a survey of local group galaxies currently forming stars

A Survey of Local Group GalaxiesCurrently Forming Stars

Phil Massey

Lowell Observatory

April 14, 2003

The Team:

• Paul Hodge, Univ. of Washington• Shadrian Holmes, Univ. of Texas• George Jacoby, WIYN• Nichole King, Lowell Observatory• Phil Massey (PI), Lowell Observatory• Knut Olsen, CTIO/NOAO• Abi Saha, KPNO/NOAO• Chris Smith, CTIO/NOAO

Overview

We are imaging all of the galaxies of the Local Group that are currently forming stars– broad-band (UBVRI) – narrow-band (H, [OIII], [SII])

with the KPNO and CTIO 4-m telescopes and Mosaic CCD cameras.

Motivation: Our Science

The galaxies of the Local Group serve as our laboratories for studying star formation and stellar evolution as a function of metallicity, Z. (Z varies by a factor of 17 from WLM to M31.)

Why should the metallicity matter?

• Star Formation:– Lower metallicity gas should have a lower

cooling rate, and hence higher temperatures larger Jeans’ mass, leading to a top-heavy IMF (Larson 1998).

But over the limited metallicity range (3x) SMCLMCMW this effect isn’t seen!

IMF Slope in OB Associations

From Massey (2003)

Z=0.018Z=0.008Z=0.004

IMF Variations that are seen in the IMF slope are

statistical, not physical (Massey 1998, Kroupa 2001)

But what would happen if we extended this to

one-tenth solar (WLM) to 2x solar (M31)???

The answer is important for understanding the integrated properties of galaxies at large look-back times.

Star Formation/metallicity (cont)

• Some expect that the upper mass limit will vary as a function of metallicity– True only if radiation pressure acting on grains is the

limiting factor in determining the mass of the highest mass star that can form.

• So far we find that the “upper mass limits” are purely statistical, and not physical. What ever it is that limits the ultimate mass of a star we have yet to encounter it in nature (cf. Massey & Hunter 1998 ApJ 493, 180).

Why should the metallicity matter ? (continued)

• Massive Star Atmospheres and Evolution– Stellar winds are driven by radiation pressure

through highly ionized metal lines. Mass-loss rates will depend upon Z, where 0.5-1.0

– This mass-loss has a profound effect on the evolution of high-mass stars.

Relative number of red supergiants(RSGs) and Wolf-Rayet stars (W-Rs)

log

[Num

ber

RS

Gs/

WR

s]

log (O/H) + 12

From Massey2003, ARAA 41 (in press)

Relative number of red supergiants(RSGs) and Wolf-Rayet stars (W-Rs)

log

[Num

ber

RS

Gs/

WR

s]

log (O/H) + 12

From Massey2003, ARAA 41 (in press)

Need good observational database

• New generation of high mass evolutionary models are becoming available, which include the important effects of rotation (mixing introduced by meridional circulation and shear instabilities).

• Need solid observational database to help “guide” the theorists.

Our Science (continued)Along the way we’ll find:

• The most massive supergiants.

• Luminous Blue Variables and other luminous stars with H emission.

• Star formation rates for massive stars.

• Distribution and numbers of evolved massive stars (RSGs, WRs).

• HII regions, SNRs, PNe, and the extent of the diffuse emission.

Your ScienceThis survey will provide the source list (“finding

charts”) for spectroscopy with 8-10-m telescopes for decades to come. Our data products include:

• “Stacked” images (UBVRI, H, [OIII], [SII])• Individual dithered images (suitable for

photometry).• Calibration• Catalog of UBVRI photometry of roughly 300

million stars

What We’re Doing: The Sample

M31 (10 fields) Pegasus Dwarf

M33 (3 fields) Phoenix

IC 10 IC 1613

NGC 6822 Sextans A

WLM Sextans B

How Are We Doing?

M31 (10 fields) Pegasus Dwarf

M33 (3 fields) Phoenix

IC 10 IC 1613

NGC 6822 Sextans A

WLM Sextans B

What We’re Doing (continued)

Aiming for a S/N of 3 at U=B=V=R=I=25,

in 1” seeing.

Also imaging in H, [OIII], [SII]

Each field 5 ditherings, then stacked.

Hasn’t All This Been Done Before?

Yes, but not with our depth, area, photometric accuracy and resolution!

Photographic plates had the area coverage and (usually) the resolution*, but neither the photometric accuracy nor depth.

CCD studies had the depth and accuracy but not always the resolution and certainly not the area coverage.

*Wal Sargent story...

Comparison of M31 CCD Surveys

Basic Processing

Generally following the Valdes IRAF “pipeline” but with some enhancements.

• Better flat-fielding techniques.

• Better determination of sky and scaling in the stacking process (via scripts using aperture photometry).

Details, and software, can be found at our web site: http://www.lowell.edu/~massey/lgsurvey

Photometry

For the purposes of photometry, we treat eachMosaic camera as 8 separate instruments:• PSF variations within a single chip modest

compared to chip-to-chip variations.• Different DQE-wavelength dependence for

each chip means different color terms and even different zero-points (despite flat-fielding efforts).

U flat divided by I flat

Variations 30%

Photometry software

• It’s a factor of 40 times more work (8 chips x 5 ditherings) but at least when we’re done we have 1% photometry.

• We’ve developed a series of IRAF scripts and FORTRAN programs that allow us to do the photometry “automatically”, chip-by-chip, dither-by-dither.

• All of this is freely available from our web site: http://www.lowell.edu/~massey/lgsurvey

How we’ve solved the calibration problem

Lowell’s dark-sky site at Anderson Mesa

External Calibration using Lowell ’s 1.2-m Hall Telescope

• Can use only the most pristine, photometric nights.

• Select the best calibrated Landolt standards covering a complete range of colors– Investigate gravity effects on the U-band filter

U solution always squirrelly near U-B=0.

U-B

It’s a matter of some gravity....

Progress Report---How are We Doing?

• All images for M31 (10 fields), M33 (3 fields), NGC 6822, IC10, WLM, Phoenix, Sextans A, and Sextans B are now released, and sitting in the NOAO “NSA” archive, as well as our own dedicated ftp site (which makes bulk downloads easier).

• Poor weather in early September prevented us from completing the project: still need IC1613 and the Pegasus dwarf, plus repeat of poor seeing frames.

• Calibration in progress and catalog should be complete on schedule, release Jan 2004.

Did We Achieve our 1.0” seeing goal?

• Not really...

1.3”

0.76”

1.3”

0.76”

Poor seeing matters!

• To redo the images with seeing >= 1.3” would require only a few additional nights.

Sadly...

We’ve been told that our time has run out, and we aren’t eligible for additional time via the survey TAC.

So, we’ve made our best case to the standard TAC and we’ll see what happens.

(Wal Sargent Cautionary Tale)

M31 in10 fields

M31 in10 fields

M31 Fields 2 +3

M33-North

M33-Center

NGC 6822

Phoenix

WLM

What’s Next?

• Spectroscopy!

M31

N206 in M31

ob78-231

HST/ FUV ob78-231

Bianchi, Hutchings, Massey (1996, AJ, 111, 2303)

To take high S/N optical spectra at B=19 requires a really big

telescope...

The 6.5-m MMT

Optical (blue) spectrum ob78-231

Spectrum incollaborationwith Kathy Eastwood

OB78-231 at H

Spectrum incollaborationwith Kathy Eastwood

Meanwhile, these data are already being used...

• Ben Williams’ PhD thesis (Univ Washington)– Williams MNRAS 340, 143 based up a “bootstrap”

calibration.

• Forms optical basis for identifying “super-soft” Chandra counterparts (DiStefano et al., in prep)

• Images have been featured in – APOD 27 Sept 2001– Astronomy Magazine (“Sky Gem” feature), Dec ‘02– Upcoming Mercury article on “super star clusters”

(Hunter, Elmegreen, Massey 2003, in press)

Real Science

...will come once the calibrated photometry is complete this summer. (Catalog will be released at the AAS meeting in Jan 2004).

Follow-up Work In Progress

We’ll be pushing our studies of stellar winds to high metallicities (M31) using Cycle 12 time on HST (40 orbits + 80 parallels just awarded)

We also hope to begin extending our studies of the IMF to the more distant galaxies of the Local Group using DEIMOS on KeckII.