ALFALFA Team Website

ALFALFA Team Project

Proposal Title:	An H-alpha Imaging Survey for a Volume-Limited Sample of ALFALFA Sources
Project Leader:	John Salzer
Institution:	Wesleyan University
Other Co-Authors (if any):	Wesleyan students (TBD); other collaborators welcome
Time frame:	Ongoing, but expect useful results within first 1.5-2 years
Publication expected:	First results paper would be submitted Fall, 2007 or Spring, 2008. Data papers to follow closely.

Description:

We plan to obtain narrow-band H-alpha images for a large sample of HI-selected ALFALFA galaxies in selected fields. The primary goal of this project will be to measure an accurate value for the local star-formation rate density (SFRD). The sample of galaxies selected by ALFALFA will yield a SFRD value far superior to any that currently exist (or that are currently being done, such as SINGG). This is because ALFALFA will be sensitive to galaxies well beyond the local supercluster, and will have superior spatial resolution to allow for unambiguous optical IDs at large redshifts. Further, ALFALFA will catalog all galaxies capable of making stars in the volume chosen for this project (i.e., all those with HI gas), hence it will be much more complete than previous studies that have relied on emission-line galaxy surveys (e.g., Gallego et al. 1995). In addition, we expect to uncover a large number of interesting candidates for follow-up optical spectroscopy. These objects will include dwarf star-forming galaxies (a.k.a. BCDs) and ultra-low luminosity dwarfs for which obtaining nebular abundances would be highly desirable, and possible examples of low-luminosity AGN (similar to NGC 4395) which will allow us to study the AGN/host galaxy connection at the low black hole mass limit.

SAMPLE SELECTION: Since ALFALFA will detect tens of thousands of HI sources to velocities approaching 18,000 km/s, we need to limit the scope of any follow-up program of this nature. Since the primary goal is to measure the local SFRD, we would like to target a volume-limited sample of objects that is (i) sensitive to the dwarf galaxy population, and (ii) still covers a sufficient volume to include a good sampling of the luminous galaxy population. The way to achieve this is to preselect galaxies at intermediate redshifts (say 2000-6000 km/s for the sake of discussion). We purposely avoid the lower redshift galaxies, since their distances will be poorly determined from their redshifts, as well as the higher redshift galaxies, where we will only be probing the high luminosity end of the HI-mass luminosity function (HILF). By restricting observations to intermediate redshifts, we will sample the lower end of the HILF adequately (although not the extreme low end) while covering enough volume to also sample well the high mass end. The latter is key, since most of the star-formation action occurs in the higher mass galaxies.

We can construct volume-limited samples based on some as yet unspecified upper and lower redshift limits (presumably set in part by the specifics of the available H-alpha filters at the various observatories), observing all galaxies within quasi-randomly selected ALFALFA tiles. The tiles selected need not be contiguous on the sky. We would naturally avoid tiles located in regions of above-average Galactic absorption, but since ALFALFA is primarily a high-latitude survey, this should not be much of a constraint.

OBSERVATIONS: We would acquire narrow-band H-alpha images for each target galaxy, using an ON-OFF-ON pattern and using the single OFF-band image to subtract both ONs. In the past we have used both narrow-band and R-band OFFs; our current leanings would be to use R-band OFFs to save time. Our tests indicate that R-band OFFs do not lead to substantially lower-quality results, and they are the standard for many other H-alpha imaging projects (e.g., Kennicutt's group, SINGG). Our current standard exposure time is 20-min per ON, and 4 minutes per OFF using the WIYN 0.9-m telescope. These values can be modified somewhat if desired, but yield adequate quality data in a reasonable amount of time (~1 hr per galaxy, including set-up and readout overhead). The H-alpha fluxes are calibrated using spectrophotometric standard stars.

PROJECT PLAN: Wesleyan University is a partner in operating the WIYN 0.9-m telescope located on Kitt Peak. We have ~10% of the observing time (~16 nights per semester). Roughly half of this time can be expected to go to the PI for this project (more in some semesters, depending on internal proposal pressure). In addition, it is possible that we could increase our share of the time if necessary/desirable. However, the following plan does not assume an increase in telescope time.

We would plan to observe ~8 nights per semester for this project, starting in the spring 2006 semester. With reasonable luck with the weather (between 4-6 nights clear), and assuming 8-10 objects observed per night, we should net between 32 and 60 objects per run. For the sake of discussion, let's assume a middle-ground number of 45 per run. In 4 semesters, we will net ~180 high-quality H-alpha images, while after 4 years will be have well in excess of 350 galaxies. These numbers will be adequate for computing the most precise estimate of the local SFRD yet available (the oft-quoted value published by Gallego et al. (1995) was derived from only 176 galaxies).

The data will be processed at Wesleyan by the PI, his students, and possibly by a postdoc we plan to hire. We have already developed the methodology for carrying out the reductions, and see no problems in completing the reductions and measurements rapidly. Once completed, the measured H-alpha luminosities can be turned directly into SFRs using standard conversion formulae (e.g., Kennicutt 1998).

ADDITIONAL COLLABORATORS: While we are prepared to carry out the project described about using Wesleyan resources and students, we would be happy to include others in the project who wish to collaborate. Others with small/intermediate telescope access would be welcomed, as this would speed up the data acquisition and lead to larger samples for the SFRD computation.

ALFALFA region:	TBD, exact location farily arbitrary (can pick to piggy back with other projects)
ALFALFA data to be used:	ALFALFA catalog
Other data sets to be used:	H-alpha imaging, obtained at WIYN 0.9-m telescope
Products to be delivered:	Archive of continuum-subtracted H-alpha images, plus tabulation of H-alpha fluxes.

Milestones (if duration is > 1 yr):

Milestone	Description	Goal
Team assembly	Recruit collaborators from among H-alpha groups where overlap is possible	Jan 2006
Data acquisition	Observe	Spring 2006 --
Data processing	Just that....	Complete within 6 months of completion of the observations
Prelim. Results	After 3 or 4 semesters, assuming a fair return from the observations	1 Jun 2008
Data Release	Data placed in ALFALFA archive on regular basis	Public release as agreed by rest of collaboration

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Last modified: Sun Oct 2 12:43:17 EDT 2005 by martha