling of the collaboration, and then at that point John Gizis at Delaware would likely rotate off.

The Science Working Groups

How do we structure? I spent some time giving you a taste of the kind of science that’s involved with the Milky Way and Local Volume and historically what we have done is we have broken this up into seven distinct science working groups. Going from the nearest neighborhood out to the farthest reaches of the local volume in order they are the solar neighborhood, star clusters, variable stars, study of the galactic bulge, study of the galactic structure and the interstellar medium, then the Magellanic Clouds, and finally near field cosmology. I am going to briefly touch on these in the next section.

For the solar neighborhood this example image is showing detections of a very cool brown dwarf.
The current chairs are Ben Burningham at the University of Hertfordshire and Sebastian Lepine at CHARA. This group is well underway and is focusing on categorizing stars and everything else within a couple hundred parsecs

Moving out to star clusters. This is actually a Hubble Space Telescope photograph of a cluster in the Magellanic clouds. At the present time this particular working group is reforming and we will be looking for more interested parties who might want to chair this group which will be looking at both open clusters and global cluster with the LSST data.

The variable stars group is in a similar position [in regards to leadership] but it has a very tight synergy with the Transients and Variable Sky science collaboration. Here is a classic example: this is RR Lyrae itself and a typical RR Lyrae light curve for comparison. Part of synergy with TVS is using variable stars to map the structure of the galaxy in the halo. Also to look at indicators for accretion and outflows like for x-ray binaries and protostars, to look at microlensing events, and so on because these are all inform us about the structure and components of the Milky Way Galaxy and the local volume. There is a very tight connection with the TVS collaboration.

The galactic bulge group is fairly active. the chairs right now include Will Clarkson,
who is also one of our co-chairs for the collaboration, at Michigan-Dearborn so this studies the stellar populations deep within the galactic bulge. So they deal with another crowded field issue.

And then moving out to the Galactic Structure and Interstellar Medium. So like Will I am actually wearing a working group hat. Here is a gorgeous image of the Milky Way in the infrared from Spitzer.

Knut Olsen in Tucson is the chair of the Magellanic Clouds group which is also concerned with study in crowded fields especially in the LMC. So this is one group that is specifically looking at neighboring objects in the local volume so not just the structure of the Milky Way itself.

Then lastly for Near Field Cosmology we are looking at streams in the halo. Here is a classic field of streams image derived from Sloan Digital Sky Survey data. We are also looking for new leadership for this science working group and there is a strong connection with the dark matter program that was initiated within the Dark Energy science collaboration.

And there is a whole theme of course of probing the nature of dark matter by looking at he structure of halos, by looking at the structure of sub-halos, looking for evidence of dark matter from microlensing events, and looking for evidence of dark matter sub-halos by looking at the structure of these accretion streams. For example, if you had a stream and a dark matter subhalo came through it you would expect to see tidal disruptions within that stream of stars and so on.

There are many different opportunities here so that in a nutshell was just an overview of our science working groups.

Science Collaboration Activities

Now I am going to talk a little bit about of our activities so many of you have probably seen the
classic image of the LSST survey footprint and that’s always shown in equatorial coordinates. Here I am showing you what that looks like in galactic coordinates. The main curve here corresponds to our northern limit in equatorial coordinates and this region of avoidance (the round circle) f course it corresponds to the south celestial pole. Then there is a very dense region that includes towards the center of the galaxy that we’re we’re probably not going to observe either. However, you can see we we are planning coverage in different regions at very low galactic latitude.

Within the science collaboration one of the things that we are working on is that for the kind of science we want to do how do we quantify that science return when you do a survey. Even though LSST is a general survey your cadence, meaning your choice of what part of the sky do I observe when and what filter do I use actually makes a difference to certain science cases. Being able to develop metrics that basically say for this particular science case how does the way I observe the sky make a difference.

The operational simulation teams have basically constructed a series of simulations of what the data stream would look like for these different cadence or mapping scenarios. Internally we are organizing task forces, actually Will Clarkson has taken a big hand with that. Some of them are dealing with the observation strategy that would help facilitate Milky Way science.

[Brief interruption from my dogs]

There is another task force deals with understanding how to do accurate photometry and astrometry within crowded fields. There are several different teams working on that as well as the effort within the project. We want to be able to reach out and have liaisons with the other science collaborations with the project and of course we want to reach out to new potential members in the community.

We have roadmap documents with the original versions of them dating from several years ago
and there is a lot of information in the LSST science book which is always being updated. As you know several years ago there was a call for white papers dealing with different cadence concerns and doing different science. We are trying to take this information and formalize and re-update the science roadmaps for the Stars, Milky Way, and Local Volume collaboration. We also want to help our various working groups before we prepare to do science with the survey data.

Here is an example of the collaborations we’ve been doing. We had a joint virtual workshop, well in part virtual, hosted by the University of Delaware last year that was between the Stars, Milky Way,
and Local Volume collaboration and the Transients and Variable Sky collaboration that was very productive.

Into the Future – an Example

Astronomy is not a static field, it moves sometimes very quickly and I want to talk about one example of this and how the Rubin observatory and galactic science may be well positioned when we start operations in a couple years. So now I am going to put on my cience working group hat and I am going to talk about galactic structure and the interstellar medium.

This has been a very active field of late, of course. Gaia has revolutionized our understanding of the locations and motion of the stars. DR2 came out a few years ago, and DR3 is coming out at the end of this year. And then in terms of understanding the three-dimensional distribution of the extinction in our Milky Way, Green et al. have been doing an amazing work mostly based on Pan-STARRS 1 data.

Here is, for example, theie most recent publication based on Gaia, Pan-STARSS, and 2MASS.
They did a fairly robust Bayesian analysis of the extinction. but I think that we can actually maybe do a little bit better when we add the LSSTdata.

Here are some examples, these are just a few of the example results from Gaia DR2 and this is a sort of a taste of the kind of things that we should be able to improve on with LSST. One example here: there is a warp detected in the disk of the Milky Way galaxy and it precesses with a period of about about three times as long as the overall [orbital] period of our sun. As seen on the right hand corner we have been able to to detect proper motions of stars in the Large Magellanic Cloud (this is with Gaia). You can see the sense of rotation of this small spiral galaxy.

Gaia has also found evidence of a major merger based on kinematics. Here in the dense group these are objects whose motions, both their circular motion and their radial motion, are consistent with essentially orbits in the Galactic disk. Gaia found a whole new distribution of stars that were inferred to come from what’s now called the “Sausage Galaxy” as a recent major merger. This is a taste of what is from Gaia.

Now from the Green et al. 2019 paper. Hereis an example, this is showing he reddening based on the r-z colors and this is for the cumulative mapping distances between one half and one kiloparsec across the Pan-STARSS footprint. Gorgeous detail here, right so there is a lot of really interesting work that is helping to lay the foundation for what we expect to find once the LSST survey starts producing data.

So what are our strengths? The Pan-STARR people have done this amazing work but some of the strengths that we will have is we will have deeper multi-band photometry. That is going to improve our priors for stellar luminosity. If you look look at the H-R diagram it is not uniformly populated. There is a clear main sequence so having this deeper multi-band photometry is going to really help nail down the initial priors for stellar luminosity.

Gaia has already given us a great temporal baseline for the brighter sources that would also be in the Gaia catalog. We will have a much longer baseline so we will get much more accurate information on parallaxes and proper motions. Since we are much more sensitive than Pan-STARRS we can conceivably probe deeper into regions of high extinction and also look at the extinction distribution at greater distances.

Not only for galactic science but for other programs if we can manage to get well calibrated extended source photometry we could look at multi-band maps of extended regions in ISM and more. There are a lot of different things we can do.

Here are some predictions of what we might see. Here is a color-color diagram showing the main sequence stars – the locus of stars here. You can pick the horizontal branch stars and you can see the white dwarfs clearly. Of course the quasars which show up as point objects. As how was learned in SDSS they are fairly identifiable on basis of the photometry. On the right we look at what SDSS found – the scale is out here about 20 kiloparsecs. And here is the stellar distribution but with LSST we are looking at way out into the halo. You can see in this cartoon image evidence of streamers and local overdensities in the halo, currently unknown dwarf-like subhaloes, and so on.

Summary

There is a lot of work to be done in galactic structure, I think we are going to be quite busy over the decade or so to come

Here is how you can get a hold of us.

Website:

• The placeholder SMWLV website is http://milkyway.science.lsst.org.

Communication:

• E-mail: The main e-mail list for the SMWLV science collaboration is lsst-milkyway-etc@lists.lsst.org. 
• Slack: The LSST science collaborations use the LSSTC slack workspace (http://lsstc.slack.com) The channel specific to SMWLV concerns is #milkyway.
• Discussion Forums: The LSST community discussion forums are hosted at http:///community.lsst.org. 
• SMWLV topics can be found at https://community.lsst.org/c/sci/milkyway.

Like pretty much every other field of astronomy the Rubin observatory is going to totally revolutionize it. I have only touched on a few of the possible science themes dealing the local volume and Milky Way evolution and structure.

One thing that has been pointed out is that in the first year of operations LSST will basically acquire more information about the sky than has ever been uncovered. By the time the survey is done we are going to have at least an order of magnitude (if not more) information and knowledge about the universe that we live in!

This is actually a nice picture of the Whirlpool galaxy – beautiful view of a majestic spiral galaxy!

Now at the very end I want you to want to give you just a construction update. Obviously with the global pandemic things have been on hold but limited construction activities have resumed as of the end of last month and work will be wrapping up gradually over the next few months.

I think the project still needs to discover what the impact for the commissioning and operations schedule is but at least construction has restarted and in a very safe manner. I am going to close with this image taken off of the web camera this morning. It is a nice view of Cerro Pachon summit with Rubin observatory in the foreground. Construction is still
underway and it is looking more and more like an operational facility.

I think we have got 10 minutes remaining in the time slot so I will open the floor for questions either by voice or by chat.

Moderator:
Okay, well thank you very much Peregrine for the very interesting talk.

Peregrine:
you’re welcome

Moderator:
Well we have some time for questions from the audience. If you are with us on YouTube you can use the chat and I will redirect your questions to the speaker – so questions?

Okay well, I have some questions. [Why wait for more people?]
Actually I am very interested also and although I am not working now precisely with stellar astronomy. When we compare what we will have in the LSST with what we have in Gaia I assume that LSST will not measure distances or parallaxes ?

Peregrine:

So LSST will go a lot deeper than Gaia and obviously we are limited by our ground-based facility with no adaptive optics. We are just limited to natural seeing, However, we will be able to leverage off the astrometric data from Gaia. We will be able to produce proper motions and and parallaxes.

Moderator:

Okay that is nice, also I assume that some people will study binary systems that to some extent you can also measure distances from stars once you know the orbital period of the system that is very
interesting,

What is the newest and most exciting thing in your field that you are expecting from LSST? Like what is coming that you can only get with LSST?

Peregrine:

I think there is a million different answers to that but I am going to pick one. In terms of galactic structurebeing able to map out the halo of the Milky Way and also of some of our nearest neighbors in detail I think it is going to tell us a lot about the underlying dark matter distributio, So basically more of a focus on the near field cosmology.

Moderator:

That is nice, I mean this is only only one topic right, so yes there is so much that it is coming with LSST. It is very exciting. We have time for more questions. Don’t be shy! More question.? Assume that will be it. Thank you, Peregrine!

This presentation will be available on our website linea.com.br and LIneA’s YouTube channel.

For the next week’s webinar we will have Cyrille Doux from the University of Pennsylvania. He will talk about how to the blend galaxies using a joint multi-band and multi-instrument approach and thank you everyone for attending and a special thanks to Peregrine.

Peregrine:

Thank you and thank you so much for the invite and I will send you a pdf version of the presentation today.

Moderator:

That’s great okay.

Peregrine:
You’re welcome.

Moderator:

Thank you so much everyone. Have a great day and see you next time.